Freddie Starr Exploded My Solar Panels

 

Get Used To It - The Mainstream Media Will Not Play Nicely

*An explanation of the above image follows later for younger readers 

An off-duty firefighter notices smoke coming from the roof of a home in Wellingborough and alerts the occupants who safely evacuate the premises.  The fire brigade extinguishes the fire, but only after the whole roof has burned away.  

This story was first reported by The Northants Telegraph, (26th May 2026, 12:26 BST) together with timelapse footage from a doorbell cam on the house opposite taking pictures every few minutes and animating them into a video.

This article reports the incident in a factual way, noting that the house has solar panels installed.

The story was picked up by everyone's favourite tabloid, The Sun, a day later (22:53, 28 May 2026)

In this telling of the story the doorbell footage shows the "terrifying! moment" that the "solar panels EXPLODED! ripping a hole! through the roof" and that the family "scrambled! to safety" as neighbours "watched in horror!"

The Sun article also includes the timelapse footage from the doorbell which, when combined with the misleading headline, does indeed look like the roof exploded.

Next up  the so-called quality press gets involved.  On 8th June The Sunday Times published an article written by Matthew Young of 21 Degrees in its 'Home Help' column offering advice on best practice installation of solar panels.  Matthew noted that solar is an electrical installation and comes with risks similar to those of other electrical appliances in the home, before outlining his advice such as choosing quality products from established manufacturers and having the system designed and installed by a company registered with the Microgeneration Certificaiton Scheme (MCS).  Despite all the sensible content, the article is published under the headline "Should I worry about solar panels exploding in hot weather?"

Meantime, back in the real world, fire investigators from Northampton Fire and Rescue Service return to the site to understand the cause of the fire.  The investigation concludes that the fire was likely due to an electrical fault in the loft, but it was impossible to narrow down the cause further due to challenges with access to the now structurally unsound roof.

So even after the investigation all we really know is that a house with solar panels on it had a fire and the fire started in the loft - maybe from elements of the solar installation, maybe from other electrical equipment.  Can we expect a retraction from the tabloids?  That solar panels don't explode blowing holes in roofs?  Not likely.

An old school friend of mine who had worked on the news desk at the Daily Mail once told me how they would know when their work was done.  Their readership would be too terrified to get out of their beds, at which point he would run a story about the dangers of bedsores.

Just like the bogus headline about a UK comedian putting a friend's pet hamster between two slices of bread to create a tasty late night snack, the news cycle has already moved on by the time the truth emerges.  People remember that solar panels can explode suddenly and our industry is just a little bit worse off.

What should the Solar Industry do?

We need to recognise two irrefutable facts.

1. There will be fires on more and more buildings that have a solar installation.  

The number of homes and commercial buildings with solar has grown to become a meaningful proportion of all buildings (more than 2 million solar installations in the UK).  Some of these buildings will suffer from fires - whether started by the solar or not, but people still see the 'new' thing on the roof and reach for a quick explanation - or maybe in the case of some of our national press, an explanation that fits in with a particular world-view.  The number of stories like this will grow.

2. In some cases the solar installation will be the cause of the fire.

Solar is a very safe technology, but a solar PV installation is an electrical installation and has all the risks that come with that, and perhaps adds in a few more of its own.  Solar remains energised while the panels have daylight, and the DC current side suffers from higher arcing risks due to the comparative stability of the DC arc.

An Action Plan for the Solar Industry

1. Reach a consensus on current best practice for fire safe solar installations, promote its uptake across the industry.  There is already work going on in this area, notably by Jonathan Bates at  Solar Energy UK and the team reviewing the RC62 document. 

2. Consider adopting as mandatory some of these best practice measures through changes to MCS standards.

3. Gather quantitative evidence on the relative fire risks from solar installations.  The most recent studies are more than 10 years old and the technology has evolved greatly since then.  The recently launched PV-FIN initiative is building evidence on the causes of fires involving solar equipment.  Crucially this work aims to add near-miss evidence, which is especially useful since collecting meaningful physical evidence from the aftermath of a well developed fire is difficult.  This work should be welcomed by the industry and its findings, when they come, studied carefully before further updating the best practice guides.

What the industry cannot do is just hope that the news cycle moves on and reporting of "solar fires" will be forgotten.  Parts of the UK media continue to promote negative news about electric vehicles, heat pumps and utility scale solar.  These outlets are unlikely to let the facts get in the way of a good story that is damaging to the solar industry.

*Freddie Starr was a stand up comedian and entertainer in the UK at his peak of fame in the 1970s.  In 1986 one of the best-known British tabloid newspaper headlines of all time featured Starr. The Sun ran its front page with "Freddie Starr Ate My Hamster". The man behind the hamster story was Starr's agent, the British publicist, Max Clifford - at that time Starr's agent.

Future Homes Standard - Homes that Cannot Meet the 40% Solar Requirement

 

The Future Homes Standard (FHS) will make solar PV panels mandatory on all new homes from 2027, with the government announcing that panels will have an area of at least 40% of the ground floor area of the building.  

In this article I'll take a look at how the calculation works, what amount of solar it might result in and how exceptions are dealt with when roofs don't have enough room.  I'll also share an early estimate of how many houses might not be able to meet the requirement in full.

How Much Solar?

The actual requirement for the amount of solar is, quite rightly defined in terms of the total rated power, which means that if you use higher performance panels, you can reduce the area or conversely if the panels you use are of low power rating then a greater area must be installed.  

The actual target is to install solar PV that has the same annual output as a solar array with the following characteristics: an area of 40% of the ground floor area, a specific power of 0.22 kWp/m2, facing SE or SW at a pitch angle of 45 degrees and with little or no shading.

So for an 85 m2 semi detached home, arranged across two floors we might have 42.5m2 of ground floor area and a target solar installation of:

40% x 42.5 x 0.22 = 3.74kWp

This figure could be met for example with ten 405Wp solar panels, or nine 445Wp panels.  It's worth noting that the current average installation we see under Part L 2021 is between four and five panels per house, so the Future Homes Standard represents a rough doubling in the quantity required.

Any amount by which the solar provided exceeds the target counts towards achieving the overall energy targets for the building, so there are benefits to going higher if the roof can fit more.

What About Complex Roofs?

The regulations makes allowances for roofs that are an interesting shape or that have features that limit the amount of solar that is possible, and this is outlined in Appendix B9.  The developer must provide evidence to the building control body to show that it is not possible to install sufficient solar on the roof to meet the requirement.

Roof diagrams showing the roof with and without solar panels showing that the installation provides the maximum PV capacity and annual generation possible for the roof orientation and layout must be provided.

The diagram showing the maximum possible roof area for solar PV has to be made using specific guidance given on the minimum allowable offset distances from other roof features.

An example of a suitable diagram showing the maximum solar possible on a house is shown at the top of the page.

The target for solar PV then becomes a system that produces an amount of energy each year that matches that of a system with the area of solar in the drawing, a specific panel power of 0.22kWp/m2 and the orientation and angle of the actual roof pitches.  This figure is then also used in the Notional House specification so there is no penalty there for having to use a smaller solar system.

At present there are no restrictions on how close to roof edges a solar panel installation can be made (subject to having a sufficient wind resistance to install in an edge zone and a sufficient fire rating to install adjacent to a party wall).  Consequently it might be possible to provide a B9 drawing showing a maximum solar area, and install something different -  so long as it matches or exceeds the annual power generation 

For example this could be achieved by using higher efficiency panels than those in the standard, and covering more of the most advantageous roof pitch, while dropping solar off the less advantageous roof pitches, to meet or exceed the annual energy yield of the B9 drawing.

However, on the same day that the FHS was revealed, the Health and Safety Executive opened a consultation on Approved Document B - the building regulations for fire safety.  This includes what would become mandatory provisions for minimum offset distances for solar panels, so reducing the possibility to use tighter margins to outperform the regulated max-fit.

Something that has been missed in the guidance is to define what size of solar panel should be used in the assessment. This really needs to be clarified because otherwise people will get very different outcomes depending on what panel size is chosen. 

How different panel sizes give different maximum-fit outcomes

 I have discussed this point with colleagues at the Future Homes Hub and the plan is to develop an industry guide in conjunction with representatives from building control bodies to fill in some of the missing detail from Appendix B9.  This way we can hopefully have a consistent approach across industry.

What Proportion of Homes Will Fit 40%

As part of our work at Viridian Solar in support of our housebuilder partners, the design team has performed studies on sample developments to assess which house types can meet a 40% requirement and which cannot.

So far we have seen around 80% of houses on a typical site can fit the full requirement of solar panels on the best elevation.  Of the houses that could not fit the requirement, on average one half of the requirement could be fit on the best elevation, so around 20% of the ground floor area. 

Under the approach in Appendix B9, these homes would need to use all the possible other elevations as well as the best elevation to maximise the solar generation possible, including North facing elevations if necessary.  Depending on the size of the less advantageous roof pitches, this might result in a greater number of solar panels being used due to the penalty that comes as panel orientation moves further away from South facing.  

Of course, this analysis was done before the final regulations were published, and so was using our own manufacturers' guidelines on offset distances which are tighter than those in the FHS.  It is likely that a higher proportion of houses will struggle to meet the 40% target using the Appendix B9 guidance.  No doubt the design team will continue to be working with housebuilders re-evaluating their house types to the final version of the standard.  I will post an update when we have got more data from this work.

When will Homes be Built to the Future Homes Standard?

 

Accompanied by much excitement in the solar industry, the Future Homes Standard (FHS) consultation response was finally published on 24th March 2026, and little wonder because the new regulations make renewable electricity generation (solar PV) mandatory on nearly all new homes.  But how long will it take for the new regulations to feed through into bricks and mortar on the ground, and more importantly solar panels on the roof?

The transitional arrangements published alongside the new regulations give developers some breathing room to get themselves ready to deliver the new specification at scale.  The rules come into force in 12 months.  From March 24th 2027, any new development that applies to Building Control for approval will see all homes on that site built to the new regulations.

A second deadline follows in March 2028.  After this, all homes not yet "commenced" will need to be meet the FHS, irrespective of when the Building Control application was lodged.

The transitional arrangements are identical to those that applied to the last update to the building regulations for energy efficiency - Part L 2021.  So we can use the last building regulations transition as a model for the next. 

What Went Before - Part L 2021

Part L 2021 was announced in that year, with a start date of July 2022 for new sites and July 2023 for houses not yet commenced.  The graph at the top shows the number of housing starts in England (black line) and completions (blue line) from 2018 to 2025.

The solid orange line shows the number of housing completions that were built to Part L 2021.  This data is provided by the Future Homes Hub and inferred from the software version used to generate the Energy Performance Certificates for the homes.  The dotted orange line is shifted three months as an estimate for roofing works going on, and solar panels being installed.

In December 2025, only 60% of homes that completed in England were built to Part L 2021 - some three and a half years after the regulations came into force.  How is this possible?

The clue to the answer to this question is found by looking at the black line.  There was an unusual spike in housing starts in the run up to the June 2023 cut-over date.  Housebuilders commenced nearly 30,000 more houses than in a normal quarter.  Strip foundations were excavated and concrete poured to classify plots as commenced.  These foundations were then mothballed for many, many months before being built on later - and locking in the older, and lower cost regulations.

What This Means for the FHS

The FHS imposes higher costs on developers, around £5,000 per house according to the Government's own Impact Assessment, so the incentive to delay the change over will definitely be there, just as it was for Part L 2021.  

There are a couple of potentially important differences this time around which may influence how long the transition takes.  

Firstly, the 2023 rush to lay foundations and beat the deadline for commencement occurred as mortgage rate increases precipitated a house buyers' strike.  The foundations lasted longer than they would have if the housebuilders were working at their usual rates.

Secondly, what counts as a "commencement" changed in late 2023 and now requires the first floor structure to be complete.  So for a house with a concrete slab ground floor, to the cost of the foundation you can now add the cost of building the below ground walls, a base-layer of hardcore and blinding and then pouring a concrete slab, plus all the drainage and service ducts required.

Will this second change be enough to change the financial calculations of the housebuilders - it's too early to say, but given the size of the prize from the cost saving, it's hard to see why the housebuilders won't rinse and repeat what they did for the 2021 regulations.

A sober assessment of how quickly the FHS will feed though to demand for more solar PV puts the start of the ramp in June 2028 and runs through to the end of 2030.

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.

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.

New Scottish Building Regulations Torpedo a Solar Success Story

On December 22nd 2023 Scottish Government published an update to the Building Standards Technical Handbook which will apply to new building sites where a building warrant is applied for after April 1st 2024.

The update to the handbook was made to implement the New Build Heat Standard which bans the use of ‘Direct Emission Heating Systems’ - gas or oil boilers in plain English - from newly built homes and some conversions of existing properties.  Instead, developers must now choose from a heat pump, direct electric heating (storage heaters or infrared panels) and connecting to a heat network (if available).

In 2015 Scottish Government introduced solar PV into its building regulations - well ahead of England which was to take until 2021 to catch up.  This has resulted in a thriving solar industry in Scotland, installing far more solar to domestic properties per head of population than the rest of the UK (see my earlier blog: How Progressive Building Regulations Made Scotland a Solar Powerhouse).  

Unfortunately, Scottish Government has ignored repeated warnings from Solar Energy Scotland that the introduction of the New Build Heat Standard without an accompanying adjustment to the Building Standards could threaten the success story of Scottish solar.

Working in Silos

To understand how the new regulations could harm the solar industry in Scotland we need a little background on how the building regulations work.  The regulations are not prescriptive, they aim to give the designer freedom to choose how to build the house - instead of defining each and every building element, the set a level of  energy performance that the house must achieve.

A developer must show that the house they are planning to build uses no more energy than a home of the same size and shape built according to a defined specification called the Notional House Specification.  (For more details on how this works see my earlier post on Energy in Building Regulations).  Over time the regulations have made new homes more and more energy efficient by changing the Notional House Specification to have a better and better energy performance.

The last major review of Building Regulations in Scotland (in 2021) introduced two specifications for the notional house - one for homes with heating by a gas boiler, the second for a house with a heat pump.

In order to 'nudge' developers towards using more heat pumps and away from polluting gas boilers the specification with the heat pump included a number of cost-saving relaxations in other areas - notably the omission of solar PV panels which were included in the gas heating specification. 

Although it might have been an admirable intention to nudge developers towards heat pumps (it didn't work by the way - gas boiler and solar remained the preferred design choice), it was clear that if the New Build Heat Standard was to come in without changing the notional house specification at the same time, then the only legal specification becomes the one with the heat pump, and solar would be dropped from the notional house in Scotland for the first time since 2015.

Unfortunately we were talking to two different sets of officials from Scottish Government - one working on the building regulations and the other working on the New Build Heat Standard.  Our concerns were ignored.  The latest regulations have enacted the boiler ban and left the Notional House Specification unchanged.

The Impact on Solar  for New Homes in Scotland

This unwelcome development is not necessarily all bad news for solar.  

First of all, housebuilders may choose to combine solar PV with a heat pump in their designs, not least as a way of keeping a lid on energy bills for their customers.  This change to the building regulations is probably the first ever update to result in higher bills for consumers.  Electricity costs far more than gas does - and the enhanced efficiency of heat pumps does not make up for the difference.  Developers can offset this rise in bills by keeping solar in their design.

Secondly, solar has become a common sight in new developments across Scotland and customers have come to expect it on new homes and increasingly see energy efficiency as a reason to buy new rather than in the general housing market.

Third the gas boiler plus PV specification still applies to homes with direct electric heating.  Developers may explore this option, adding better insulation to the point where space heating demand is reduced to an absolute minimum - especially for smaller properties.

Finally, since new regulations only come into force from the point of applying for a building warrant it is likely to take a year or so before sites begin construction under the new regulations, and 2-3 years before the majority new homes being built are to the new standard.  What could change in this time?  Here are some thoughts:

• Alex Rowley MSP proposed that Scotland move to a Passivhaus basis for building regulations, and in December 2022 Scottish Government announced that it would legislate for this by December 2024.  We may have a new version of the regulations in less than 12 months time.
• The European Commission announced a strengthened Energy Performance of Buildings Directive in December 2023 in which "Installing solar energy installations will become the norm for new buildings".  The current Scottish Government is very keen on aligning its regulations with the EU with its goal of quitting one union and re-joining another.
• A new Future Homes Standard for England is embracing smart energy, time of use electricity tariffs, energy storage and solar generation with a new half-hourly calculation method, and is beginning to make the Scottish approach look rather passé.  A strong Future Homes standard specification with heat pump and solar will encourage the Scottish Government to surpass it.
• Who knows how far battery storage, solar and smart energy technology will developed by this time?  If we were to go back four years, battery storage was only for dedicated enthusiasts and off-grid applications - now it's included with around half of all retrofit solar installations.  The performance and cost of solar and energy storage continues to make the technology more widely applicable and attractive.  More recent innovations such as time of use tariffs and electric vehicle to grid charging will only add to the advantages of having solar on your home.

Whether the omission of solar PV from the notional house specification in Scottish building regulations slows the adoption of solar PV in Scotland remains to be seen.  What is clear is that, at worst, it will only temporarily slow its rise.