Reading emission factors in a planning energy statement

The carbon factor for grid electricity in the assessment methodology behind Part L 2021 is roughly 0.136 kilograms of carbon dioxide per kilowatt hour, against about 0.210 for mains gas. Under the previous version of the methodology, electricity carried a factor of around 0.519. That single change reversed the relative carbon position of the two fuels, and it is why a heat pump now beats a gas boiler comfortably in a regulated emissions calculation where it used to lose. An energy statement that still argues from the old position is arguing from a document that no longer exists.

Why the electricity factor moved

The factor reflects the average carbon intensity of the electricity a building will draw over a period, and the generating mix behind it changed faster than the assessment methodology did.

The older figure was set when coal and gas dominated generation. The current figure reflects the growth in wind, solar and nuclear output and is projected forward over an assessment period rather than taken as a snapshot of today.

Two consequences follow for anyone writing planning documents. Electrical heating stopped being penalised. And the arithmetic of any percentage improvement over a baseline changed, because both the target and the design figures moved.

Check the factor in the current version of the methodology rather than quoting a number from an older scheme. The values are revised, and the version in force for building regulations purposes is the one the statement should use.

What changed for heat pumps against gas boilers

Take the delivered energy first. A gas boiler at high efficiency delivers slightly less than a kilowatt hour of heat per kilowatt hour of gas. A heat pump with a seasonal performance factor around three delivers roughly three kilowatt hours of heat per kilowatt hour of electricity.

Combine that with the emission factors and the gap is wide. The gas boiler's heat carries something close to the gas factor. The heat pump's heat carries the electricity factor divided by the performance factor, which lands well below it.

That is the whole explanation for the shift to all-electric schemes, and it has consequences beyond carbon. Peak electrical demand rises, the incoming supply capacity has to be checked with the distribution network operator early, and the absence of a gas connection changes the cost plan. The decision is worth taking deliberately, as the note on all-electric new homes and the gas connection decision sets out.

Performance factor assumptions are where statements get challenged. A figure taken from a manufacturer's best case at a flow temperature the emitters cannot work at is not evidence. State the flow temperature and the assumed performance, and make sure the emitter sizing supports it.

How the regulated emissions baseline is set

The target rate for a dwelling comes from a notional building of the same size and shape, given a specification set out in the methodology. Your design is modelled, and the resulting rate is compared with the target.

Part L 2021 introduced three tests running together for new dwellings: a carbon dioxide target, a primary energy target and a fabric energy efficiency target. All three have to be met, and it is possible to pass the carbon test while failing on primary energy, which catches schemes relying heavily on low carbon electricity without improving the fabric.

Regulated emissions cover heating, hot water, ventilation, fixed lighting and, in non-domestic buildings, cooling. Unregulated emissions from appliances and equipment sit outside the regulations, and several local policies ask for them to be reported separately. Report them separately, and never mix them into a percentage improvement figure without saying so.

Presenting the percentage reduction a local plan asks for

Most policy requirements are expressed as a percentage improvement over the building regulations baseline, and many are structured around an energy hierarchy of demand reduction, then efficient supply, then renewables.

Set the results out as a table with regulated emissions in tonnes of carbon dioxide per year at each stage:

State the baseline edition explicitly. A policy written against an earlier edition of the regulations and applied to a scheme modelled against the current one produces a meaningless percentage unless the statement explains which baseline it used and why. Where the authority's guidance sets out the conversion, follow it and cite it.

An illustrative example. A scheme reports a 40 per cent improvement without naming the baseline. The officer asks which edition it refers to, the answer turns out to be the superseded one, and against the current baseline the improvement is materially smaller. The application loses six weeks over a missing sentence.

Arithmetic errors officers notice

The same handful of mistakes recur, and they are all avoidable.

1. Applying the current emission factors to a baseline calculated under the previous methodology, which inflates the apparent improvement.
2. Counting a fabric improvement once in the demand reduction stage and again in a later stage.
3. Mixing regulated and unregulated emissions between stages of the same table.
4. Quoting a percentage against floor area in one row and against dwelling numbers in another.
5. Reporting site-wide totals that do not reconcile with the sum of the individual blocks.
6. Confusing primary energy with carbon dioxide, which are different tests with different units.

A cross-check that takes ten minutes: add the individual block figures and confirm they equal the site total, then divide the site total by the number of dwellings and see whether the result is plausible.

Where the figures sit in the submitted statement

Put the numbers in one table near the front, with the modelling outputs in an appendix. Officers and their consultants read the table first and go to the appendix only when something looks wrong.

The statement should name the methodology version, the emission factors used, the regulations edition forming the baseline, the assumed system efficiencies and performance factors, and the software used for the modelling. Where an overheating check is required, cross-reference the overheating risk assessment rather than repeating it, because glazing and ventilation decisions affect both documents and they must agree.

Embodied carbon sits in a separate document with a different method, and mixing the two in one table is a common source of confusion. The whole life carbon assessment covers that ground.

Keeping the statement consistent with the building control submission

The planning energy statement and the compliance submission to building control are produced at different times by different people, and they drift apart.

That drift is picked up at discharge of conditions, when the authority asks for evidence that the approved reduction has been delivered. If the as-designed model shows a smaller improvement than the planning statement promised, the condition is not discharged.

Two habits prevent it. Record the assumptions behind the planning statement in a register issued with the tender information, in the same way a daylight case is protected. And re-run the model when the specification changes materially, rather than at the end when the only options left are expensive.

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This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.