Defending the assumptions inside an energy statement

Energy statement assumptions get challenged in a predictable order. Baseline selection first, then renewable capacity against the roof area actually available, then plant efficiencies quoted at test conditions, then the treatment of unregulated energy. A reviewer working for the authority will go to those four before reading anything else, because that is where the percentage improvement is usually manufactured.

None of this means the document was dishonest. Most of the time the optimistic assumption was made early, by somebody who expected to revisit it, and nobody did.

Baseline selection and why it matters

Every percentage improvement is measured against something. In England that something is normally the notional building generated by the national calculation methodology under the current Part L, and the first question is whether the right version was used.

Two errors recur. Using a superseded version of the methodology because the model was set up at the start of a long project. And comparing against a baseline that is not the notional building at all, but the applicant's own earlier design, which produces a flattering number with no policy meaning.

State the methodology version, the software and the compliance route in the first page of the assessment. A reviewer who has to work out which baseline was used will assume the worst.

Where local policy sets its own target, check whether it is expressed against regulated emissions, primary energy or carbon, because the three give different answers from the same model. A target written against an older Part L and carried into a new plan without updating is a real problem, and the honest response is to report against both.

Renewable capacity against roof area available

This is the assumption that fails most visibly, because anyone can check it with a scale rule.

The energy model asks for an installed capacity. The design team enters a figure that delivers the required percentage. Nobody then checks whether that array fits on the roof once plant, lift overruns, safe access zones, maintenance gangways, roof lights, green roof areas and any terrace have taken their space.

Do the layout drawing. A roof plan showing panel positions, inter-row spacing at the chosen tilt, the plant zone and the access strategy is the single most effective piece of evidence in an energy statement, and it takes an hour. Where the array competes with a green roof or amenity space required by another policy, say how the conflict was resolved rather than counting the same square metre twice.

Shading and orientation taken optimistically

Yield calculations run on assumptions about orientation, tilt and shading. A south-facing unshaded array on a flat roof in central London is rare.

Model the actual context. Neighbouring buildings, the scheme's own taller elements, parapets, plant enclosures and any consented development next door all cast shadows, and a yield figure that ignores them overstates output through the winter months when the shading is worst.

Where panels are mounted east and west to increase density, say so and use the right yield. Inter-row shading within the array itself also matters at low winter sun angles, and a tightly packed layout loses more than the panel count suggests. The trade is more panels at lower output each, which can still be the right answer, and presenting it honestly is far stronger than claiming south-facing performance from an east-west layout.

Plant efficiencies quoted at test conditions

Heat pump coefficients of performance are quoted at standard test conditions. Real seasonal performance depends on flow temperature, source temperature and how the system is controlled, and a system designed around high flow temperatures for existing radiators will not achieve the headline figure.

But communal systems deserve particular care. Distribution losses in a heat network with long horizontal runs and poorly insulated risers can undo a meaningful share of the generation efficiency, and reviewers in London have been asking about them for years.

Unregulated emissions and their treatment

Building Regulations cover regulated energy: heating, hot water, ventilation, lighting and cooling. Cooking, appliances and small power sit outside it.

Most local policy targets follow the regulated definition, which means a building with high unregulated loads can hit its planning target while consuming considerably more energy in use. That is a limitation of the method rather than a fault in the assessment, and saying so plainly is better than ignoring it.

Where a policy asks for unregulated emissions to be reported, use a stated method and name it. Where it does not, a short paragraph acknowledging the gap gives the document credibility. The related question of which modelling inputs officers query is covered in energy modelling assumptions officers challenge.

Sensitivity testing that pre-empts the challenge

The best defence is to make the challenge yourself, in the document. Run the model with three or four less favourable inputs and report the result.

1. Photovoltaic array reduced to the capacity the roof plan actually supports.
2. Heat pump performance at the design flow temperature rather than test conditions.
3. Shading from consented neighbouring development included.
4. Distribution losses at the higher end of the plausible range.

If the scheme still meets the target, the assessment is close to unchallengeable. If it does not, you have found out during design rather than at committee, which is the point of doing it.

Sensitivity testing also protects the client at condition discharge, when as-built evidence has to match what was claimed. A scheme that was approved on optimistic inputs has to be built to them, and the variation application that follows is more expensive than the modelling would have been.

Presenting uncertainty honestly

Write the assumptions register as a table at the front rather than burying it in an appendix. Each assumption, its value, its source and who is responsible for holding it through procurement.

That last column is the one nobody fills in, and it is the one that matters on site. An airtightness target owned by nobody drifts. A panel count owned by nobody shrinks when the roof plant grows.

An Energy Statement written that way reads as a professional document rather than a compliance exercise, and officers treat it accordingly. Where the energy case interacts with overheating, the same discipline applies to the Overheating Risk Assessment, because glazing and ventilation assumptions feed both.

And accept one honest limitation. Modelling predicts compliance, not performance in use, and the gap between the two is well recognised in the field. A statement that acknowledges it, and explains what the design does to narrow it, carries more weight than one that does not mention it at all.

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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. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.