Friday 26 January 2024

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.


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.


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 .


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.


Friday 19 January 2024

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.

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

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


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.


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.

solar energy yield with locaion

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.

Solar yield vs panel tilt angle


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.


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.

Thursday 18 January 2024

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.


Tuesday 2 January 2024

The "Home Energy Model" - The Artist Formerly Known as SAP


Alongside its consultation on the Future Homes Standard building regulations, the government has revealed sweeping changes to the calculation underpinning Part L of the building regulations (Conservation of Fuel and Power), and launched a consultation on the new approach.

The StandardAssessment Procedure (SAP) has been the government approved methodology to estimate the energy performance of homes in the UK since 1993, a time when it was felt to be important that the method be simple enough to be completed with pen and paper and calculator. 

Inevitably, computer software emerged to make the job of energy assessors more convenient.  Provided by third party companies, these applications needed to be checked by the Building Research Establishment (BRE), the body responsible for the development of SAP, before they could be used to demonstrate compliance with building regulations.

Over subsequent versions of SAP issued in 1998, 2001, 2005, 2009, 2012 and 2022 the complexity of the model increased.  Building elements were dealt with in more sophisticated ways to improve the accuracy of the model (for example the treatment of junctions in thermal insulation or - a personal favourite – an improved treatment of the performance of solar thermal systems in 2005).  New technologies were more widely adopted in construction and needed to be added (for example battery storage in 2022).

SAP's Growing Pains

Shortcomings of this approach had begun to emerge over time but were brought into sharp focus by the 2022 implementation.  Developers found themselves struggling to work out how to build new homes that complied with the new building regulations already in force even as the third-party software was unavailable due to delays in the certification process.

Another reason for a wholesale review of the model, flagged by the Climate Change Committee, was the emergence of key technologies that couldn’t be easily or accurately added to the existing framework in a timely way:

  • Solar PV and self-consumption of generated electricity
  • Battery storage of electricity
  • Solar PV diverters
  • Time of use energy tariffs
  • Smart energy controls – timing the use of energy to coincide with cheap tariffs and the availability of renewable energy.

The SAP model was based on a monthly time resolution.  This meant that the impact of new technologies had to be demonstrated in real life studies and an average performance across multiple households derived before a simplified month by month impact could be added to the model.

A case in point was the introduction of battery storage in the 2022 version.  Data was scarce because the technology was relatively new.  A simplified average performance that linked installed solar capacity, total energy use and battery storage capacity was derived by applying a line of best fit to the available data.  This formula was incorporated into the monthly SAP model.  No data was available on homes that combined battery storage with solar PV diverters, so SAP only allowed one of the technologies to be used at a time.


 A New Approach

The government is consulting on a fundamental re-working of the model.  It’s such a big change that SAP has been dropped in favour of a new name ‘The Home Energy Model’.  Changes include:

  • The model will be available as a cloud-based software ‘core engine’, with the source code published on GitHub
  • ‘Wrappers’ will be published for different applications – comprising different starting assumptions as inputs which will then feed into the same core engine.  The first wrapper to be published will be for the 2025 building regulations, followed by a wrapper for the generation of Energy Performance Certificates (EPCs) for existing homes.
  • The software runs on a 30-minute time resolution, allowing better modelling of smart technologies such as solar, battery storage and time-of-use energy tariffs
  • An updated solar PV generation calculation is based on the hourly methodology in BS EN 15316-4-3:2017, which includes the effect of ventilation on the rear of the panels.

Impact on Solar

 The solar industry should welcome the change from SAP to the new Home Energy Model.

The move to cloud-based software brings the approach up to date.  The separation of a core engine, based on best available building physics modelling, from the ‘wrappers’ which clearly surface the assumptions and inputs into the model for specific applications such as building regulations or EPCs makes it much easier to interrogate how the ‘black box’ is working.

The change to a half-hour resolution better supports enabling technologies that work with solar PV – battery storage, smart energy controls and hot water from PV fed immersion heaters.  This will further cement the position of solar PV as a normal part of any new home built in the UK.

The change to the hourly methodology for solar generation needs to be carefully assessed, I will be writing about this in my next blog.