GCV/NCV in 2026: fuel conversion ratio for accurate pricing

Understanding GCV and NCV for each fuel (without mixing up the basis)

What GCV measures: heat including condensation of water vapour

GCV (gross calorific value) measures the energy released by a fuel including the recoverable heat when the water vapour in the flue gases condenses. In other words, the "hidden" heat in this condensation is also counted. This is the basis that corresponds to the efficiency stated for a condensing boiler.

What NCV measures: heat without recovering condensation

NCV (net calorific value) measures the energy available without counting this condensation heat. It is assumed that the water vapour stays as vapour and leaves through the flue. This basis is often used to compare non-condensing equipment and to express fuel consumption.

Why the same fuel doesn't give the same result in GCV and NCV

During combustion, the hydrogen in the fuel forms water. The more it produces, the higher the "condensable" share, and so the greater the GCV-NCV gap. The moisture content of the fuel (wood, pellets) also plays a role. The result: GCV is always greater than NCV, but the gap varies depending on the fuel and actual conditions.

GCV/NCV conversion ratio: principles and factors that make it vary

The role of moisture and hydrogen in the fuel

GCV counts the energy recoverable if the water vapour in the flue gases condenses. NCV ignores it. The more hydrogen a fuel contains, the more water it forms during combustion, and the greater the GCV/NCV gap. Initial moisture content (wood that's not dry enough, biomass stored outside) adds even more water to be evaporated. The result: NCV drops and the ratio varies.

Conversion: when to use a coefficient, and when to check the supplier's data sheet

An average coefficient is enough for quick estimates, preliminary sizing or comparing offers on the same basis. For billing, a contract, a performance calculation or a certificate, check the supplier's data sheet. The actual quality (gas composition, moisture content, origin) shifts the ratio and therefore the kWh.

Case of common fuels: gas, heating oil, wood (logs, pellets)

Orders of magnitude: natural gas around 1.10 to 1.12, propane a little lower. Heating oil often close to 1.06. Wood is highly variable: wet logs can exceed 1.20, pellets are more stable, often around 1.10 to 1.15. Keep these values as an order of magnitude, then adjust with the product's data.

GCV ↔ NCV conversion in your quotes and sizing: simple method and points of vigilance

Converting from NCV to GCV (and vice versa): formula and units to check

On a quote, keep a clear rule. GCV includes the recoverable heat from condensing the water vapour in the flue gases. NCV does not include it. Conversion is done with a coefficient specific to the fuel. Formulas: GCV = NCV × k. NCV = GCV ÷ k. Also check the basis stated in the product data sheet (kWh NCV or kWh GCV), otherwise sizing starts off wrong.

kWh, m³, tonne, litre: avoiding unit errors by fuel

Energy and volume get mixed up quickly. For gases, the supplier often bills in kWh (GCV or NCV) calculated from corrected m³. For heating oil and propane, you see litres or kg, then kWh. For firewood, pellets or wood chips, a tonne doesn't hold the same energy depending on moisture content. Systematically note the purchase unit, the calculation unit, and the NCV or GCV basis.

Impact on the stated efficiency of a boiler or a biomass appliance

Efficiency changes depending on the basis. A condensing appliance can show more than 100% on an NCV basis, with no magic involved. On a GCV basis, it drops back below 100%. To compare two appliances, bring everything back to the same basis, especially when calculating consumption or energy savings from the kWh figures.

GCV/NCV and grants in 2026: securing your files depending on the fuel used

CEE: consistency of kWh and evidence (invoices, technical data sheets, certificates)

For the CEE, the sensitive point is the gap between kWh billed on a GCV basis (often for gas) and equipment performance expressed in NCV. Align the value used with the operation fiche and keep the chain of evidence: invoice, technical data sheet (efficiency), statement of honour, and, if needed, a dated conversion note.

MaPrimeRénov' 2026: choosing the right energy reference in the supporting documents

For MaPrimeRénov', avoid files that mix GCV and NCV from one document to another, especially when the fuel changes (gas to heat pump, heating oil to biomass). On the quote, invoice and product data sheet, clearly state the reference used and stay consistent with the DPE or audit submitted. To frame the supporting documents and points of vigilance, also rely on this guide to avoid file rejections.

RGE certification and inspections: tracking the conversion used and its sources

In the event of an inspection, what reassures is traceability. Add a short RGE annex to the file: formula, coefficient, official source (grid operator, CEE fiche, public guide), and the date. You save time, and you reduce back-and-forths on compliance.

Tools and field best practices to make fuel conversion more reliable

Internal tables by fuel: standardising your conversion coefficients

Create an internal table by fuel with your actual units (litre, kg, stère, m³) and a single coefficient in kWh. Source your values from public references (ADEME, France Rénov') and lock in a dated version. You avoid "rough guess" conversions and keep simple, clear traceability.

Verification checklist before sending the quote and the grant application

• Consistent unit between invoice, reading and quote.
• Value used in NCV or GCV, written down in black and white.
• Current and future equipment efficiency recorded, with supporting evidence.
• For wood, moisture content specified (logs, chips, pellets).
• Rounding kept to a minimum and calculations reviewed by a second person.

Common mistakes among tradespeople: confusing calorific value and efficiency, forgetting moisture content

The classic trap is mixing up NCV, GCV and efficiency. The result: overestimated or underestimated consumption, and therefore distorted gains. Another point that changes everything is the moisture content of the wood. The wetter it is, the less it "lights up" your kWh calculation. To stay safe, note the assumption and keep the supplier's proof in the file. To go further on the topic, also see how to calculate efficiency with a standard method.