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A new draft version of SAP has more than halved the CO2 emissions factors for electricity. Alex Smith looks at how changes to the energy use calculation could accelerate a move from gas to electric heating in homes

CIBSE Journal

The updated methodology for calculating energy use in new residential developments is set to have a significant impact on the way consultants design heating systems for the purpose of Building Regulations compliance.

One of the biggest changes in the draft Standard Assessment Procedure (SAP 10), published in July, is to the lower carbon emissions factors for electricity, which reflects the rapid decarbonisation of the National Grid.

Other key updates include higher distribution loss factors in heat networks, and measures taken to reduce risks of overheating (see panel ‘Important changes in SAP 10’).

It is not known when SAP 10 will come into effect. The new methodology will only supersede SAP 2012 when the Building regulation conservation of fuel and power: Approved Document L, is next updated, which will be in 2019 or 2020.

Many of the SAP changes were outlined in BEIS’ Proposed Changes to Government’s SAP (November 2016) and its response to the consultation a year later, but figures for CO₂ emissions factors in SAP 10 are far lower for electricity than in 2016.

What is SAP?

The purpose of SAP is to assess the energy and environmental performance of new homes to ensure developments meet energy and environmental policy initiatives, as well as building regulations.

The assessment is based on standardised assumptions for occupancy and behaviour. This enables a like-for-like comparison of dwelling performance. Related factors, such as fuel costs and emissions of carbon dioxide (CO2), can be determined from the assessment. These indicators of performance are based on estimates of annual energy consumption for the provision of space-heating, domestic hot water, lighting and ventilation.

SAP is used to assess energy use and carbon emissions for Building Regulation Approved Document L in domestic buildings, while SBEM is the equivalent tool for non-domestic buildings. Reduced data SAP (RdSAP) is used to produce Energy Performance Certificates.

Current SAP assumes that electricity used produces 2.4 times the carbon emissions of mains gas. The 55% reduction in the CO₂ emissions factor for electricity means homes heated by direct electric systems will produce virtually the same CO₂ emissions as gas, while heat pumps will produce even less. The SAP 10 factor is also far lower than the carbon factor proposed in 2016 (see table ‘Change in CO₂ emissions factors’).

‘SAP 10 has aligned the carbon factors with reality,’ says Julie Godefroy, acting head of sustainability development at CIBSE. ‘The new carbon factors and distribution losses will change the appraisal of low carbon heating options.’

Phil Jones, past chair of the CIBSE CHP-DH group, carries out feasibility projects for CHP schemes. ‘CHP is still the most economic, but it is increasingly falling down on CO₂ emissions, particularly where lower carbon factors for electricity are already being used.’ This is the case in schemes supported by the Heat Networks Delivery Unit, set up to administer government grants for heat networks, says Jones.

The lower carbon factors mean heat pumps only need a coefficient of performance (COP) of 1.1 to have carbon emissions lower than a gas boiler, says Elementa senior engineer Clara Bagenal George, MCIBSE.

However, she warns of an unintended consequence of a lower carbon factor – higher energy bills. ‘Electricity is more expensive than gas, so to be running cost neutral, the COP of a heat pump would need to be four. Hence, further guidance will need to be introduced to protect a building’s occupants from increased energy bills.’

The shift in carbon factors will change how design teams prioritise carbon reduction measures, she says, because the relative cost/carbon tonnes of different measures has changed. For instance, CO₂ emissions from grid electricity falling by 55% means the relative cost per carbon tonne of PVs increases substantially.

‘It is still important to install PV on buildings to help to continue reducing the carbon emissions of the UK grid,’ she says. ‘Further incentives or metrics may be needed to ensure this.’

Jones says the lower carbon factors are overdue, and should be used now in making SAP calculations. ‘I’m very concerned about carbon factors in the SAP proposals. I welcome the much lower electricity grid factor of 0.233, but we are close to this already. Why do we need to wait until 2020? Are we still going to be assessing heat pumps at 0.519 then?’

This view is supported by Bean Beanland, vice-chair of the Ground Source Heat Pump Association. ‘If the carbon factor in SAP and Part L is not changed soon, we will be faced with buildings still going up in 2022 that have been designed on false principles. Significant damage is being done to the heat pump sector every day that this situation is allowed to perpetuate.’

Hanaé Chauvaud de Rochefort, senior policy research manager at the Association for Decentralised Energy, says: ‘The changes to the grid electricity emission factor are appropriate. However, it is problematic for all power generation as the grid emissions are a blend of all sources of generation on the system.’

‘Gas CHP – even at very low grid emission factors – continues to displace higher emission generation, such as gas engines and turbines without heat recovery.