New Research Published - Rooftop Solar Reaction to Fire

Above Roof Solar PV Panels over (BRoof) Plastic Tile Roof - 8 Minutes into Test.
  Image: Health and Safety Executive 

The UK's Health and Safety Executive (HSE) has published a new report detailing a series of fire tests performed on pitched roof installations of solar PV panels.  The results (and the pretty spectacular photos) should give the solar and construction industries much to consider, as the findings challenge long-held assumptions about how fire safety regulations apply when solar is fitted to buildings.

In this blog we'll take a look at the tests and what they may mean for future regulation of the safety of  solar installations.

You can access the full HSE report on this link: Fire Spread Over Pitched Roofs Fitted with Solar Panels 

Key Findings

1. Above-roof solar is not immune to fire.  Above-roof solar contributes to the spread of the fire by providing fuel to the fire, and by changing the fire dynamics of the roof covering for the worse.
2. Roof-integrated solar systems based on plastic trays performed far worse than those without plastic trays.  The combustible plastic tray accelerated the fire which spread twice as far and twice as fast when compared to a proprietary roof integrated solar system of panels with a push-fit aluminium flashing that avoids the use of plastic trays.
3. Glass-glass modules perform better than those with a plastic back sheet but still got involved in the fire once the glass broke and plastic encapsulant material was available to the fire.
4. Current fire regulations do not address the risk posed by fires initiated by electrical faults in the solar installation (for example faulty DC-DC connectors) which highlights the value of fire safety products like ArcBox.

This project by the HSE is an extremely helpful contribution to the ongoing debate about how to make solar installations safer, and should encourage industry participants to consider solar practice that does more than just meet the current (insufficient) regulations instead of waiting for regulations to catch up.

None of the findings should have come as a surprise to folks in the solar industry - see the featured news articles below, but the fact is that most people in the solar industry would  just prefer not to spend time thinking about this topic.  

Which another way to say that we've probably lost most of our readers already, but lets plough on and dive into the detail of the report...

The Tests

A crib of burning wood was placed behind the bottom left panel in an installation of four solar panels.  The intention is that the crib simulates an electrical fault in a DC-DC connector.  The panels are on a test roof pitched up at 45 degrees.  Fans at the base of the roof are activated after 2 minutes to simulate the effect of a wind pressure on the roof.

A range of sample combinations were tested which combined above roof systems with different panel types and roof coverings as well as testing different roof integrated systems.  The time taken for the fire to spread to the top of the sample roof was recorded.  If the fire did not reach the top of the roof within 17 minutes the fire was extinguished and the spread of the fire spread was inspected after disassembling the installation.

Above Roof Solar

The working assumption in the solar industry has been that if you install solar on racks above a traditional roof covering, that the fire performance of that roof covering is unaffected and a new fire classification is not required for the roof build up including the solar panels.

The HSE report blows a massive hole in this.  

Firstly, the tests with solar panels installed above incombustible concrete tiles showed that the backing sheet of plastic on the rear of the panels will contribute to the development and spread of a fire.  Glass backed modules were found to perform much better than those with plastic back-sheet but still provided fuel from the cell encapsulant after the glass shattered due to the heat.

Second, the test with panels installed above plastic roof tiles had the fastest spread of flame of any of the tests and needed to be stopped after only 8 minutes because of safety concerns (this is the image at the top of the page).  These plastic tiles have a BRoof (T4) fire classification - the highest possible fire classification for a pitched roof covering. 

The presence of the solar panels above the combustible roof covering changes the fire dynamics, trapping heat and reflecting it back onto the fire as well as funnelling air over the fire (a chimney effect).  

The tests make clear what has been evident from real-world fires - that above-roof solar is not immune to fire and this is especially the case when installed over combustible roof coverings.  Note also that none of the tests attempted to simulate the real world situation where combustible material such as bird nests or wind blown leaves have accumulated behind the panels.

Real World Fires

St. Martin's Hospital, Bristol, UK, 22/05/2025 - fire spread behind above roof panels on flat roof of this maternity hospital, necessitating the evacuation of the building.

Bow Wharf, Bethnal Green, London, UK, 2/7/2017 -  fire behind panels installed above slate roof during the refurbishment of this building.

Roof Integrated Solar

Roof integrated solar systems replace tiles or slates on the roof.  Two types of system were tested.  One type that covers the roof with overlapping plastic plates before fixing the panels above, and a second that does not use plastic trays and fixes the panels straight to the roof with an aluminium surround (flashing) that pushes into the panels.

The system using plastic trays performed far worse in the tests, with the fire spreading to cover about twice the roof area in half the time compared to the system without plastic trays.  The explanation for this is the large quantity of accessible fuel for the fire provided by the plastic trays.  

A number of test were run with the plastic tray systems that varied the type of roofing membrane below the trays using membrane with differing fire rating.  The finding that this made no difference to the outcome suggests that the plastic tray is the main contributing factor for the very rapid spread of the fire.

Real World Fires

House Fire, Roden, Netherlands, 04/09/2023 - fire spreads on plastic tray roof integrated solar system with the seat of fire near the top of the array which limited the area of damage.

North Prospect, Plymouth, 03/05/2022 - fire destroys roof of new build residential property in Plymouth.  Fire partitioning prevents spread to joined house.

Does the Rate of Fire Spread Matter?

The average total response time by the Fire Service for a house fire in England in the period from 2013 to 2023 was an amazing 7 to 8 minutes.  The length of time the tests ran for, and the growth of the fire in that time is representative of how developed a fire would be when the Fire Service arrives at the building. The differences between the reaction to fire of the different systems could make a big difference to the scale of the challenge that the fire service faces upon arrival and the amount of damage done to the building by the fire and the measures to extinguish it.

Source: UK Home Office Official Statistics

Fire Regulations

The fire regulations relating to roofs were designed for a world where roof coverings were passive materials like concrete, clay, and slate and the risk that needed to be managed was to stop a fire next door spreading to your building.  Roofs are tested and classified for their reaction to an external fire outside the outermost layer of the roof covering.  Depending on the classification achieved, there may be limitations placed on the use of the roof covering (its proximity to adjacent buildings, the maximum continuous area of roof covered).

When solar panels are added on top of an existing roof covering, the interpretation has so far been that the roof covering below the panels performs as if the panels were not there and you can use the fire classification for the roofing material when tested alone.  

This work by HSE is just the latest in a line of research that has been sounding the alarm that this approach is not a safe one (see list below).  The presence of solar panels above the roof covering clearly does change the fire performance of the roof.

• Experimental Study of the Fire Behaviour on Flat Roof Constructions with multiple Photovoltaic (PV) panels. Fire Technol. 2018 Nov;54(6):1807–28.
• Experimental study of fire propagation on sloped roof with building applied photovoltaics. J Phys: Conf Ser. 2024 Nov 1;2885(1):012047.
• Impact of flat roof–integrated solar photovoltaic installation mode on building fire safety. Fire and Materials. 2019 Dec;43(8):936–48.

The current regulations are insufficient and should be urgently reviewed, but  regulatory change is painstaking and slow.   In the meantime schemes like the Microgeneration Certification Scheme in the UK could require fire classifications for above-roof solar systems, and put this in place more quickly than traditional regulations will manage.

Mitigation Measures

While we wait for the regulatory environment to catch up, should the solar industry just carry on as it is, sheltering behind the argument that its practices are 'compliant with regulations', regulations we now know to be insufficient for the situation?

Risk = Likelihood x Consequence

The research has focused in the consequence of a fire that starts in the solar installation and how fast it spreads in different situations.  You can also reduce the risk by lowering the likelihood a fault occurring, or if it does occur stop it spreading to start the fire in the first place.  Clearly higher-quality installations with fewer errors is a good starting point, but technical mitigation measures can be specified that in the event of an electrical fault, help prevent the development of a fire:

• The ArcBox DC connector enclosure protects the solar connectors in the installation from external damage and in the event of a fault contains arcing to prevent the initiation of a fire.
• Arc Fault Circuit Interrupt (AFCI) is a technology that uses electronic monitoring of the current in the solar circuit to detect the presence of an arc and disconnect the power.
• Micro inverters reduce the DC voltage in solar systems to a level below that which can cause arcing.

Conclusion

The findings of this research, and the other studies mentioned, make it hard to avoid the conclusion that the solar industry needs to do more to reduce fire risks.

• Solar installations above combustible roofing materials (including building integrated solar installations) could be required to adopt additional mitigation measures against situations where the fire starts in the solar installation itself.
• A new test for external fire performance of roofs could be developed to allow the fire classification of a roof including above-roof solar panels and mounting system. 

Warm Homes Fund to Go All-in on Solar

 Announcement in January Expected to Include Grants for Solar PV

Zero Bills Homes by Keepmoat and Platform Housing Group (C) Viridian Solar

It is widely expected that a £13bn 'Warm Homes Fund' to be announced by Ed Miliband in January will include grants for the installation of solar PV and batteries.  If so, this will represent a big departure from previous policy in this area which have almost exclusively emphasised retrofit insulation measures.

Until recently the accepted wisdom has been coined 'fabric first’.  This catchy phrase summarised a prevailing energy-efficiency orthodoxy which held that until you fix the insulation and airtightness of a building (its fabric) there is no point using ‘expensive bolt-ons’ like solar.  

Fabric first became sacrosanct for some in the energy efficiency industry (and not only those who manufacture insulation), with every consultation on building regulations met with howls of criticism from some quarters for not going far enough on required insulation levels and any inclusion of solar or other technology criticised as 'green bling'.  

Numerous government policies have been influenced by fabric first thought – with ECO, Low Carbon Buildings Programme, and most recently the ‘Scottish Passivhaus’ rabbit hole that building regulations north of the border appear to be about to disappear down all prioritising insulation over renewable energy.

This dismissal of solar bling as unserious and insulation as the only ideologically pure approach to decarbonisation misses three important points: 

First, that thermal efficiency is a game of diminishing returns.  

Second, (and linked to the previous point), having run out of easy targets such as loft and cavity wall insulation, more ambitious retrofit approaches aiming for big improvements in thermal efficiency can be highly invasive, complex and risk unwanted side effects like damp and mould.

Third, that the orthodoxy arose at a time when our energy system was dominated by fossil fuel and renewable energy was expensive.  This is now out of date.  What matters more today and in future is when you use energy not how much you use.

Diminishing Returns

It's physics.  The more you insulate a building the more difficult becomes the next improvement in performance, until you are adding large amounts of insulation for only marginal gains.  Building regulations for new homes appear to have now reached this point since the Future Homes Standard consultation proposes no change to the fabric performance of new homes over those of current (2021) regulations.

In retrofit scenarios, the payback for simple low cost measures like loft insulation chimney balloons, lagging hot water tanks and pipes and draft excluders is measured in months while more expensive improvements like external wall insulation for solid walls can take many years to pay back their costs.

Unwanted Side Effects

Once you've lagged every hot water cylinder, topped up loft insulation where you can and blown insulation into walls with cavities, you're left with a large number of hard-to-treat properties with solid brick or block walls.  These require a layer of insulation to be fixed to the external walls either on the inside face which makes the rooms smaller or on the outside face which needs to be carefully protected against the weather. 

The challenges with solid wall insulation really became apparent once we moved from theory and pilot studies to pushing into volume in the real world.  The work is complex, expensive and intrusive and has sadly proven to be easy to get badly wrong at scale, with unwanted side effects such as damp and mould widely reported. 

A National Audit Office review of works done under the ECO4 and the Great British Insulation Scheme found that an amazing 98% of homes fitted with external wall insulation and 29% of those with internal wall insulation had major issues that need fixing.

The Economics Has Shifted

Fabric first approaches to energy conservation in buildings emerged at a time when renewable energy was ruinously expensive and the energy supply system was dominated by coal, oil and gas.  At this time, the careful conservation of energy was the only logical way to reduce emissions and lower energy bills.

Renewable energy is now the cheapest form of energy.  It getting more and more plentiful as investments in new solar and wind capacity expands.  The nature of renewables is that the timing of generation cannot be controlled in the same way as it can for fossil fuel based energy, but the falling cost of battery energy storage and advent of smart controls that react to time-of-use pricing signals are combining to overcome the weakness of intermittency in renewable generation.

Those who can adjust their energy demand to use power when energy is in over-supply can now take advantage of these tariffs to pay very low (sometimes zero, sometimes negative) prices for their power.  Space heating, domestic hot water and electric vehicle charging are all amenable to time-shifting or rate shifting.

New housing developments such as Hollymead Square in Essex and Beeston Canalside in Nottingham offer so-called Zero Bills Homes where the combination of solar PV, battery energy storage and electric heating with time of use tariffs and smart energy controls allow the energy supplier, Octopus Energy, to guarantee that the householders will pay nothing for energy for ten years after moving in. 

In this approach to low-carbon living it's not how much energy you use, its when you use it and how you combine that with maximising the use of low cost renewable energy you generate for yourself.

Learn to Love the Bling 

The case for using energy sparingly has not gone away, and simple, low-cost insulation improvements will always be high up the to-do list.  

However renewable energy is combining with smart energy management, electrification of transport and heating and battery energy storage to offer an alternative vision in which the when of your energy use is as important as the how much.  If the Warm Homes Fund recognises this fundamental shift, then that is to be welcomed.

Fire Classification of In-Roof Solar PV

Housebuilders, building control inspectors and Microgeneration Certification Scheme (MCS) auditors should be alert to the increasing risk that roof-integrated solar mounting systems that use interchangeable solar panels from third-party manufacturers will be installed with solar panels for which the combined system has no fire classification.

Building Regulations and Fire Safety

In the early hours of Sunday morning, Thomas Farriner was awoken by smoke coming under the bedroom door.  His bakery downstairs was on fire.  It was September 2nd, 1666 and over the next four days this fire would spread, quickly jumping across the narrow lanes from one timber-framed building or thatched roof to another.  From its small start in that bakery in Pudding Lane, the Great Fire of London as it became known went on to destroy almost all of the medieval centre of the city.  

The responses to the disaster included the emergence of the first insurance companies, together with their own private fire-fighting organisations.  In addition there was a new focus on creating building codes with the aim of preventing the spread of fire between buildings.

The Rebuilding of London Act 1666 laid out restrictions on the type of building materials that could be used ("all Walls in front and reere as high as the first Story be of the full thicknes of the length of two Bricks").  It also increased the distance between buildings by requiring London's streets to be widened. 

You can draw a dotted line all the way from this first law regulating the construction of buildings to today's Building Regulations.

The UK Building Regulations Approved Document B outlines the legal requirements for fire safety. 

If on reading the Requirement you're wondering how you go about demonstrating compliance you wouldn't be alone - it's woolly isn't it?  Fortunately the regulations go on to lay out "Guidance".  In practice, for most situations, most people will seek to show that their building meets the Guidance because meeting the Guidance is a Matter of Fact, whereas trying to meet the Requirement by other means is a Matter of Opinion.

The Guidance Requires a Fire Classification

Section 12 of Approved Document B deals with roofs, outlining the situations in which roof constructions with varying fire classification can be used.  For example, roofing materials with a lower fire classification may only be allowed when installed more than a certain distance from boundaries and with limitations to the maximum area of roof covered.

The resistance of roofs to external fire exposure is measured in terms of penetration through the roof construction and the spread of flame over its surface.

Roof constructions are classified within the European system as BROOF(t4), CROOF(t4), DROOF(t4), EROOF(t4) or FROOF(t4) in accordance with EN 13501-5. 

BROOF(t4) indicates the highest performance and FROOF(t4) the lowest, whereas the (t4) indicates the use of Test 4 in the standard.

An alternative route to classification involves testing to a British Standard, BS476-3, with Approved Document B providing a 'transposition table' that allowed a classification to 476-3 to be treated as if equivalent to a given classification to 13501-5.

• If a roof covering does not have a fire classification, it is not possible to follow the guidance in the Approved Document to demonstrate compliance with the building regulations.
• The classification is for the whole roof system, so it is not possible to add fire-resistant layers to an unclassified system and assume a performance – the whole system including the fire-resistant layer must be tested together as a roof build up.  
• Building applied PV (on-roof) currently lives in a grey area where a fire classification for the roof without solar panels is assumed to be representative of the classification of the roof with solar installed above it - despite a growing body of evidence that building applied solar does change the fire dynamics of a roof.  (See, for example,  1,  2,  3)
• Building integrated PV is not in a grey area – it must have a fire classification to be lawfully used, unless an Alternative Approach is used, requiring a report from a Chartered Fire Engineer for each building the system is used on. 

Interaction with the Microgeneration Certification Scheme 

The Microgeneration Certification Scheme (MCS) approves and lists both solar panels and solar panel mounting kits.  To register a solar installation with the scheme, an MCS certificated solar installation company must use a certificated panel and combine this with a certificated mounting kit. 

As a condition of insurance (for example those offered to buyers of new homes by the likes of NHBC, LABC Warranty), many housebuilders require an MCS certificate for the solar installations on their developments.

The MCS12 standard deals with solar mounting kits and it requires that where such kits either replace roof coverings or create excessive gaps in roof coverings that a fire classification to BS EN 13501-5 of BS476-3 is obtained and declared.

Some roof-integrated solar systems are proprietary, combining a dedicated solar panel and mounting kit into a single system whose component parts cannot be interchanged, so that once tested and issued with a fire classification, non-compliant installations are not possible.

Other roof-integrated solar systems consist of a mounting system from one manufacturer that can be interchangeably combined with a solar PV panel from any number of other manufacturers.  The MCS12 standard requires that these mounting systems have a fire classification for each solar panel family it can be used with and that these panel families are listed on the MCS12 certificate.  A fire rating achieved with one panel family is not portable to another family, even those from the same panel manufacturer, because differences in Bill of Material for different panels have been shown to result in different performances in the fire tests.

The online system of registering an installation with MCS (called the MID) only allows the issue of a certificate if the installation comprises both an MCS certificated panel and an MCS certificated mounting system, but crucially it does not check that the solar panel model used with the mounting system is listed on its MCS12 certificate as having a combined fire classification.  This means that it is left to the solar installer to ensure that only listed solar panels are used in combination with the in-roof mounting kit.

• For roof-integrated solar systems that allow interchangeable panels, a loophole in the registration software makes it possible to obtain an MCS certificate for a non-compliant combination of panel and roof-integrated mounting kit for which there is no combined fire classification
• Consequently, the existence of an MCS certificate for roof-integrated solar installation proves neither that the system is compliant with MCS nor meets building regulations.

Recent Difficulties Obtaining New Fire Classifications for Solar PV Systems

In 2023 EN 15725 was updated.  This standard deals with the “extended application of fire performance of building products and building elements”.  The new version removed the ability of fire test laboratories to use expert judgement to extend fire classifications under EN 13501-5 beyond that which was tested,  and instead limited such extended application only to those situations specifically dealt with in the standard.

The new version of EN 15725 did not include extended application guidelines for the fire classification of solar PV roofing systems.  

Some fire experts have interpreted that the new version of EN 15725 completely prevents the fire classification of solar roofing, arguing that expert opinion is required to define the test conditions.  Other experts believe that testing can still proceed but that a classification can only be issued for the exact system that was tested –in effect limiting the fire classification to a single solar panel model rather than a whole solar panel family with a range of electrical powers as was previously the case.

In September 2023, many of the limited number of test laboratories that are capable of testing withdrew from issuing fire classifications for solar roofing systems, further refusing to test and classify to BS476-3 even though EN15-725 has no direct bearing on this standard.

Hopes were raised that a written opinion from the British Standards Institute (BSI) committee with responsibility for BS476-3 (FSH22-8) would convince reluctant fire testing laboratories to re-open the BS476-3 route to classification, but these  subsequently dashed when a change to Approved Document B removed any reference to BS476, leaving only the European classification as a means of compliance with the Building Regulations.

Both routes to demonstrating compliance with MCS012 and Building Regulations were closed. 

Meantime, the solar industry marches on with its relentless technological progress.  New solar panels are constantly being launched and older panels withdrawn from the market.  The in-roof solar market is insufficiently large to influence the product strategy of global solar manufacturers, for whom utility scale solar drives the greater part of demand.  

Existing fire classifications remain valid, but without a means to test and add new solar panels to their list, the number of products that are still available in the market with which those roof-integrated solar systems that use interchangeable modules have a combined fire classification is diminishing month by month.

Furthermore, the interest of housebuilders in innovations such as Octopus Energy's ‘zero bills homes’ programme has created a pressure to maximise annual solar generation from the roof by using the most up-to-date solar PV panels with the highest power-density (N-Type or TOPcon panels).  In almost all cases, these newer panels post-date the withdrawal of testing, so do not have a combined fire classification with the mounting systems.  

Confusingly, some of these newer panels have a power output in a smaller size (108 cell) that matches older, larger-area (120 cell) panels for which the roof-integrated system may have a combined fire classification.  Solar installers have mistakenly concluded that these newer panels have a fire classification when they do not, by not checking beyond the brand name and power rating to consider whether the actual product code for the panel is listed as having a fire classification. 

• The publication of a new version of EN 15-725 in 2023 caused test laboratories to suspend new fire classifications for new PV panel families with roof-integrated solar mounting systems
• The number of products that are still available in the market with which roof-integrated solar systems that use interchangeable modules have a fire classification is diminishing.
• Housebuilders, building control inspectors and MCS auditors should be alert to the rapidly rising risk that roof-integrated solar mounting systems that use interchangeable panels from third party manufacturers will be installed with panels for which they have no fire classification in combination.
• It is insufficient to check that the manufacturer and power level is listed as having a fire classification with such systems – the product code must match – as the classification listed may be for an older panel of the same power, but larger format (area).

How to Get Through This?

The MCS is leading the response to the challenges created by the publication of the new standard and the withdrawal from solar fire classifications by test laboratories.

One possibility, now underway, is to create a industry guidance document for the testing and certification of solar PV roofing systems.  The idea is that this document will outline a consistent approach that test laboratories can then use to apply the tests in TS1187 to solar roof samples.  It is important that, for it to be widely accepted, the guidance should be developed in consultation with all relevant stakeholders including the solar industry, fire test laboratories, the safety regulators (for all devolved governments) and building control.

The development of this document may require a series of fire tests to demonstrate the approach is rigorous.  Given this, and the number of bodies involved it is becoming obvious that this will be no quick fix.

However, you cannot finish what you do not begin, so it is good that there is an agreed approach and that a start has been made.

What Should the Industry do in the Meantime?

Choosing a proprietary roof-integrated solar system that does not allow the interchangeable use of solar panels from different manufacturers greatly reduces the risk, but you should still ask for evidence of the fire classification for the system.

When using a roof-integrated solar system where third party solar panels can be interchangeably substituted, ask the mounting system manufacturer to provide evidence that the exact model of solar panel you intend to use has a UK fire classification (not an indicative classification) in combination with this mounting system, and take steps to ensure that this model is not substituted at any time during the project.

In both cases the MCS012 certificate for the product concerned will have a list of the solar panel product codes with which the system has a fire classification.

Moving Fast - Solar Uptake on New Homes

 

New data provided by the Microgeneration Certification Scheme (MCS) shows that the proportion of new homes built in England that come with solar PV has more than doubled in the last 12 months.

In the last quarter of 2023 it was estimated that 13% of new homes that completed construction had solar PV fitted by the developer.  This figure has risen to 29% in the most recent quarter and will continue to grow as housebuilders in England further transition to the 2021 version of Part L of the Building Regulations.

As the number of new homes with solar PV built by private developers has increased, the average installed power (kWp) per new build installation is falling.  

Prior to the new Building Regulations coming into force, solar on new homes was driven by either local planning requirements or fitted by self-builders as a personal preference.  Self-builders would size a system for a cost-effective contribution towards their own energy requirements.  Solar to meet a planning condition was on a site-wide basis and tended to be concentrated onto a few homes on the development to meet the condition in the most cost-effective way.  

By contrast the new Building Regulations apply individually to each plot but once the required energy performance of the building is met, developers generally see little reason to extend a PV system further - resulting in generally smaller PV installations.  The effect of this can be seen in the kWp/install column where the average has reduced over the period from 3.8kWp per installation to 2.8kWp.

Also worth noting is that solar on new buildings has risen from being 19% of MCS certified solar installations in England to 34% over the period.  As already mentioned, this transition has further to run and I predict that by the end of 2025, new build solar will account for at least 50% of MCS solar installations in the UK.

Notes on the Analysis

The number of new build MCS solar PV installations in the quarter was expressed as a proportion of new build housing completions in the same period to arrive at a percentage of new homes built with solar PV.

When registering a new solar PV installation with the MCS, the installer must tick a box to say whether the installation is on a new building.  The brilliant, publicly available MCS data dashboard does not currently allow users to filter the data on this basis, but the team at MCS responded to a request from Solar Energy UK and kindly provided us with the data.  I hope this functionality can be added in future.

The MCS certificate is issued only when the system is commissioned, which occurs after second fix.  This creates the potential for a timing difference between the MCS data and the data based on practical completion of the building (which may come a few weeks later).  The error this creates is mitigated against by aggregating to quarterly data.

It is also worth mentioning that it was not possible to split the MCS new build data into residential and commercial installations, so I have had to assume that the total is all residential, which will result in an over-estimate, but one that reduces as the number of new build homes with solar grows.  On the other hand not all solar installations will run through the MCS so this mitigates the over-estimate, as does focusing on the number of installations rather than the installed power (commercial solar tends to be larger systems in lower number).

Data for housebuilding completions is taken from this ONS dataset, which is quarterly, and only available up to Q2 2024.  The dataset was extended by one quarter by reference to this NHBC data which was used to scale the figure for Q2 to Q3.

The Future Homes Standard Consultation

 

The Future Homes Standard (FHS) consultation is here and while the consultation itself seems at first glance to be relatively straightforward, the whole thing is, in fact, an absolute beast.  Why?  Because at the same time as issuing the FHS consultation, the Department for Levelling up, Housing and Communities (DLUHC) also dropped consultations on its plans for a wholesale redesign of the underpinning calculation methodology that the building regulations require to demonstrate compliance with the energy performance of new buildings.

I decided to tackle the meal in bite-size portions, starting with the underpinning calculations and working my way up to this, my final blog on the consultation.  You can find my earlier blogs looking at the changes to the calculation on the links below:

The Home Energy Model

The Solar Calculation in the Home Energy Model

The FHS 'Wrapper' to the Home Energy Model

In summary the solar industry should welcome the change to the Home Energy Model which moves from a monthly calculation of energy to a half-hourly one, better positioning the building regulations to properly account for the benefits of technological advances such as solar PV, battery storage, solar diverters, and time of use electricity tariffs. 

At the same time, I found a concerning calculation error in the HEM that under-represents the energy generation of solar PV as its orientation shifts away from south-facing.  This needs to be corrected before the HEM can be used, and should also be taken into account when the government is assessing the responses from housebuilders, who will have been taking the results from the HEM at face value.

What's in the FHS consultation Itself?

The FHS consultation itself is on the face of it a pretty simple choice between two options for each of domestic and non-domestic buildings.  

Domestic Buildings

It will not be possible to meet the standard with gas heating, so all new homes built after the regulations come into force will be electrically heated or use a heat network.  For clarification this exclusion also applies to so-called 'hydrogen-ready' boilers that have been proposed by those advocating for the interests of the gas and gas boiler industry.  

In this way, as the electricity grid is decarbonised over time, homes built under the FHS will naturally become zero carbon.  (Note the circularity of this argument if the FHS itself only increases electricity demand without renewable generation then the job of decarbonising the grid becomes harder and takes longer, a point we return to later).

Government does not propose to change the minimum building fabric (insulation) standards for homes compared to the 2021 standards.  It believes that the 2021 standards provide a good basis for the Future Homes Standard.

The Option 1 specification is based on an air-sourced heat pump for domestic heating, solar PV covering an area of the roof equal to 40% of ground floor area, an enhanced air-tightness compared to current regulations, decentralised mechanical extract ventilation (dMEV) and a waste-water heat recovery (WWHR) system on any showers not on ground floor.

Option 2 removes the solar PV, heat recovery and ventilation and relaxes the air-tightness requirement.

The table below summarises the two options and compares the key specifications to the current building regulations.  At this point it is worth mentioning that housebuilders do not need to slavishly follow the specification in the building regulations.  They simply need to meet or exceed the performance given by a house of that specification.  For a fuller explanation of how the 'notional house specification' in the building regulations works please see my earlier blog on the topic.

Selected Elements in Notional House Specification by Building Regulations Update

Non-Domestic Buildings

Similar to housing, the new requirements for non-domestic buildings will be for electric heating and will have the same fabric as in current regulations.  There is an increase to airtightness for top-lit buildings to better support new requirement for heat pump plus more efficient lighting and heat recovery.

The two options

Option 1 (recommended) solar PV to 40% of foundation area for side lit spaces and 75% of foundation area for top-lit spaces

Option 2 (not recommended) solar PV to 20% of foundation area for side lit spaces and 40% of foundation area for top-lit spaces

Analysis and Comment

Domestic Buildings

If you assume grid electricity will soon be zero carbon, you could heat an open cave electrically and it would be a zero carbon home.  Once you've mandated electric heating you've met your goal and all other energy efficiency choices simply come down to the trade off between construction costs and running costs for the occupants.  

The consultation lists the government's desired policy outcomes from FHS in order of priority as follows:

1. Protect occupants against high energy bills

2. Reduce energy demand of homes

3. Reduce operational carbon emissions

4. Simple to understand and use

5. Consider peak electricity demand

The consultation also presents an analysis of the estimated extra building costs and the energy bills associated with the regulated energy - that is the energy demand in the building for heating, hot water, lighting and pumps and fans, not counting energy used for electrical appliances such as TVs, dishwasher, fridges and freezers.

It concludes that Option 1 (with solar) imposes additional build costs of £6,100 compared to current building regulations but reduces regulated energy bills by between £910 and £2,120 compared to a typical existing home.

By contrast Option 2 (without solar) imposes additional build costs of £1000 while resulting in a saving of between £210 and £1,420 on regulated energy bills compared to a typical existing home.

The consultation fails to compare the regulated energy bills from Option 1 and Option 2 to those from a house built to current building regulations (I suspect deliberately, and to flatter Option 2).  I have added a column on the right side of the table below showing this figure.

If the government chooses Option 2 then this will be the first time ever that a change to the building regulations on the conservation of fuel and energy results in an increase to householder's bills compared to the previous regulations, and at £580 per year, the increase is not small, in fact it nearly doubles the regulated energy bill compared to new homes being built today.

Stating that Option 2 is better than a 'typical house' is like saying that a new regulation that allows water companies to discharge 50% of sewage into rivers untreated is just fine because back in the 1920's we used to allow them to dump all of it.

Why the increase?  Well simply put although heat pumps generate heat at a far higher efficiency than a gas boiler, the energy supply (electricity) is more expensive than gas, which offsets the benefit completely.  Keeping solar PV in the specification generates energy that offsets the increase in bills.

Measured against the stated highest priority desired outcome from the regulations, Option 2 simply fails to deliver.

It should be remembered that tens of thousands of new social housing properties each year are also built to the building regulations.  Option 2 increases energy bills for these homes too, putting some of society's most vulnerable people at increasing risk of energy poverty.

Furthermore, the entire premise that these FHS homes will be zero carbon ready is based on an assumption that grid electricity becomes zero carbon pretty quickly.  Adding hundreds of thousands of electrically heated homes to the grid without at the same time taking an opportunity to add millions solar PV panels to the grid each year on the roofs of those homes will delay and make more difficult the job of decarbonising the grid, so Option 2 fails on the measure of reducing operational carbon emissions as well.

How Much Solar?

The consultation document says the amount of solar is 40% of ground floor area, but tucked away in an uncharted corner of the associated documents (the full notional house specification) is the calculation that turns the area into something more meaningful - the total panel power in kilowatt-peak (kWp), and there has been a significant change here too.

While 40% of ground floor area is unchanged from the current building regulations, the conversion factor from area to power has changed, see the illustration below. 

Solar PV provision in the Notional House of Selected Building Regulations

The previous conversion factor (1/6.5) assumed solar PV panels have a power density of 153Wp/m2.  The new factor (1/4.5) is a figure of 222Wp/m2.  So while at first glance the solar PV provision in the specification has not changed, it has in fact increased by 45%.

Is this justified?  Well the simple answer is yes.  Solar panel power density has increased over time from around 150Wp/m2 in 2015 to 207Wp/m2 today, with 220Wp/m2 looking probable by the time the regulations are in force.  (The table below shows the increase in specific power for Clearline fusion solar PV panels since 2015).

Specific Power Density of Clearline fusion solar PV panels since 2015

The challenge that is being voiced by colleagues in the housebuilding industry is that some of their house designs do not have sufficient roof space to accommodate the amount of solar called for in the notional house specification in Option 1.  These houses will have hipped roofs, dormer windows or other roof designs that limit the roof area available for solar. 

Under current regulations these house types can be compliant because the amount of solar in the 2021 regulations is lower than the FHS and also because other improvements can be made to the specification in excess of the notional house.  With the addition of higher air tightness, ventilation and heat recovery in Option 1 as standard, the opportunities to make up for less solar with improvements elsewhere are reduced.

The fact that the orientation of the solar in the notional house is assumed to be South, rather than following the orientation of the actual house makes it even harder to comply with the specification, the whole further exacerbated by the error in the Home Energy Model mentioned earlier, that grows as orientation deviates from South.

For this reason, housebuilders are saying that they cannot get behind Option 1 and will (regretfully and with heavy heart) be shouting loudly for the cheaper Option 2.

If government is minded to agree with housebuilder's arguments that they should not be required to ever change the design of houses they offer, then there are a number of potential adjustments that would make Option 1 more feasible for housebuilders:

• Fix the HEM so that it correctly accounts for solar orientations away from South
• Change the notional house specification so that the solar orientation is as per the actual house, or the best elevation of the actual house rather than assuming always south
• Change the notional house specification so that the actual roof design is taken into account when setting the % of floor area to solar.  It is far more expensive to build a complex roof than to put solar on a simple roof so the risk that this could become a loophole is slim.  This approach would better follow the logic of the notional house - the actual shape of the building below the roof is taken to be the same as the actual house, why not the roof?  A suitable formulation would be to ask for 40% of floor area or a max-fit power based on the available roof, agreed by the energy assessor, whichever is lower.
• Finally, regulators could go for Option 1.5, half way between Option 1 and 2 - retain solar, which is the measure with the highest impact on primary energy and protects residents from increased and volatile energy bills, but to leave out the other additional measures in Option 1.  This would give housebuilders design choices to allow them to 'flex' their specification for homes that struggle to accommodate the solar to match the notional house by increasing specification elsewhere.

I believe that many people that work for housebuilders genuinely want to be part of the solution to the climate crisis, and that with a few simple changes, Option 1 can and should be made to work for both the housing industry and the solar industry.

The Future Homes Standard Unwrapped

A Review of The FHS 'Wrapper' for the Home Energy Model 

The Future Homes Standard (FHS) consultation includes proposals for a wholesale revision to the underpinning calculation method by which the energy efficiency of new homes is evaluated.

I have already written about the Home Energy Model (HEM) in an earlier blog.  It is planned that this will replace the Standard Assessment Protocol (SAP) currently in use by Energy Assessors to calculate whether a new home design specification meets building regulations.  An FHS version of the Home Energy Model will be used to demonstrate compliance with the building regulations, by preloading the Home Energy Model calculation with a set of assumptions and inputs and defining the outputs it needs to provide, collectively called a 'Wrapper' for the HEM.

The consultation document on the Future Homes Standard wrapper can be found here.

In this blog I go through the most significant changes the wrapper introduces compared to the current version of SAP.

Occupancy

When a new home is built you probably don't know how many people will live in it and even if you do it will change over time, so calculations for building regulations require a standard occupancy.

The number of occupants is an important factor - affecting the amount of energy used for lighting, appliances and hot water use.

In SAP the standard occupancy was taken to be a function of the total floor area of the building, but for the FHS this will change to be driven by the number of bedrooms, apart from 1 bedroom dwellings which will have an occupancy driven by floor area.

In the graph above the dots show data from a national survey and indicate that SAP 10 occupancy (wide yellow line) is not a good match.  the coloured horizontal lines show the new occupancy level based on the number of bedrooms.  

A higher occupancy will increase the hot water demand (although this is offset by other changes, see below), and electricity demand for lighting and appliances.

Hot Water Demand

As in SAP 10.2, hot water demand is driven by occupancy, but demand per occupant is lowered based on new evidence from a 2021-22 study of 45,000 combi boilers in UK homes, which suggests that measures taken to reduce hot water use (for example low flow taps and shower heads) have had an effect on hot water use.

For contrast typical consumption in SAP 10.2 is 120 litres per day for a two person household and 160 litres a day for a three person household.  This reduction of nearly 20% will be offset by the higher occupancy for three and four bed homes.

Weather

The FHS standard is consulting on the use of regional weather data.  Historically housebuilders have been against this as it means they cannot build the same house all round the country (or rather they can but would need to meet the regulations in the most arduous location and therefore over-provide in others).

Another possible change is to use 'future' weather files based on Met Office climate projections relating to the assumed use period of the standard (2025-29).  

Electricity Demand

The wrapper also contains assumptions on electricity use for lighting and other appliances (dishwasher, tumble dryer, fridge, freezer and electronic goods).  

The FHS wrapper has to support the 30 minute time resolution of the Home Energy Model so both lighting and appliance electrical use has been split into daily profiles.  For now, these profiles are aggregated and averaged whereas in real life electricity demand is more 'peaky' with kettles being boiled for only a few minutes and freezers cycling on and off.  This smoothing will over-state self consumption of electricity for solar PV.

Compared to SAP 10.2 the demand for lighting is increased, reflecting the fact that homes have more and brighter lighting than in the past, although the efficiency gains from LED bulbs more than offsets this.

Emissions Factors

In the FHS, the emissions associated with mains electricity has fallen substantially from the value used in SAP 10.2, reflecting the decarbonisation of the generation mix providing UK power, and also a change in approach to predict the carbon intensity of the grid for the time the standard is in use rather than fixing it at the consultation value.

The big difference in the emissions factor for renewable generation is due to an 'accounting change'.  As explained in this document Fuel factors within the Home Energy Model: FHS assessment, the emissions factor for on-site generated is deducted from the emissions factor for grid electricity when calculating a reduction in dwelling emissions and primary energy.

This differs from SAP 10.2 where the onsite generated energy was a negative value and was multiplied by the renewable emissions factor to get a saving to take away from the total emissions or primary energy for the period.

A key factor to note is that solar generation exported to the grid produces the same benefit to carbon emissions and primary energy as solar generation used in the home.  The logic is that this exported unit of electricity is preventing the need to generate electricity at the grid factor and the benefit accrues to the dwelling.  This is in stark contrast to Scottish building regulations which since the 2023 revision have deliberately excluded the benefits of exported energy from contributing to the assessment of the dwelling performance.

Another feature of this approach is that energy storage in batteries produces no benefit on emissions or primary energy scores.  In fact the addition of a round-trip efficiency to energy stored in a battery for later use actually reduces the benefit of battery storage compared to export.  This will remain the case until the model can take account of the fact that grid emissions and primary energy varies during the day and a strategy of avoiding export by storing surplus solar generation for use in the evening will not only reduce energy bills but also result in a net reduction in carbon emissions and primary energy use because renewable energy is generally less of the grid mix at this time of day .

Conclusions

The addition of a time of day value for grid emissions and primary energy would be a welcome addition to the FHS Home Energy Model, but apart from this omission, the changes introduced compared to SAP 10.2 look benign from the point of view of the solar industry.

  

A New Solar Calculation for Building Regulations and EPCs

 An Assessment of the Solar Energy Calculation in the Home Energy Model

The HEM introduces a new variable - the degree of ventilation of the solar panels

As part of its consultation on the Future Homes Standard, UK Government has revealed details of its proposed replacement of the associated energy calculator.  You can read more about the new 'Home Energy Model' in my earlier blog on the subject.

Alongside the consultation on the Home Energy Model (HEM) a paper was published describing how solar PV generation and the proportion of energy used in the property (self-consumption) would be calculated.

Modelling PV electricity generation and calculating self-consumption within the Home Energy Model - A technical explanation of the methodology

Also available for the consultation is a prototype of the calculator implemented as a web page which can be played with.

Home Energy Model: Future Homes Standard assessment consultation tool

The solarblogger has been busy checking how this new tool will treat solar energy and in this article I'll be sharing my findings.

The Method

The energy output of the solar PV system is calculated according to BS EN 15316-4-3:2017 using the hourly procedure described in the standard.

Inputs to the calculation are:

• rated peak power (kWp) of the solar array under standard test conditions
• location of the house (which selects a climate file with irradiation data)
• orientation  
• tilt angle 
• the area of the solar array, and its height above the ground
• shading (captured as part of the general shading of the building)
• 'ventilation strategy' of the solar panel

This last input captures the difference between above-roof (rack mounted) solar that is rear-surface free, classed as Moderately Ventilated and in-roof (roof integrated) solar which is classed as Unventilated.

Since the HEM is modelled on a half-hour time slice, it can account for real-time variation in the PV generation and the energy demand in the property to estimate how much solar energy is used in the home, or available to charge a battery, divert to a hot water cylinder or export to the grid, as appropriate.  Generated electricity is assumed to be allocated in this order of priority 

1. To meet household demand
2. Into battery storage (until full)
3. To a PV diverter (until the hot water reaches maximum set temperature)
4. Exported to the grid

Consumed electricity is assumed to be taken in this order of priority:

1. From solar PV generation 
2. From battery storage
3. From the grid

Testing

The online tool provided with the consultation helpfully comes with two case studies - a detached house with two bedrooms, 82m2, with a heat pump serving both hot water and space heating and a bungalow with one bedroom, 40.5m2, direct electric heating and hot water.

The detached two bedroom house was selected as the base model and features of the solar PV system were varied and the annual energy generated was derived for each case.  This figure was compared with:

• the solar calculation in SAP 10.2, the predecessor to the HEM
• the solar calculation used for the Microgeneration Certification Scheme (MCS)

Panel Ventilation

The HEM introduces a new variable ignored in both the SAP10 and MCS calculations - the degree of ventilation of the rear of the panel.  Solar PV panel power output decreases with increasing temperature of the panel, so a panel installed with an open back side should produce more energy than the same panel with less ventilation to the rear.

Choosing 'Moderately Ventilated' produced around 2% more energy than SAP10, whereas 'Unventilated' produced 3% less (see graph at top of article).  The difference between Moderately Ventilated and Unventilated - 5% - is in broad agreement with this study by Viridian Solar / Cambridge University into the difference in yield between roof integrated solar and above roof solar which found a difference of 3%.

All the following comparisons are made with the ventilation set at Moderately Ventilated.

Location

The HEM solar yield prediction was compared with SAP10 and MCS at five different locations in England (rest of UK is not offered in the consultation version, which is for English regulations).

The HEM follows SAP10 closely.

Tilt Angle

The annual energy yield from a solar panel in the UK is optimal at around 35 degrees tilt angle from horizontal.  The HEM model follows the shape of the MCS prediction albeit at a lower predicted energy, closer to SAP 10.

Orientation

A solar panel facing south will generate the most energy yield each year in the UK, with progressively less energy the further from south it is facing, though the effect is less pronounced than most people expect due to the very high level  (around 40%) of diffuse light - that reflected from clouds, sky, surroundings - in the UK.

The HEM deviates very significantly from both the MCS and SAP10 predictions as the panel orientation moves further from south.  It starts matching SAP 10 closely when facing due south, but by north facing SAP 10 predicts 64% more energy yield.

This aspect of the HEM model is very concerning and warrants further investigation to check for a bug.

Self Consumption

The output from the software also shows the amount of solar energy used in the property and the amount exported to the grid, so it was possible to derive a scatter plot from all of the results generated in the above analyses and take a look at how the model predicts self consumption.

The plot below shows how the predicted proportion of solar generation that would be self-consumed changes as the size of the solar installation increases.  In this scenario, there is no battery storage or solar PV diverter in the house.

Comparing with some work done previously on SAP10 self consumption prediction shows that the HES predicted self consumption ratio drops more quickly than was the case in SAP 10.  It is worth noting that SAP 10 was based on a very small data set and it is possible that there is more and better data available against which to test the HEM both with and without battery storage.

Conclusion

The testing given to the HEM on its solar energy prediction has only raised one serious red flag - that the modelling of panel orientation looks off and should be checked.