SAP 10 carbon factors and the heat pump case
The revised electricity emission factor changed the arithmetic of every energy statement. A heating strategy that failed on carbon under the old figure can pass under the new one.
The electricity carbon emission factor used in SAP fell sharply when SAP 10 was introduced, from the SAP 2012 value of 0.519 kgCO2 per kWh to 0.233 kgCO2 per kWh, reflecting the decarbonisation of grid generation over the intervening period. Mains gas stayed close to 0.210 kgCO2 per kWh. Electricity went from roughly two and a half times as carbon intensive as gas to very nearly the same, and because a heat pump delivers several units of heat per unit of electricity, electric heating became the lower carbon option for most new residential development overnight.
That single number change is why energy statements written before it read so differently from those written after.
Why the factor fell
The emission factor for grid electricity is a weighted average of the generation mix, and the mix changed. Coal generation effectively ended, gas generation fell as a share, and wind and solar capacity rose substantially.
SAP 2012 used a factor derived from data that was already out of date when it was published. SAP 10 recalculated it on a forward-looking basis over the life of the building, which is the right approach for an asset lasting 60 years and an obviously contestable one for a compliance calculation.
The Greater London Authority moved to the SAP 10 factors in its energy assessment guidance before they were adopted nationally in Part L 2021, which is why London schemes were making the heat pump argument earlier than schemes elsewhere.
What it did to the gas against electric comparison
Under the older factor, a gas boiler at around 90 per cent efficiency produced roughly 0.233 kgCO2 per kWh of delivered heat. An air source heat pump at a seasonal coefficient of performance of 2.5 produced roughly 0.208 kgCO2 per kWh of heat under the SAP 2012 electricity factor. The margin was slim enough that a mediocre installation lost it.
Under the SAP 10 factor the same heat pump at a seasonal coefficient of performance of 2.5 produces around 0.093 kgCO2 per kWh of heat. The comparison is no longer close.
| Heating system | Delivered heat carbon, SAP 2012 factors | Delivered heat carbon, SAP 10 factors |
|---|---|---|
| Gas boiler at 90 per cent | about 0.233 | about 0.233 |
| Heat pump at SCOP 2.5 | about 0.208 | about 0.093 |
| Direct electric | about 0.519 | about 0.233 |
These figures are illustrative arithmetic from the published emission factors and boiler efficiency, not modelled results for any scheme. Direct electric resistance heating now matches a gas boiler on carbon, which is worth knowing when a developer proposes panel heaters in small flats. It matches on carbon and loses badly on running cost.
Effect on the percentage reduction a scheme can claim
Most local plan energy policies ask for a percentage improvement on the Part L target emission rate, often with a fabric-first stage before any low-carbon heat. London policy sets a staged energy hierarchy and asks for a stated on-site reduction before any offset payment.
With the revised factor, the on-site reduction a heat pump scheme can demonstrate rises substantially, which is why some schemes now reach a policy target without photovoltaics that would previously have needed a full roof of panels.
Two cautions. First, the baseline moved too: Part L 2021 tightened the notional dwelling, so the target emission rate against which you are improving is already lower, and a percentage saving against the new baseline is harder won than the same percentage against the old one. Second, a policy drafted against SAP 2012 and carried into an adopted plan without amendment can produce odd results, and officers vary in how they handle that. Ask at pre-application.
Peak demand and the distribution network question
Carbon is not the only constraint. Replacing gas heating with heat pumps moves the entire heating load onto the electricity network, and the site connection capacity has to carry the winter peak.
For a scheme of any size, the distribution network operator's connection offer becomes a programme item rather than a formality. Reinforcement can take many months and carries a cost, and on constrained parts of the network the offer may require a flexible connection.
Make the enquiry early. An energy strategy that assumes 100 per cent heat pump provision with no connection enquiry is an incomplete document, and a Utilities Assessment covering the capacity position belongs alongside the energy work. Diversity assumptions matter here: the simultaneous demand of 60 flats with heat pumps is not 60 times the single unit demand, and the assumption used should be stated.
Where a heat network still makes sense
Individual heat pumps are not the answer everywhere. A heat network wins where there is a low-carbon or waste heat source available, where the scheme is dense enough for the distribution losses to be acceptable, where a designated heat network zone covers the site, or where plant space and acoustic constraints make individual units impractical.
Ambient loop networks with individual water source heat pumps in each dwelling combine the two approaches and are increasingly the answer on tall buildings, where external unit siting is awkward and the facade cannot carry the equipment.
The weakness of a traditional high-temperature heat network is distribution loss. A network running at high flow temperatures through a lightly loaded summer system can lose a large fraction of its heat, and the energy statement should give the modelled annual loss rather than assuming a token figure.
Presenting the comparison so an officer can check it
Set out, in one table, each heating option considered, the assumed efficiency or seasonal coefficient of performance, the emission factor used with its source and version, the resulting regulated emissions, and the percentage against the target emission rate.
State the SAP version and the emission factors explicitly. An officer or their consultant checking the work needs to know whether 0.233 or 0.519 sits behind the numbers, and a statement that gives only outputs cannot be checked.
Include the seasonal coefficient of performance source. A manufacturer figure at a flow temperature of 35 degrees does not apply to a system designed around radiators at 55 degrees, and the gap between those two conditions is where most overclaiming happens.
The Energy Statement should carry this comparison and the Sustainability Statement should not repeat it. Where overheating is assessed, a Overheating Risk Assessment covers the summer condition that a well-insulated heat pump dwelling can suffer from.
Pitfalls when reusing an older energy statement
A statement prepared for a scheme in 2019 and resubmitted on a re-application is probably using SAP 2012 factors, a superseded Part L baseline and a policy reference that has changed. Reusing it produces a document that fails on all three counts.
Check in this order: the SAP version and emission factors, the Part L edition and notional dwelling, the adopted policy wording and percentage target, the carbon offset price if the authority operates one, and the flow temperature assumption behind the seasonal coefficient of performance.
Then check whether the scheme still needs the photovoltaic array it was originally given. Under the revised factors, the carbon value of exported electricity fell along with everything else, and an array sized to close a carbon gap that no longer exists is money that could go into fabric.
Enter your postcode on PlanScope to see which reports a site is likely to need.
This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What changed between the SAP 2012 and SAP 10 electricity factors?
The factor fell from 0.519 kgCO2 per kWh under SAP 2012 to 0.233 kgCO2 per kWh under SAP 10, reflecting the decarbonisation of grid generation. Mains gas remained near 0.210. The practical effect is that electric heating, and heat pumps in particular, became the lower carbon option for most new residential development.
Does a lower emission factor make the policy target easier to meet?
For the heating strategy, yes. But Part L 2021 also tightened the notional dwelling that sets the target emission rate, so the baseline you are improving on is lower than it was. A given percentage saving is harder to achieve against the newer baseline, and the two changes partly offset each other.
Is direct electric heating now acceptable on carbon grounds?
On carbon alone it is now roughly equivalent to a gas boiler, which is a genuine change. On running cost it remains poor, and most local plan policies and officers will press for a heat pump instead. Where panel heaters are proposed in small flats, expect to justify the choice on space and cost grounds and to lose the argument on fuel poverty.
What seasonal coefficient of performance should I use?
One that matches the actual system design, not the manufacturer headline. A figure quoted at a 35 degree flow temperature does not apply to a radiator system running at 55 degrees. State the source, the test condition and the design flow temperature in the energy statement so the assumption can be checked by the authority's reviewer.
When does a heat network beat individual heat pumps?
Where a low-carbon or waste heat source is available, where a heat network zone covers the site, where density makes distribution losses acceptable, or where plant space and acoustics rule out individual units. Ambient loop networks with water source heat pumps in each dwelling are a common answer on taller buildings.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.