Natural gas NCV: 10.4 kWh/m³, your bill counts 11.6

The net calorific value (NCV, called PCI in French) of the natural gas distributed in Belgium is about 10.4 kWh per m³. Your bill counts 11.6, because it is drawn up on the gross calorific value (GCV, PCS in French). Both figures are correct. Mixing them skews an efficiency calculation by 11% and a comparison with heating oil by several points.

What is the net calorific value of natural gas in Belgium?

Around 10.4 kWh per m³ for high-calorific gas, known as H gas.

The CREG, Belgium's federal energy regulator, publishes a reference value of 11.6 kWh per m³ for this gas, and that value is a GCV. For natural gas, the NCV sits about 10% lower, according to the tables used by Selectra (updated 27 May 2026): 11.6 × 0.90 gives 10.4. Low-calorific L gas, no longer distributed since the end of the conversion from lean to rich gas, was worth 10.3 kWh GCV, or about 9.3 kWh NCV.

The real value moves. ORES quotes a GCV between 10.81 and 12.79 kWh per normal m³ on its network, which puts the NCV between 9.7 and 11.5 kWh.

NCV or GCV: what is the difference?

A matter of water vapour. When it burns, methane produces CO₂ and a lot of water, which leaves the flame as vapour. If that vapour condenses, it gives back its latent heat. The GCV counts that heat. The NCV assumes it goes up the chimney, which was true of every boiler until condensing technology arrived, and that historical reason is why the heating industry long reasoned in NCV, while meters, contracts and regulators reason in GCV.

Which basis is your Belgian bill calculated on?

The GCV, in all three regions.

The conversion factor printed on your statement multiplies your m³ to obtain kWh, and that factor is the gross calorific value of the gas delivered to your receiving station, averaged over the billing period. The rule is the same in Wallonia (ORES, RESA), in Brussels, where Sibelga publishes its GCV values as open data, and in Flanders with Fluvius. Only the value changes from one network and one month to the next. Our article on converting gas m³ to kWh details the calculation and includes a converter.

The EU energy label uses the same basis. The note from the Walloon air and climate agency AwAC on Ecodesign Regulation 813/2013 states it plainly: efficiencies are expressed relative to the gross calorific value. The 86% minimum seasonal efficiency, in force since 26 September 2015 for boilers of 70 kW or less, is therefore a GCV figure.

Why does a boiler show 108% efficiency?

Because the brochure calculates on the NCV, and a condensing boiler recovers precisely the heat the NCV did not count. The figure is not a lie; it is simply expressed in a different unit from your bill. Converted to the GCV, a 108% efficiency becomes about 97%, and an old atmospheric boiler advertised at 90% NCV drops to around 81%.

The mistake is costly in estimates. Take the CREG standard profile D3a, 17,000 kWh of gas billed per year. With a condensing boiler, the heat actually produced is close to 16,524 kWh. Applying the brochure's 108% to the billed kWh promises 18,360. That is 1,836 kWh of heat that does not exist, 11% too much, enough to distort any payback calculation for a heat pump or insulation.

How much does a kWh of gas cost on each basis?

We took the all-in variable price from the Eneco Gaz Naturel Flex price card of September 2026 on the ORES network, 13.31 euro cents per kWh including VAT, excluding the standing charge and the fixed network term, and broke it down on the three useful bases.

This price is re-indexed every month on a variable contract. Brussels and Flanders have different network tariffs, but the ratio between the bases stays the same everywhere: 0.90.

Gas, heating oil, propane: the calorific value of each fuel

H gas values: CREG and calculation at 0.90; other fuels: Selectra tables, May 2026.

Heating oil shows the trap. At the maximum price of 1.4227 euros per litre set from 8 October 2026 for orders of 2,000 litres or more, reported by L'Avenir, a kWh of heating oil costs 14.37 c NCV and 13.42 c GCV. Against gas at 13.31 c GCV, the usual comparison, gas from the bill against heating oil from NCV tables, makes gas 7% cheaper. On a common basis, the gap shrinks to under 1% on GCV and reverses slightly on NCV. The full trade-off, fixed charges and efficiencies included, is in our comparison of heating oil versus gas.

Four mistakes that skew a calorific value calculation

• Applying an NCV efficiency to kWh billed on GCV, the 11% error described above.
• Comparing two fuels on two different bases, typically gas from the bill and heating oil from a table.
• Using a French conversion factor, calculated on other networks, to check a Belgian bill.
• Still using the GCV of lean gas, 10.3 kWh per m³, on a recent bill.

For each situation, the basis to use:

Does "town gas" have a different calorific value?

No. In French, "gaz de ville" today simply means the natural gas distributed by the network, the gas in your meter.

Calorific value cannot be negotiated; the price per kWh can. To place your rate, compare the total annual cost of your contract in our energy supplier ranking, and locate your profile with the average gas consumption in Belgium. Any work on a boiler or gas installation must be carried out by an approved professional, and periodic servicing remains compulsory under your region's rules.

Sources. Reference GCV of H gas (11.6 kWh/m³) and L gas (10.3 kWh/m³): CREG, "Conversion du m³ de gaz en kWh", accessed 28 October 2026. GCV range on the ORES network (10.81 to 12.79 kWh/m³(n)): ORES, "Le pouvoir calorifique du gaz". GCV per receiving station in Brussels: Sibelga open data. Ecodesign efficiencies expressed on GCV and 86% minimum: AwAC and DGO4 note on Regulation 813/2013, updated 22 March 2016. NCV and GCV gap (about 10% for natural gas), NCV and GCV values for heating oil, propane and butane, efficiencies above 100% on NCV: Selectra, "pouvoir calorifique" and "pouvoir calorifique inférieur" guides, updated 27 May 2026. Gas price: Eneco Gaz Naturel Flex price card, September 2026, ORES network, 13.31 c/kWh incl. VAT excluding fixed charges. Maximum heating oil price (1.4227 €/L from 2,000 L, from 8 October 2026): Energy Administration, reported by L'Avenir on 7 October 2026. The 0.90 factor and the 97% and 81% GCV efficiencies are conversions made by us on 28 October 2026; as variable contract prices are re-indexed every month, redo the calculation with your own rate. No affiliate links.

Camille