SAP emission factors and their effect on heating choices
Under the Part L 2013 factor set a gas boiler was the low carbon answer. Under the Part L 2021 set the comparison reverses, and primary energy pulls the other way.
The sap emission factors heating question matters because the numbers changed direction. Under the version of the Standard Assessment Procedure used for Part L 2013, grid electricity carried a carbon factor several times higher than mains gas, and a gas boiler was the low carbon answer on paper. Under the version used for Part L 2021, electricity carries a lower carbon factor than gas, so the same comparison reverses. Any energy statement, planning policy target or fuel decision that relies on the older factor set now gives the wrong answer, and it gives it confidently.
Where the factors come from
The approved calculation methodology assigns each fuel two numbers. A carbon dioxide emission factor, in kilograms of carbon dioxide per kilowatt hour of delivered energy. And a primary energy factor, a multiplier reflecting the energy consumed in extracting, generating, transmitting and distributing that delivered energy.
Neither is a measurement of your building. Both are national average figures set within the methodology, revised when a new version is published, and the version matters as much as the specification. Two assessors modelling the same house under two versions will report different carbon results and both will be correct for their version.
Grid electricity is where the movement has happened. Its carbon factor reflects the generation mix, which has changed substantially as coal has left the system and wind and solar capacity has grown. Its primary energy factor has moved far less, because generation and network losses are physical rather than political.
How far the electricity factor has moved
In the factor set used for Part L 2013 compliance, grid electricity was around 0.519 kgCO2 per kilowatt hour and mains gas around 0.216. In the factor set used for Part L 2021 compliance, electricity sits near 0.136 and gas near 0.210. Check the figures in the version of the methodology your assessment actually uses, because these are published values that change with each release.
The ratio is what matters. Electricity went from roughly two and a half times the carbon content of gas per delivered unit to roughly two thirds of it. Nothing about the buildings changed. The grid did.
Primary energy factors went the other way in relative terms. Electricity carries a factor around 1.5 against roughly 1.13 for mains gas, reflecting losses in generation and distribution that no amount of renewable capacity removes from the accounting method.
Gas against electric, worked through
This example is illustrative and uses the published Part L 2021 factor set. It is arithmetic anyone can repeat, not a claim about a particular building.
Take 1 kWh of useful heat delivered into a dwelling.
| Route | Delivered energy for 1 kWh heat | Carbon, kgCO2 | Primary energy, kWh |
|---|---|---|---|
| Gas boiler at 90 per cent efficiency | 1.11 kWh gas | 0.23 | 1.26 |
| Direct electric heating | 1.00 kWh electricity | 0.14 | 1.50 |
| Air source heat pump at seasonal efficiency 2.8 | 0.36 kWh electricity | 0.05 | 0.54 |
Read across the rows and the point becomes obvious. Direct electric heating beats a gas boiler on carbon and loses to it on primary energy. A heat pump wins on both by a wide margin, because reducing delivered energy improves every metric at once.
That is precisely why the 2021 edition of the Approved Document added a primary energy target alongside the emission target. Judging buildings on carbon alone would have allowed inefficient electric heating to pass on the strength of a decarbonising grid, and the way the two targets interact is set out in target emission rate and target primary energy rate compared.
One caution on heat pump figures. The seasonal efficiency used in the calculation depends on flow temperature, emitter sizing and control strategy, and an optimistic figure in the model that the installation does not deliver is the commonest gap between a compliance calculation and a building in use. Design the emitters for a low flow temperature or do not claim the efficiency.
Regulated emissions and what they leave out
The figures reported against the targets cover regulated energy only: space heating, hot water, ventilation, pumps, fans and fixed lighting. Cooking, appliances, lifts in some contexts, and anything an occupier plugs in are unregulated and sit outside the calculation.
Two consequences follow for a planning submission.
The percentage carbon saving you report is a saving on regulated emissions, and it should say so. A scheme reporting a large reduction is reducing part of the building's actual energy use, and a reader who assumes otherwise will be surprised by the operational data later.
And measures that reduce unregulated demand, such as efficient appliances or communal laundry, do not improve the compliance figure at all. Where a local policy also asks for whole life carbon, those measures do count, which is one reason the sustainability statement and the energy assessment need to be written together rather than by two teams in sequence.
Why a percentage saving depends on the baseline
Planning policies routinely express a requirement as a percentage improvement over the Building Regulations target. The London Plan 2021, for instance, requires major development to achieve a minimum on-site reduction beyond the Part L baseline as part of its net zero-carbon policy.
The problem is that the baseline is a moving object. It is set by a notional building defined in the current Approved Document, calculated using the current version of the methodology, with the current factor set. Change any of those three and the same building reports a different percentage.
So a 35 per cent improvement against a 2013 baseline and a 35 per cent improvement against a 2021 baseline describe very different buildings. The 2021 notional building is already substantially better than its predecessor, so the same percentage is a much harder target. Applicants who quote a percentage without its reference point invite an officer to assume the easier reading, and consultees who spot it will say so.
State the baseline in the same sentence as the percentage. Every time.
Stating the factor set in the submission
An energy statement should make the following explicit on its first page, because a reviewer cannot check anything without them.
- The Approved Document edition the assessment complies with.
- The version of the calculation methodology used, including the point release.
- The carbon and primary energy factors applied to each fuel, quoted as numbers.
- Whether the assessment is at design stage or as-built.
- The assumed seasonal efficiency of any heat pump, with the flow temperature it depends on.
- The photovoltaic area and orientation assumed, and the proportion of the result that depends on it.
Item three feels pedantic and it is the one that resolves arguments. Where a consultee challenges a carbon figure, a stated factor set turns the discussion into arithmetic instead of a dispute about methodology.
Item six is the one that protects you after permission. Panels are the first thing value engineered off a scheme, and a compliance result that depends heavily on them is fragile. If the roof later carries plant, the calculation has to be redone.
Sensitivity where a target is marginal
Where a scheme lands within a few percentage points of a policy target, run the calculation under more than one assumption and report both. That means a sensitivity run with the alternative heat pump efficiency, a run without the marginal photovoltaic area, and where a methodology update is expected, a view on how the factor set would move the result.
Two situations make this worth the fee. Where the permission will be implemented two or three years after approval, the compliance version at building control may differ from the one used at application, and a scheme passing by one per cent at application may fail later. And where a condition requires the approved carbon saving to be delivered, a marginal calculation becomes a discharge problem rather than an academic one.
The direction of travel is a falling electricity carbon factor and continued attention to delivered energy. Government has consulted on a replacement calculation methodology alongside the Future Homes Standard, and whether it has been adopted should be checked against the current position rather than assumed, as at September 2026.
Two related decisions worth thinking through in the same exercise. Where a communal combustion plant is proposed, the carbon case and the air quality case pull against each other, as set out in combined heat and power and the air quality objection. And where a ground source system is proposed, the borehole itself brings consenting questions covered in ground source heat pumps and borehole permitting.
Enter your postcode on PlanScope to see which reports your site is likely to need. 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.
Frequently asked questions
Why did the electricity carbon factor fall so far?
Because it reflects the national generation mix, which changed as coal left the system and wind and solar capacity grew. In the factor set used for Part L 2013 compliance grid electricity sat near 0.519 kgCO2 per kilowatt hour against roughly 0.216 for gas. In the Part L 2021 set electricity sits near 0.136 and gas near 0.210.
Does direct electric heating now beat a gas boiler?
On carbon, yes, using the Part L 2021 factor set. On primary energy, no, because electricity carries a factor around 1.5 against roughly 1.13 for mains gas and direct electric heating converts one unit of delivered energy into one unit of heat. That is the outcome the primary energy target was introduced to prevent.
What do regulated emissions exclude?
Cooking, appliances and anything an occupier plugs in. The reported figures cover space heating, hot water, ventilation, pumps, fans and fixed lighting only. So a percentage saving describes part of a building's actual energy use, and measures reducing unregulated demand do not improve the compliance result at all.
Why is a percentage carbon saving unreliable on its own?
Because the baseline moves. It is set by a notional building defined in the current Approved Document, calculated with the current methodology version and factor set. The 2021 notional building is substantially better than its predecessor, so the same percentage against a 2013 baseline and a 2021 baseline describe very different buildings.
What should an energy statement state about factors?
The Approved Document edition, the methodology version including point release, the carbon and primary energy factors applied to each fuel quoted as numbers, whether the figures are design stage or as-built, the assumed heat pump seasonal efficiency with its flow temperature, and the photovoltaic area the result depends on.
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.