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.

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. 

 

 

 

 

 

 

 

Solar PV and Primary Energy in Building Regulations

The Future Homes Standard consultation has proposed that a new requirement based on Primary Energy use should be brought into the next building regulations .  In this article we look at how the Primary Energy use of a house might be calculated, and what is fair or desirable for solar.

Primary Energy is energy found in nature that has not undergone an artificial (man-made) transformation process.  Electricity generated from gas, oil, coal, nuclear or biomass is secondary energy - the original fuel found in nature has been extracted, transported and converted to electricity.  All the way along the process, energy is used or lost.  So to deliver one unit of electricity to your home, a greater number of units of primary energy is consumed.

In an earlier blog I explained the concept of Primary Energy and Primary Energy Factors, see What is Primary Energy.

The Primary Energy Factor (PEF) is a measure of how many units of primary energy are needed to get the unit of final energy to your house.

So, for example in SAP 10.1 (the draft calculation method for the next building regulations) the primary energy of natural gas from the gas grid is given as 1.13, meaning that for every kWh of gas delivered to your house, gas of energy content 1.13kWh needs to be taken out of the ground.  This figure is a weighted average of the PEF for all the different sources of natural gas that make up the UK supply - for example gas extracted from wells in the North Sea, Russia, USA and Qatar.

Primary Energy of Electricity

Electricity is even more complex.

The generation mix includes power stations that use gas, oil, coal, plutonium and wood as their feedstock, each with different Primary Energy Factors once they have been converted in a thermal power station and transmitted across the power distribution network to your consumer unit.

In addition to these thermally generated electricity sources you can add direct conversion renewables such as wind turbines and solar PV panels.  The convention is that, since the natural energy they convert is limitless, the PEF for energy generated this way is 1.0 at the point of generation.

So the electricity generation mix results in an average Primary Energy Factor that depends upon which types of electricity generation are in use at any time.  SAP 10.1 makes assumptions about what the UK's electricity generation fleet will look like in 2020-25 and estimates what the average combination will be in each month of the year.  The average figure through the year for grid electricity at the point of use is a PEF of 1.51.

A home fitted with solar PV panels will generate solar electricity.  At some times the solar electricity will exceed the electricity use of the house and electricity will flow back onto the distribution network where other buildings will use it, so called export.

Increasingly, homes with solar are also fitted with other technology that allows the building to retain more of the solar generated electricity and minimise amount exported to the grid.  Devices include PV power diverters that send excess generation to an immersion heater to heat water in a hot water cylinder, battery storage to keep the electricity generated during the day for use in the evening and smart car chargers that optimise car charging to use self-generated renewable power to the max.

Of these three technologies, SAP 10.1 includes provision for PV diverters and battery storage, but does not give credit when both are used in the same house.

The Primary Energy Use of a Dwelling

The Primary Energy used by a house is calculated by adding up the total of the different energy types used by the house, each multiplied by the Primary Energy Factor (PEF) for that energy type.

The building regulations is rightly focused on the Net Primary Energy use over the year - which takes into account both the flows of energy into and out of the building and allows for energy generation in the building.  Since the solar PV system is part of the building the solar energy flows that cross the boundary consist only of the solar generation exported to the grid.  (See image)

Solar energy generated by the building and used in the building reduces the electricity needed by the building.  So the first benefit of putting a solar system on a house for the net Primary Energy used is the solar energy kept in the building multiplied by the Primary Energy Factor of the electricity use that was avoided - the PEF of grid electricity.

The second benefit of a solar system on a house is that the solar energy exported from the house is a negative flow of energy and should reduce the net Primary Energy use of the house.  The government has suggested in the consultation that the PEF for this exported electricity should be 0.51.

Why not a PEF of 1.0?  This is the primary energy factor for solar generated electricity at the point of generation (before transmission losses).  A query to the team in charge of SAP received this explanation:

Since grid electricity has a PEF of 1.51, and solar electricity has a PEF of 1.0, the net benefit of the exported electricity is 1.51 - 1.0 = 0.51 per unit of electricity exported.

In effect what they're saying is that this unit of solar generated electricity with PEF 1.0 flows into a nearby building and saves that building from using electricity from the grid generation mix with PEF 1.51, so the net benefit (to the grid) of the exported electricity is 0.51

I can see that there is a logic to this, but on balance I think the net Primary Energy of the building can and should consider the building and not the grid - this means that the flows of energy that cross the building boundary are the ones that matter.  Unlike carbon emissions where you cannot apply a carbon saving to the exported electricity without considering the carbon emissions avoided where that energy is used, Primary Energy for solar energy has an agreed value for the PEF, and that value is 1.0.

Grid connected PV feeds into the grid at 1.0, why should microgeneration be treated any differently?

The government is keen to promote the uptake of smart technologies such as PV diverters, smart hot water tanks and battery storage, and so is the solar industry.

Does a lower PEF for exported solar energy not create a stronger driver for the uptake of these technologies in new build?  Surely the lower the PEF of exported electricity the greater the value of installing equipment to use the solar electricity in the building?

The answer to this question is yes, but only up to a point.  If the calculation is set up in such a way that solar starts to look less appealing as a technology to achieve building regulations, housebuilders may not use it at all, and then there will be zero incentive to add in smart technologies that increase solar energy utilisation.

Giving solar export a PEF of 1.0 still creates an incentive to use smart technologies, but without so significantly diminishing the benefits of solar PV.  In addition, there will be a new affordability criteria in building regulations and this will create an incentive to keep energy in the building (saving 16p/unit) rather than exporting it (yielding only 5p/unit).

BRE and DCLG should reconsider the logic behind the treatment of solar PV in the net Primary Energy calculation in building regulations as the approach being consulted upon runs the risk of unfairly under-reporting the benefits of solar electricity.

The Future Homes Standard Consultation

Where next for Building Regulations?

In the week where Extinction Rebellion activists were arrested for hosing the Treasury in 'blood' in protest at the lack of progress on tackling a climate emergency, the consultation on the Future Homes Standard came out.  There's talk of solar panels for all new homes - so let's take a look under the hood of the consultation.

The consultation itself consists of two main parts - consideration of the Future Homes Standard due to come into force in 2025 which is intended to deliver "world-leading levels of energy efficiency" for new homes and  an update to the Building Regulations Part L (energy efficiency) and Part F (ventilation) in 2020 to provide a "meaningful but achievable" uplift in energy efficiency as a first step towards the 2025 vision.

There's also a raft of supporting documentation

The Standard Assessment Procedure (SAP) calculation version 10.1
An Impact Assessment, which includes details of cost assumptions
Approved Documents L and F 

2020 Part L - a Stepping Stone to Future Homes 2025

There's a lot to talk about here.  This is no 'tweak' but a significant revision, at least in part forced by the significant changes to the carbon intensity of grid electricity, but also by the Grand Challenge Mission for Buildings, announced by Theresa May about a year ago.

1. Primary Energy Use is the new Gold Standard

Until today, Part L has always used carbon dioxide emissions as its measure of compliance with regulations.  Buildings had to achieve a certain Dwelling Emissions Rate (DER) in kgCO2/m2.

DCLG has rightly concluded that as the electricity provided by the grid comes with a lower and lower carbon intensity, developers could switch to electric heating and hit a carbon target without improving the energy efficiency of buildings.  If energy efficiency of buildings is not improved, then decarbonising the grid becomes more challenging and costly.  So a new measure is required and primary energy, which has the benefit of aligning UK regulations with the measures chosen in the EU Energy Performance of Buildings Directive, is added as a new metric.

(See this article on the rapid progress made in decarbonising the grid.)

The latest revision to the government's Standard Assessment Procedure (SAP) version 10.1 has been published alongside the consultation.  This is the calculation used to demonstrate a house complies with the building regulations.  In this version of SAP the carbon intensity of electricity is set to 136gCO2/kWh, a projection of the average from 2020-2025, and a massive reduction from the value of 519gCO2/kWh in the current version of SAP 2012.  Electricity now produces less than 65% of the carbon emissions of mains gas (which is at 210gCO2/kWh).

By contrast, the primary energy content of a unit of electricity is 1.501 compared to gas at 1.130.

This document explains primary energy and how the values were arrived at

Fitting solar PV to a property reduces the grid electricity that is needed by the house, solar PV generation used in the building (self-consumption) reduces both the carbon emissions and primary energy by the same factor as grid electricity.

Electricity sold to grid also reduces both the carbon and primary energy use of the dwelling but it's primary energy factor is only 0.501.

The impact of this is that a unit of electricity generated by PV and used in the building would save 1.501 kWh of primary energy use, but a unit of PV generated electricity exported to the grid would only save 0.501 kWh of primary energy use in the calculation.

Since the benefits of battery storage (SAP Appendix M) and PV diverters (SAP Appendix G4)  have also been added to this update to SAP, the combination of using primary energy as the main regulatory target and the low primary energy factor for PV export has the effect of incentivising measures such as these to use as much PV-generated electricity within the building.

The trouble with this is that

(a) developers prefer combi boilers so there's no hot water cylinder in most new homes for a PV diverter to divert excess electricity into.
(b) batteries are approaching cost effectiveness but are likely to be seen by developers as an additional cost and not a sellable benefit.

We understand that the logic for choosing this value for exported energy is that the exported energy has a primary energy factor of 1.0 (renewable energy), and displaces a unit of energy from being fed into the grid at the grid average of 1.501, so the net benefit to primary energy added to the grid is 0.501.

The solar industry might argue that considering things from the point of view of the building produces a different logic (and after all what we're supposed to be modelling is the energy performance of the building).  The net primary energy consumption of the building is the electricity imported at a primary energy factor of 1.501 less the PV generated electricity exported which should have a primary energy factor of 1.0. 

A minimum carbon emissions requirement is retained in addition to the primary energy requirement as this remains an important consideration for government and there is concern that certain solutions could produce low primary energy figures with high carbon emissions - for example heating oil and coal both have low primary energy but  high associated carbon emissions.

Finally, the current fabric efficiency requirement is dropped to make way for a new householder affordability target, with fabric efficiency now considered adequately protected by tougher minimum heat loss standards for building elements.  As discussed, electricity has low and falling primary energy and carbon emissions factors, and government is concerned that direct electric heating would be a viable option for meeting both the carbon and primary energy targets, but with the side-effect of saddling occupants with too-high energy bills.  To guard against this the new affordability rating is likely to be set at a level that means direct electric heating would only be an option when combined to other measures to reduce electricity bills such as increased thermal insulation, PV panels or battery storage.

2. Uplift of the Minimum Standard

The minimum performance standard is defined by publishing a build specification (insulation levels, heating system, light fittings, microgeneration technologies) to be used by the developer to model a 'notional house'.  The developer then has to design the house they plan to build to produce modelled carbon emissions and primary energy lower than that of the notional house.  It's an elegant way to allow the developer complete freedom in design but control the outcome.

 The consultation proposes two options for the minimum performance standard:

Option 1 - "Future Homes Fabric"

This specification would produce a 20% reduction in CO2 emissions when compared against the specification in current building regulations .  The standard is based on a notional home with improved insulation measures (including triple glazing) plus a gas boiler and waste water heat recovery.

The estimate given in the consultation is that this option adds £2557 to the build cost of a semi-detached house and saves households £59 a year in energy bills.  (Payback 43 years)

Given that by 2025 the Future Homes Standard needs to be at a 75% of the carbon emissions of 2013 regulations, 20% does not seem like a big enough step - it only brings England roughly to the level that  Scotland's developers have been achieving since 2015.  DCLG appears to agree, stating that it's preferred option is Option 2.

Option 2 - "Fabric plus Technology"

In this option, the specification of the notional house is set at a level to produce a 30% reduction in carbon dioxide emissions across the build-mix.  The specification has slightly lower insulation than Option 1 plus waste water heat recovery and a solar PV system.

SAP 10.1 Appendix R outlines the specification for the notional house.  The size of the PV system in kWp for the notional house is 40% of the building foundation area divided by 6.5.  So for example for a typical two-storey semi-detached house of total floor area 85m2, this would be

[40% x (85/2) ] / 6.5 = 2.6kWp (around 9 or 10 panels)

DCLG's modelling estimates that building to this new notional home adds £4847 to the building costs and saves £257 a year in energy bills.  (Payback 19 years).

The costs used in the accompanying impact assessment for solar PV are £1,100 fixed costs plus £800 variable per kWp installed.  This implies the following installed costs:

1kWp  £1,900  £1.90/kWp
2kWp  £2,700  £1.35/kWp
3kWp  £3,500  £1.17/kWp

Solar is a fast-paced technology and it would be unusual if a government consultation were to use up-to-date cost information.  My understanding is that solar installers operating in the new-build sector are typically charging an installed price the range of £1.10-£1.20/kWp for four or five panel systems (1 -1.25 kWp).  So it is likely that the costs of Option 2 are over-stated relative to Option 1.

If the solar industry can provide evidence that costs in Option 2 are over-stated, it will make it easier for government to hold the line on its preferred option.

DCLG reckons that Option 2 might result in developers moving away from gas boilers to air-sourced heat pumps.  A specification based on ASHP alone over-shoots the Option 2 target at a lower cost than the notional house (£3,134), which would allow some relaxation of the fabric for further cost savings.  The experience in Scotland suggests that housebuilders will avoid ASHP for as long as possible because customers neither like nor understand them.

3. Heat Pumps - "Lord Make Me Chaste - but not yet!"

The consultation steps away from banning gas heating in 2020, this change is timetabled for 2025.  However it does impose extra conditions on wet space heating systems to ensure that they are 'future proof'.  In practice this will mean that 'emitters' (normal people call them radiators) will be increased to a size that would work at lower temperature, and so the house would be suitable for later conversion to a heat pump heating system without the cost of replacing all the radiators.

A side effect of this requirement is that increasing the cost and space requirements for wet systems could push developers towards direct electric heating with panel heaters, simple underfloor electric or radiant heat panels.  The removal of the entire cost of the wet heating system would offset a considerable chunk of the costs for the additional measures (PV solar, more insulation) needed to stay within the householder affordability target.  A house without a wet heating system would be low on maintenance and low cost to build, coupled with better insulation plus lower cost PV and battery storage to keep bills down this could become a favoured option for new homes.

4. Transitional Arrangements

This proposed change is likely to cause significant concerns at housebuilding companies.

The current situation is that as new Building Regulations come into force, they apply only to whole developments as new planning applications are lodged with local authority planning offices and work has started on site.

The practical outcome of this rule is that new homes are still being built to versions of building regulations in force many years ago, because:

(a) Developers rush to submit planning applications in the run up to new regulations coming into force, banking large numbers of homes to be built under the old regulations
(b) Large sites of many hundreds of homes are built out over many years, but there is a site-wide application of the regulations.

This was clearly demonstrated by the 2015 Scottish building regulations change, where it is only now (nearly 4 years later) that pretty much all new sites coming forward for tender require solar.

The consultation proposes moving from a site-based application of building regulations to one based on specific buildings.  Large developments spanning many years would have to redesign to meet new building regulations that apply as the building is being built.

Housebuilders will be alarmed by this proposal because all developments still under construction under 2013 regulations will be caught in this net.  The land for these sites would have been bought at a price based on the construction costs expected under those 2013 regulations and the housebuilders will argue that this measure is a retrospective action that will harm their profitability.  How much sympathy there is for the housebuilders having to shoulder the extra costs remains to be seen, when government has been subsidising the housing market through the Help to Buy scheme and the chief executives of some companies have been given bonuses amounting to £10,000 per house built .

 5. Other Stuff

Solar PV on Apartment Blocks

 In the original SAP10, PV on apartment blocks connected to the landlords' supply did not improve the DER of the individual apartment, whereas in SAP 2012 the carbon savings were apportioned across apartments by floor area.  The Solar Trade Association argued that connection to Landlord's supply was often by far the most cost-effective and practical way to install solar on apartment blocks, that the changes would force systems to be split into mini-systems serving each apartment at great cost, and that the carbon savings were real.  It seems that this argument has prevailed as SAP 10.1 has changed the treatment of solar PV in apartment blocks back to as it was in SAP 2012. 

Heat Networks Get a Free Pass

SAP 10 introduced punitive heat losses on district heating networks, based on evidence that large amounts of heat are lost in the underground pipework of these systems (40-50% even for best practice new ones).  It seems that government thinks that heat networks will be an important part of the energy future, and that their drawbacks should be ignored.  So a fudge-factor (they call it a 'technology factor') is applied to buildings that use a heat network.  These are allowed to emit 45% more carbon for heating and 5% more primary energy.

The Government's enthusiasm for heat networks is baffling considering that there is a perfectly good electricity network that loses far lower energy in transmission and is already connected to every single property.  A heat network is not of itself low carbon - it depends what you're doing to make the heat.

The Future Homes Standard - for 2025

The second part of the consultation is some early range-finding questions for the Future Homes Standard due to come into force in 2025.

The government reckons a 70-80% reduction in carbon emissions compared to current housing is possible.  This will be achieved by adding low carbon heating (heat pump or district heating) to the Option 1 fabric proposed in the 2020 regulations, and relying on further decarbonisation of grid electricity to do the rest.  Government is seeking views on whether this is achievable.

Local authorities which have been using planning powers under the Planning and Energy Act 2008 to require developers in their region to build to standards above those of the current building regulations.  This role for local authorities has been crucial for pushing forward on energy efficiency during a period of inaction from Westminster.  The consultation considers whether these powers should be removed alongside the 2020 regulations, the 2025 Future Homes Standard or not at all.

Summary

This change is significant and there's still some modelling to be done to figure out which packages of technology developers are likely to favour, but given the simplicity and popularity of solar it seems unlikely that the technology will not be a big winner from these changes to building regulations.

The Impact of Solar PV and Solar Thermal on EPC Ratings

The EPC Rating is a score between 1 and 100

Introduction

The Energy Performance Certificate (EPC) is a fundamental plank of the government’s strategy to improve the energy performance of the UK’s building stock.

• Since 2007 it has been a legal requirement that homes for sale have a report of their energy performance for potential buyers.
• From 2018 it will be a requirement that rental properties have a rating higher than E.
• In Scotland, all social housing will have to achieve an energy rating of C or D (depending on house type) from 2020
• To access the Feed in Tariff for solar PV, it is necessary that the building achieves an EPC D rating
• To access the domestic Renewable Heat Incentive, it is necessary to undergo a Green Deal Assessment, which is essentially an EPC with added extras to factor in the way you use energy.

The certificate rates buildings with a score from 1 (least efficient) to 100 (most efficient), with the scores divided into bands A through G as shown.

How it is Worked Out?

An approved calculation called the Standard Assessment Procedure (SAP) is used to calculate the energy used to heat the home, provide hot water to occupants together with electricity for lighting, pumps and fans.  Electricity used by other appliances is not considered.

The actual number of people in the house and the way it is heated is ignored.  A standard occupancy and a fixed indoor temperature is assumed – the idea is to compare the building with other buildings, not compare one set of occupants with another.

The energy used (gas, electricity oil) is then multiplied by fuel cost factors to produce a calculated energy cost for the property.  The energy cost is normalised by the floor area of the property to give a score between 1 and 100, the higher the score, the lower the building costs to run (with a house with a score of 100 nominally costing nothing)

For new homes the current calculation is SAP 2012 and the calculations are updated every time the building regulations change.

For existing homes, assessors use RdSAP 2009 (the Rd stands for Reduced Data).  This is an extra appendix to the SAP 2009 calculation which provides guidance on what assumptions to make when you don’t know or can’t see the exact specification of insulation and equipment.

Solar PV

SAP 2009 gives an unshaded, south-facing solar PV installation around 858kWh/year per kWp installed, irrespective of its the location in the UK.

Its calculation of energy costs assumes that 50% of the energy from a solar PV system is used in the dwelling and 50% exported (and this assumption does not change with the size of the PV installation).

The saving on the energy bills is then:

50% x energy generated x cost of purchased electricity

+

50% x energy generated x payment for exported electricity

In SAP 2009 the cost of purchased electricity is set to 11.46p/kWh for homes on a standard tariff, and the payment for exported electricity is also set at 11.46p/kWh.

Solar Thermal

A solar thermal installation could benefit the household energy bills in two ways.  First the solar system will generate heat that the boiler or electric immersion heater no longer needs to supply.  Second, a new solar hot water cylinder with better insulation will result in a reduced escape of heat from the stored water.  Although this heat loss contributes towards space heating in winter, but is wasted energy in summer.

SAP 2009 calculates out a solar energy input of 1,316 kWh per year for an unshaded 4 square metre flat plate installation, facing south and heating a 250 litre cylinder in an 85m2 house.  This translates into a fuel saving of 1,586kWh when the replaced heating system is a gas or oil boiler (taking into account their lower efficiency in summer months).

In addition the replacement of an old cylinder (where there is one) with a new would reduce heat losses.  For example, replacing a 50mm jacket insulated 180litre cylinder with a new 210 litre solar cylinder with 105 litre auxiliary heated volume and declared loss of 1.8kWh/day will save 444kWh/year based on a boiler winter efficiency of 90%.

Impact on EPC Rating

The impact on EPC rating of solar PV and solar thermal was calculated as follows.  For every SAP 2009 rating from 1 to 100, the implied Energy Cost Factor (ECF) was calculated by rearranging equations (10), (11) and (12) –  Section 12, page 33.  A house of 85 m2 floor area was considered, being the UK average size.

The energy cost that had resulted in that SAP rating could then be calculated from equation (357).

The saving on the energy cost was calculated by multiplying the energy savings from solar (discussed above) by the fuel prices in table 12.  The reduced energy cost was then converted back to a SAP rating.  The table shows the improvement in energy performance score for solar thermal and PV.

The impact of solar on a home's EPC energy score

NOTE: Figures shown are for a starting EPC score of 40 or higher (beginning of EPC band E), below this the size of the improvement decreases a little.

Conclusion

There are a growing number of drivers that are pushing building owners to improve the energy performance of their buildings.

Until recently, much of the focus has been on insulation measures to achieve these goals, but as more and more of the available cavity walls and lofts have been treated, the remaining insulation options such as external wall, internal wall and under-floor become disruptive and costly.

Solar thermal and solar PV are low hassle – high impact measures that can help increase the energy performance of homes.

Solar PV can give a significant boost to the energy rating of homes, particularly those with a clear roof of adequate size.

Solar thermal can be extremely cost-effective when combined with other heating system works such as boiler or hot water cylinder replacement and is more suitable for smaller roofs and partial shading.

SAP 2012 Gives Solar Heating a Boost

What the Government's new Calculation Means for Solar

Credit: Viridian Solar

Ahead of the announcement of the new Building Regulations later this summer, BRE has published the accompanying energy calculation, the Standard Assessment Procedure, or SAP.  

If you think this document is only relevant to the new build sector, you would be wrong. SAP is also the basis for Energy Performance Certificates (EPCs), and therefore is also highly influential in the refurbishment of existing homes, for example under the Green Deal. SAP Appendix H is also used by the Microgeneration Certification Scheme (MCS) for installers of solar heating to provide customers with an estimate of their likely energy savings. Finally, whichever of the proposed routes to calculate payments under the forthcoming incentive scheme for renewable heat, the domestic RHI, is chosen, SAP lies behind it somewhere.

This document is not for the faint-hearted, comprising a thick booklet of dense calculations and notes. Fortunately the solarblogger has done the heavy lifting, so you don't have to...

A detailed assessment has been published as a briefing document available from the Viridian Solar website, but here's a quick round up of the headlines:

►  New postcode-based irradiation data (PV&T)

The UK average irradiation remains the same, but the value varies depending on location with northern Scotland around 10% below the average and southern England around 10% above.  This feeds directly into the solar pv estimate, but results in a smaller +/- 5% change across the country for solar heating.

► Updated fuel carbon emissions factors (PV&T)

Carbon savings from solar electricity reduce by 2%, carbon emissions avoided by solar thermal replacing natural gas increase by 9%

► Addition of hot water use factor (T)

There is a 29% increase in hot water used from the hot water cylinder in the solar thermal calculation if no electric showers are present, and a reduction of 36% if only electric showers are present. This produces a 20% increase in solar heating energy in SAP Appendix H compared to the previous version

► Reduction in solar pump electricity consumption (T)

The 75kWh flat rate for the solar pump electricity consumption is reduced to 50kWh for mains powered solar pumps, based on evidence from EST solar thermal field trials submitted by EST and Solar Trade Association

► Addition of second order coefficient for thermal panel efficiency (T)

Creates a level playing field (for more information see my paper on this issue here)

So What Does it all Mean?

Photovoltaics

For solar PV, the main impact will be on new build housing.  Here the changes are relatively neutral, being a 2% drop in carbon savings on average due to the lower emissions factor for solar electricity. Of course, compared to the previous version of SAP, which used a single figure for irradiation for the entire country, some areas have increased significantly while others have fallen. (See map, below)

Credit: Viridian Solar

For retrofit PV the recent MCS PV installation Guide has already implemented a new energy calculation based on a different set of  irradiation data.  This produces slightly higher energy outputs than SAP 2012.  The discrepancy could be explained by the different uses of each calculation, with the MCS method taken to predict year one energy (with solar panel performance degradation taken into account later for any financial forecasts).  By contrast, SAP is more aimed at producing a whole life average estimate.

Solar Thermal

For solar heating, the calculation impacts both new build and retrofit, and in a really positive way. (See map, top of post)

In new build, carbon savings will increase significantly for almost all homes with solar heating.  The new adjustment factor for electric showers rightly produces a disincentive to install these alongside solar heating.  I reckon that overall the boost to solar heating carbon savings from all the changes is worth between 17% and 31% depending on where the house is.

In retrofit situations, the MCS installer will perform an energy calculation  for the customer using SAP Appendix H. The new version of SAP will an average of a 20% boost to the solar energy estimate, again differing depending on the location of the installation.  

Further changes are coming down to road to improve things even more in MCS, more on this soon....