Standard Assessment Procedure (SAP) calculations are used to calculate an EPC rating for UK homes. This enables understanding of the energy efficiency of and carbon emissions from a home, as well as allowing comparison between different homes.
The key result of SAP calculations is an Energy Efficiency Rating (EER) between 1-100, which is then converted into an EPC rating between A-G – which is the standard way that the energy performance of a UK home is measured.
Since 2008 it has been a legal requirement to have an EPC when selling or letting a home in the UK. EPCs are also used as the basis for climate regulations that aim to drive property decarbonisation, such as MEES.
So, if you work in the property sector it’s useful to understand the basics of how SAP calculations actually work.
In this article we’ll take a closer look at SAP calculations, covering:
- What is SAP?
- How do the SAP calculations work?
- What do SAP ratings mean?
- When will SAP be updated or replaced?
What is SAP?
Standard Assessment Procedure (SAP) refers to a set of two methodologies used to calculate the energy performance of houses in the UK:
- SAP or ‘full SAP’: used to assess the energy performance of new build homes once construction is complete and ensure compliance with Part L of the buildings regulations via a Predicted Energy Assessment (PEA). SAP was introduced in 1993 and last updated in 2023 with SAP 2010.
- RdSAP or ‘reduced SAP’: used to assess the energy performance of existing homes and produce an Energy Performance Certificate (EPC) – RdSAP cannot be used for new build homes. RdSAP was introduced in 2005 and last updated in 2012 (i.e. the version known as RdSAP 2012), with the update to RdSAP 2010 due to be implemented in 2024.
SAP consists of calculations which take into account many different factors that contribute to the energy costs of running a home.
The SAP methodologies were developed by the Building Research Establishment (BRE) for the UK government.
How are EPCs calculated using the SAP methodology?
There are a few step involved in using SAP calculations to determine an EPC rating.
Firstly, an energy assessor visits a home and uses lots of inputs to calculate an estimate of the total energy demand (kWh) and the total energy consumption (kWh) of the property.
These differ because the energy consumption takes into account the efficiency of appliances e.g. a gas boiler might be 84% efficient with some energy lost – this is included in consumption but not demand.
The inputs the energy assessor uses include:
- Property age
- Property location
- Construction materials (walls, floors, roofs)
- Thermal insulation materials, thickness, effectiveness (walls, floors, roofs)
- Heating and hot water system type and efficiency
- Fuel used e.g. gas, electricity
- Renewable energy technologies installed e.g. solar PV rooftop panels
- Door and window types, sizes, glazing thickness, airtightness
- Thermal bridges i.e. weaknesses in the property that allow heat to escape more easily
- Ventilation system
- Internal and external lighting
- Heat network (location specific).
Standardised assumptions for occupancy of properties and the behaviour of occupants are also used as an input.
The total energy consumption is key, because this is then used to calculate the following vital elements which are included on a property’s Energy Performance Certificate (EPC):
- Energy Efficiency Rating (EER) / EPC rating. Also known as Energy Efficiency Rating (EER), this is a measure of the energy costs per square metre per year and is used as an indicator of the home’s energy efficiency, with bands from EPC A-G.
- Energy bill estimates. Calculated by multiplying the estimated energy consumption by the cost of energy. RdSAP 2012 is the latest version of the SAP methodology used for EPCs, and so these energy bill estimates are calculated using static baseline energy prices set in 2012 – with energy bills fluctuating so much, this causes considerable inaccuracies.
- Carbon emissions estimates / Environmental Impact Rating (EIR). Calculated by multiplying the estimated energy consumption by a carbon intensity factor which accounts for how dirty or clean our electricity is i.e. how much renewable vs fossil fuel energy in the National Grid. Again, RdSAP 2012 uses a static baseline carbon intensity factor set in 2012 of 519g CO2/kWh – in reality, the National Grid has become much cleaner in recent years, which the actual up-to-date carbon intensity being 173g CO2/kWh.
The EER is the key output of the SAP calculations in terms of producing an EPC – it is the headline rating used when referring to an ‘EPC rating’.
The EER is given a score against the SAP rating system (1-100) and the EPC rating (A-G) – both of which are shown on the final EPC certificate.
How exactly the SAP score and EPC rating are determined is a bit of a black box mystery due to the current lack of transparency surrounding the SAP methodology – energy assessors simply plug all of these inputs into a SAP worksheet.
We do know that the calculations only include ‘regulated energy’ i.e. energy consumed by any controlled, fixed services and systems within the building and not unregulated energy e.g. the use of appliances like a washing machine.
This is, of course, only a brief overview of how the SAP calculations work. For a more detailed explanation go to the BRE website to find the full suite of methodology documents.
But, be warned, it’s impossible to find out exactly how all of the SAP calculation inputs work – the methodology lacks transparency and is very difficult to understand, one of the main reasons behind the upcoming shift from SAP to HEM (Home Energy Model).
What do SAP scores mean?
The final output of the SAP calculation is a score between 1 and 100 – often referred to as the SAP score or SAP rating.
The higher the rating, the higher the level of efficiency of the property, resulting in lower energy usage and lower energy costs for the property.
A SAP rating of 100 would mean a £0 energy cost.
It is also possible to have a score above 100 if a property is exporting energy to the grid from e.g. solar rooftop panels, and therefore has a negative energy cost.
The SAP rating is then converted into an EPC band – which is how the SAP methodology is used to calculate EPCs:
- EPC A = 92-100 SAP points
- EPC B = 81-91 SAP points
- EPC C = 69-80 SAP points
- EPC D = 55-68 SAP points
- EPC E = 39-54 SAP points
- EPC F = 21-38 SAP points
- EPC G = 1-20 SAP points
When you look at an energy performance certificate for an individual home you’ll see the EPC rating given as the letter and alongside it you’ll also see the SAP score as a number.
In the example below, for instance, we can see the property is an EPC C with a score of 74, so it’s right in the middle of the C bracket.
The UK government uses EPC ratings as a way to set targets for the decarbonisation of housing, with the key aim to get as many homes to an EPC C or above as possible by 2035.
This might suggest that a home with an EPC C or above is seen as ‘energy efficient’.
However, in reality, an EPC C represents the very minimum standard we should be aiming for on energy efficiency – an EPC C home is still responsible for a significant amount of carbon emissions.
To put it simply: getting all homes to an EPC C standard is not enough to achieve property decarbonisation.
When will SAP be updated or replaced?
The full SAP methodology used for new build homes was last updated in 2023, with the publication of SAP 10.
This included updating the baseline energy prices and carbon intensity factors used within the methodology, to increase accuracy. However, it does not include dynamically updating these baselines – so the methodology will quickly become outdated again.
The equivalent update to RdSAP 10 for existing homes and EPCs is due in ‘early 2024’ according to BRE.
At the same time, the UK government is currently planning to replace the SAP methodology entirely through the more sophisticated Home Energy Model (HEM).
Initially, the Home Energy Model will be part of the Future Homes and Buildings Standard (FHS) which will launch in 2025 – focused on ensuring that all new buildings (including homes) are ‘zero carbon ready’ so that we can meet net zero targets and mitigate the impacts of climate change.
Whilst FHS focuses on new homes only, the intention is that the Home Energy Model will also be rolled out for use across other purposes and policies, including energy performance measurement in existing homes to produce EPCs – but the timeline on this is currently unknown.
SAP calculations are being replaced because they’re far from perfect as a methodology for determining the energy performance of a property – if you’ve seen headlines criticising EPCs, this is likely why.
We’ve already highlighted a few of these problems throughout this article, but the primary problems are as follows:
- Modelling costs not carbon impact. The output of SAP calculations is a prediction of the cost to heat a home – environmental impact is not the main focus of the methodology and so it is not a reliable indicator of energy efficiency.
- Unfair reflection of cost. The SAP rating is based on static, outdated fuel prices and so produces an inaccurate reflection of the costs of both energy bills and retrofit improvements.
- Outdated baselines. The SAP methodology was first developed in 2012 and uses baseline costs of energy and construction materials from this date – energy prices are much higher, and the costs of measures like solar panels have reduced. Until 2022 the SAP calculation also used carbon intensity factors dating from 2012 which are vastly different to todays. This has led to incorrect bill estimates and energy efficiency improvement recommendations within EPCs.
- Inconsistent implementation. SAP calculations are reliant on the expertise and accuracy of energy assessors who produce the inputs of the calculation e.g. thickness of roof insulation. Studies have found huge inconsistencies in the methods that different assessors use, damaging the reliability of the result.
For more details on all of these issues, head to our blog: 6 reasons that EPCs are an inaccurate measure of energy efficiency
The Home Energy Model (HEM) aims to overcome these key problems with key changes including:
- A centralised, open source methodology
- Wrappers to distinguish different use cases
- New technologies are recognised
- Half hourly simulations
- Updated standardised assumptions.
To read more about progress so far take a look at Government consultation documents on HEM and FHS.
