Converting delivered energy into primary energy is a single multiplication: primary energy equals delivered energy times the conversion factor. In SAP 10, the method behind an EPC, electricity carries a factor of about 1.5 against roughly 1.1 for mains gas, a far smaller penalty than earlier versions of the method applied. That factor is what decides the EPC band: at unchanged metered consumption, an electrically heated home is rated on about 1.5 kWh of primary energy for every kWh counted at the meter.
Understanding the difference between final energy and primary energy
Final energy: what the client consumes and what you bill (kWh at the meter)
Final energy is what your client sees passing through their meter. Gas in kWh, electricity in kWh, fuel oil in converted litres, wood in stères or kg. It is also the basis of the bill. For you, it is the most concrete benchmark for estimating consumption after work and explaining the savings, at the meter.
Primary energy: the energy "before delivery" and its production/transport losses
Primary energy corresponds to the energy mobilized at the source to supply the final energy. It includes losses linked to production, transformation and transport. This is particularly visible for electricity. Part of the energy is "lost" before reaching the client. This is referred to as a level before delivery.
Why this coefficient exists: comparing uses (electricity, gas, wood) on a common basis
The factor puts all uses on the same calculation rule. Otherwise, you would be comparing kWh that do not carry the same upstream energy cost. In energy balances and on an EPC, primary energy is used to classify a home on a common basis, regardless of the energy carrier.
Conversion coefficient: how to go from final energy to primary energy
The simple formula to use on site: primary energy = final energy × coefficient
On a quote or an energy balance, you often start from delivered energy (the figure measured at the meter or on the bill). To compare solutions and align with the way SAP works, a factor is applied.
| Energy | Delivered to primary factor | What to remember |
|---|---|---|
| Electricity | about 1.5 | in SAP 10, no longer penalised the way it used to be under older versions of the method |
| Mains gas | roughly 1.1 | |
| Fuel oil | not stated | |
| Wood | not stated |
Where to find the right factor: EPC, retrofit assessment, technical data sheets and official texts
The simplest approach is to use the factor stated in the EPC or in the retrofit assessment. You also find it in certain technical data sheets (particularly for heat networks) and, above all, in the SAP documentation itself. If in doubt, rely on the project's binding document, not on a value taken by feel.
Common conversion mistakes: units, reference periods and rounding
What this changes in a retrofit: EPC, assessment and choice of work
Impact on the home's energy class: why electricity "weighs" differently
An EPC does not just look at your billed kWh. It converts to primary energy. Electricity is therefore multiplied by a factor different from gas or wood, though the gap has narrowed sharply as the grid decarbonised. As a result, two homes with the same bill can display a different band. In 2026, the retrofit assessment complements this diagnosis to prioritise the measures that truly raise the rating.
Concrete cases: insulation + ventilation, heating, domestic hot water
- Insulation and airtightness first, otherwise you are heating the outdoors. Then suitable ventilation to keep the air healthy.
- Heating next, sized after the work. A better-insulated house allows a smaller, more efficient heat generator.
- Domestic hot water, often overlooked. A heat pump water heater, solar, or simple adjustments can reduce consumption year-round.
Heat pump, boiler, stove: comparing correctly thanks to the conversion
To choose, compare on the same basis. The EPC expresses figures in kWh of primary energy, but your cost of use also depends on billed kWh, efficiency and the actual need after insulation. A heat pump can perform very well if the house is consistent and well ventilated. A wood stove can help as a supplement. A boiler remains relevant if the network and usage patterns suit it.
Subsidies and obligations: talking about "energy" correctly in your files in 2026
Funding: when primary and delivered energy come into play in supporting documents
Funded measures lean on the EPC or the assessment, expressed in primary energy. On manufacturer data sheets and invoices, you instead find values in delivered energy. To avoid back-and-forth, clearly state the unit, the scope (home, system, measure) and keep kWh figures consistent from one document to another.
Registration and inspections: securing your documents (quotes, product sheets, certificates)
In 2026, the strength of a file rests on simple but rigorous documents.
- A dated quote.
- Your company number.
- Your certification number at the right point.
- A precise description of the work.
- Verifiable characteristics (the U-value for insulation, the SCOP for a heat pump).
- Product references.
Organise everything from the start, like a well-labelled consumer unit. Goal: clean evidence.
Condominiums and single-family homes: points of attention on reference consumption
In a house, the reference is the property's own EPC. In a block of flats, there can be an EPC per dwelling, an assessment, and consumption figures that mix private and communal areas. Specify the situation (communal heating or not), the floor area used, and the reference period. A clear reference avoids losing energy in paperwork.
Field method: explaining the coefficient to the client and avoiding disputes
Two-minute explanation: a simple comparison to get the message across
Explain the coefficient like an exchange rate. You start from a real figure (bill, kWh, litres of fuel oil) and "translate" it into another unit everyone understands. In renovation, this is often used between final energy and primary energy. The client remembers a simple idea. Same use, two ways of counting it.
Quoting before/after: presenting energy savings without promising the impossible
Present a before/after with a range, not a figure set in stone. Base it on 12 months of consumption, which Argile pulls from the metering operators with the client's consent, then adjust to a comparable climate (heating degree days) and consistent usage. Announce an order of magnitude in kWh and euros, while noting that weather and habits can change.
End-of-visit checklist: data to collect for a reliable conversion (2026)
- Energy sources and contracts (electricity, gas, fuel oil, wood). Meter reading and 12 months of bills.
- Actually heated surface, ceiling height, unheated zones.
- Generator (make, power, age), emitters, control system, setpoint.
- Ventilation, perceived airtightness, existing insulation.
- Assumptions noted in writing. Conversion coefficient used (2026 reference).
