SAP Conversion Tool

Bath and North East Somerset, and Cornwall, Councils recently introduced the UK’s first net zero energy policies for new housing. To enforce the policies, they needed a way for applicants to use SAP 10 calculations to produce realistic design stage estimates of operational energy use.

We have been working with the councils over the past few months to develop an excel based tool that post-processes SAP 10 results to produce design stage predictions of space heating demand, total energy use, and solar generation, which are similar to PHPP. The tool is now complete and is currently being used for new planning applications in Cornwall.

The tool is a stopgap while compliance tools and policy catchup with what is needed to achieve net zero carbon by 2050 or before.

Using the SAP Conversion Tool

The tool is owned by Bath and North East Somerset Council, Cornwall Council, and Etude. While the tool can be used for any climate region in the UK, as the tool’s development was funded by Bath and North East Somerset Council and Cornwall Council, it has only been verified for use in these climate regions.

Both the SAP Conversion Tool and accompanying methodology report are available to download and use free of charge from Cornwall Council.

Please contact Etude if you would like us to carry out testing and verification of the tool for use in your climate region. A development version of the tool can also be made available for formal third party testing and peer review, by prior agreement with Bath and North East Somerset Council, Cornwall Council, and Etude.

Planning applicants may contact Bath and North East Somerset or Cornwall Council if they have questions that relate directly to planning applications for which the tool is being used, within these regions.

Limitations and further work

While we’re confident in the ability of the SAP Conversion Tool to consistently provide predictions of space heating demand, total energy use, and solar generation that are similar to PHPP, and as a result hopefully closer to real-world values than the original SAP 10 outputs, we are aware that the tool has limitations, and that there are opportunities to improve it further. These include:

  • Testing of the tool for a wider range of building types, specifically for a range of flats and very large or small dwellings.
  • Testing the tool against other software.
  • Independent testing of the tool by third parties.
  • Comparison of tool outputs against operational energy data (though this is expected to produce similar results to comparisons of PHPP with operational energy use).
  • A range of possible refinements to the final energy use calculations, for example pump and fan energy use could be calculated within the tool.
  • Energy modelling always involves the use of assumptions, some of which ultimately must be selected in a subjective manner. Our approach has been to choose assumptions that are similar to PHPP, which often means they represent good, but achievable, practice.
  • The tool is not a replacement for PHPP, which can predict the space heating demand for very high performing buildings with a high level of accuracy and user adjustability.

How we built the SAP10 conversion tool

Work began by modelling four house types and eight performance specifications in SAP 10.2 and PHPP 10.4a. We assumed, based on various post-occupancy studies, that PHPP results provided a better estimate of real-world energy performance than SAP 10.2.

We then reviewed SAP 10’s space heating demand and total energy use calculations in detail, and came to the same conclusion as several other studies that the core approach to building physics in SAP 10 is generally sound.

We then investigated whether it was possible to make a range of targeted, evidenced, adjustments to specific parts of the SAP 10 calculation, to produce results that were similar to PHPP. We discovered that this could be done while still using most of the SAP assessor’s original inputs for the building’s geometry and thermal performance. The adjustments made to the calculation are summarised below, and explained in detail in a separate methodology report.


Space Heating Demand calibration and methodology

The SAP Conversion Tool uses SAP 10.2 outputs to predict similar levels of space heating demand to PHPP. Results shown for a terraced house with eight different performance specifications.

Climate regions assumed in SAP 10 calculations vary depending on which calculation worksheet is used. The Dwelling Emission Rate, Fabric Energy Efficiency and Energy Rating worksheets all use UK average climate data. The SAP Conversion Tool adopts the same approach as PHPP in using local climate data, by using outputs from SAP 10’s EPC Cost worksheet calculations, which are based on the building’s climate region.

Internal temperatures assumed in SAP 10 are replaced with an assumption in the tool that the internal temperature is set to a constant 20C, the same value as PHPP. Effectively this is also an assumption that the same level of thermal comfort is achieved, regardless of heating system.

Heating season – a small adjustment is made in the SAP Conversion Tool, which assumes the heating system is switched off during any months with very low levels of space heat demand. This prevents overestimating the space heat demand relative to PHPP, for dwellings with the highest levels of fabric efficiency.

Transmission losses are calculated in the SAP Conversion Tool by reusing the building’s geometry and the thermal performance of the floors, walls, roofs, openings, and thermal bridges, that are input into SAP 10.

Infiltration losses are calculated in the tool using the same approach as PHPP 10, which is based on ISO 13790:2004. The design stage air permeability value from SAP 10 is converted into an air flow rate and the n50 volume is also calculated by the tool, using the building’s geometry. This allows the equivalent air change rate @50Pa to be calculated, from which the infiltration rate can be calculated based on the site’s exposure.

Ventilation losses are calculated within the SAP Conversion Tool by first calculating the Part F minimum ventilation rate for continuous ventilation systems. This rate is assumed to be achieved for all ventilation systems. If MVHR is present, the system’s heat recovery efficiency is then taken from SAP 10 and used to calculate an effective air change rate.

Internal heat gains in the SAP Conversion Tool are generally less than half of those assumed in SAP 10. While the SAP 10 assumptions for gains from lighting, pumps, fans, and evaporative losses are used directly in the tool, the gains from occupants, appliances, water heating and cooking are all recalculated.

Solar heat gains from SAP 10 are used directly in the SAP Conversion Tool, as there is no easy way of recalculating them. Gains are capped at 50% of total annual heat losses to prevent excessive reliance on solar gains, however the cap is not expected to have an impact on the majority of dwellings.

Gains utilisation rates are calculated in the SAP Conversion Tool using the same formulae as in SAP 10. This produces very similar results to PHPP.

The combined effect of these adjustments, is that a set of SAP 10 calculations can be post-processed to produce space heating demand results that are almost identical to PHPP, with a similar breakdown of space heating losses and gains. Testing of a range of building types and performance scenarios indicated the space heating demand predicted by the tool was generally within 10% of PHPP, and often within 5%.


Total Energy Use calibration and methodology

The SAP Conversion Tool uses SAP 10.2 outputs to predict similar levels of total energy use to PHPP. Results shown for a terraced house with eight different performance specifications.

Appliance energy use in the SAP Conversion Tool is calculated using a first principles calculation that assumes the least efficient major appliances available for sale in the EU are used by occupants in a similar pattern to PHPP. Additional fixed allowances for consumer electronics are applied per dwelling and per occupant. This results in similar estimates of electricity use to PHPP 10.4a.

Space heating energy use is calculated by combining the revised space heating demand calculated by the tool, with the heat source efficiency from SAP 10. For heat pumps, the efficiency is increased by around 15%, which results in values that are closer to PHPP 10.4a and field trial data for well designed/installed systems.

Water heating energy use is calculated in the tool using PHPP’s assumption of 25 litres per person per day at 60C for initial demand, and SAP 10’s assumptions for system losses. These are combined with the heat source efficiency from SAP 10. For heat pumps the efficiency is increased by 60%, which results in values that are closer to PHPP 10.4a and field trial data for well designed/installed systems.

Pump and fan energy use from SAP 10 is used directly in the tool.

Cooking energy use from SAP 10 is used directly in the tool.

Lighting energy use is calculated in the tool using a first principles approach that is similar to PHPP 10.

The combined effect of these adjustments, is that a set of SAP 10 calculations can be post-processed to produce total energy use results that are almost identical to PHPP, with a similar breakdown by end use. Testing of a range of building types and performance scenarios indicated the total energy use predicted by the tool was generally very close to PHPP.


Solar Generation

The solar photovoltaic generation calculation in SAP 10 is straightforward and provides a rough estimate of energy production, accepting the limitations of not using site specific irradiation data. The main adjustment applied in the SAP Conversion Tool is to assume a system performance factor of 85%, which is less conservative than the 80% assumed in SAP 10. This is based on analysis of thousands of solar systems in the UK by Sheffield Solar.

Some additional adjustments may also be made for climate regions for which the tool has been verified for, to better align the irradiation data between SAP 10 and PHPP.


Next steps

Thank you for your interest in the SAP conversion tool. As we’ve said, we acknowledge that this is a stopgap and only one piece of the puzzle in terms of improving UK building efficiency policy. Improving the way compliance is assessed, the quality assurance of construction, not to mention how to deal with existing building, are all other important areas to explore.