Carbon factors in SAP and what electrification changed

The carbon emission factor for grid electricity inside the Standard Assessment Procedure fell by roughly three quarters between SAP 2012 and SAP 10, while the factor for mains gas barely moved. That single change flipped the arithmetic of domestic heating. Under the older factor set, a condensing gas boiler could beat a heat pump on calculated carbon dioxide. Under the current one, it cannot come close. Everything else in the electrification argument follows from those two numbers.

For a planning consultant reading an energy statement, the practical consequence is that any carbon saving claim is meaningless without the SAP version attached to it. Percentage improvements from different factor sets are not comparable, and a surprising number of submitted documents compare them anyway.

What a carbon emission factor is

A carbon emission factor converts delivered energy into carbon dioxide. Multiply the kilowatt hours a dwelling is calculated to use by the factor for the fuel, and you get the emissions figure that Part L compliance and the energy statement both rest on.

The factor for a fossil fuel is close to fixed, because burning a cubic metre of gas releases the same carbon whenever you do it. The factor for electricity is a function of how the grid generates, so it changes as coal leaves the system and wind and solar enter it. SAP does not update continuously. It uses a fixed set of factors for the duration of each version, which means the calculation always reflects the grid as modelled when that version was written.

That lag is deliberate and it is also the source of most confusion in planning documents. The factors in a SAP calculation are not today's grid intensity, and they are not the annual conversion factors published for corporate greenhouse gas reporting. Three different numbers, three different purposes.

How the electricity factor fell relative to gas

The published factors in the two versions most often encountered illustrate the shift.

Check the current SAP documentation before quoting either column, because the factor set is version specific and successive releases have adjusted it.

Read the table as a ratio rather than as two pairs of numbers. Under SAP 2012, electricity carried about 2.4 times the carbon of gas per unit delivered, so a heat pump needed a seasonal efficiency well above 240 per cent simply to draw level with a boiler. Under SAP 10, electricity carries less carbon per unit than gas, so any heat pump with a coefficient of performance above one shows a saving, and a realistic seasonal figure shows a large one.

Consequences for heat pump schemes

Three changes followed, and all three turn up in planning submissions.

Heat pumps became the obvious route to a carbon reduction target, which is why they appear in almost every residential energy statement now. Direct electric heating stopped being penalised as heavily, which matters for small flats where a heat pump is hard to accommodate, though the running cost argument against panel heaters has not changed at all and authorities increasingly ask about fuel poverty as well as carbon.

Gas fired combined heat and power lost its advantage. Under SAP 2012 factors, a gas engine generating electricity displaced grid electricity at 0.519, which produced a healthy carbon credit and made district heating networks with gas CHP attractive. Under SAP 10, the displaced electricity is cleaner than the gas burned to make it, and the credit largely disappears. Schemes designed around a gas CHP engine and resubmitted with an updated calculation often show a worse result than the fabric-only option.

Photovoltaics moved the other way. Each kilowatt hour generated on the roof now displaces electricity at 0.136 rather than 0.519, so the carbon saving per panel fell by about three quarters even though the panel is identical. A scheme that met a percentage reduction target through a large array under the old factors may miss it under the new ones, which is exactly the trap in an Energy Statement carried over from an earlier feasibility study.

Primary energy as a second target

Part L 2021 introduced primary energy as the principal metric for new dwellings, sitting alongside an emissions target and a fabric energy efficiency target. Primary energy factors account for the energy used to generate and deliver a unit of fuel, including generation losses, so electricity carries a higher primary energy factor than its carbon factor alone would suggest.

The effect is to stop a design claiming credit for decarbonised electricity while ignoring fabric. A poorly insulated dwelling heated by a heat pump can pass an emissions test and fail the primary energy test, which is the point of having both. For an all electric scheme, the primary energy target is usually the binding constraint rather than the carbon one.

So an energy statement should report three results against three targets, not one headline percentage. Where the submission gives only a carbon reduction figure, ask for the primary energy rate and the fabric energy efficiency rate as well. A statement that cannot produce them has not been run properly.

Where older reports give the wrong conclusion

Four patterns recur in documents that reach a conclusion the factor set does not support.

1. A carbon saving percentage quoted against a Part L 2013 notional dwelling, presented as though it answered a policy drafted around the 2021 baseline. The 2021 uplift already required a step change in emissions for new dwellings relative to the previous edition, so a percentage against the older baseline overstates the achievement.
2. A fuel comparison lifted from a pre-2018 feasibility report, concluding that gas is the lower carbon option.
3. A photovoltaic array sized to deliver a carbon saving calculated with SAP 2012 factors.
4. A district heating case built on a gas CHP carbon credit that no longer exists.

The fourth is the most expensive to unwind, because the energy centre, the flues, the plant room and the pipe routes are all in the planning drawings by the time anyone recalculates. Flue height and dispersion modelling then become an air quality question in their own right, with its own consultee and its own cost.

Reporting the factor set used

A defensible energy statement names its arithmetic. State the SAP version and release number, the Approved Document L edition the compliance check was run against, the baseline notional dwelling used for any percentage claim, and the emission and primary energy factors applied to each fuel. Where a local policy sets its own baseline or requires a particular factor set, say which one you used and why.

Include the output worksheets rather than only a summary table. Officers and their reviewers check the inputs, and the ones they check first are the assumed seasonal efficiency of the heat pump, the ventilation strategy and any overshadowing applied to the array. Overheating interacts with all of this, because an all electric, airtight dwelling with large glazing can pass Part L and fail the overheating assessment, as covered in overheating assessment assumptions.

Avoid claims about future grid decarbonisation inside a compliance calculation. Describing a design as future proofed, without a mechanism securing the future step, is the sort of statement that attracts a condition rather than approval. Our note on future proofing claims in an energy statement sets out what to say instead.

Checking the edition before comparing schemes

Before you rely on any carbon figure in a submitted document, run four checks.

• Which SAP version and release produced it, stated explicitly rather than inferred from the date.
• Which Part L edition and notional dwelling the percentage is measured against.
• Whether the same factor set was used for the baseline and the proposal, which is not always true in documents assembled over a long design period.
• Whether operational carbon claims have been confused with embodied carbon, which belongs in a Whole Life Carbon Assessment and uses entirely different data.

That last confusion is common once a scheme has both documents. Operational and embodied carbon are measured in different ways, and adding a percentage from one to a tonnage from the other produces a number that means nothing. Councils usually expect a suitably qualified author for whole life carbon work and professional review is strongly advised.

Where a permission is granted on the strength of a calculation, expect a verification condition requiring as-built evidence, and read post-construction energy verification conditions before agreeing the wording.

Enter the postcode on PlanScope to see which energy and sustainability reports your site is likely to need, and send the draft for professional review before submission.

This is general guidance, not 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.