kW or kWh: what is the difference on your energy bill?

A kW measures power, a kWh measures a quantity of energy. Your supplier only bills the second. An appliance rated at 1 kW running for one hour uses 1 kWh, and in Belgium that kilowatt-hour costs about 2,5 times more as electricity than as gas.

What is the difference between kW and kWh?

The kW says how fast you are consuming. The kWh says how much you have consumed in total.

A car makes the point better than any textbook definition. The kW is the speed on the speedometer, the kWh is the distance covered at the end of the trip. Being able to do 100 km/h costs nothing while you stay parked, and exactly the same goes for a 2 kW heater that is switched off.

The letter "h" therefore changes everything. It means the power was sustained for a certain time, and it is that duration your meter adds up, day after day, in its reading.

How do I convert watts into kWh?

One multiplication, then a division by 1 000.

The formula: power in watts × time in hours ÷ 1 000 = kWh. A 2 000 W portable heater left on for 3 hours uses 2 000 × 3 ÷ 1 000 = 6 kWh. A 10 W LED bulb has to stay on for a hundred hours to reach 1 kWh.

The reverse calculation gives you an average power. Divide the kWh by the hours: 6 kWh over 3 hours makes 2 kW. That result remains an average, and it says nothing about an appliance that might draw 3 kW for one hour and then stop.

How much does 1 kWh cost in euros in Belgium?

It depends on the energy and the region, and the gap is striking.

For electricity, the official benchmark is the average commercial price calculated by the CREG, the federal energy regulator, all-in. For Q4 2026, Securex reports the figures communicated on 4 September 2026: 32,25 c€/kWh in Flanders, 36,88 c€ in Brussels and 37,79 c€ in Wallonia. For gas, no public quarterly equivalent exists; the all-in averages published by Selectra for October 2026 give 12,10 c€ in Flanders, 12,81 c€ in Brussels and 14,78 c€ in Wallonia.

One kWh is therefore not worth another, and the useful comparison is the kWh of heat actually delivered to the room. We redid the calculation for three heating methods, with two explicit assumptions: 90 % efficiency for a condensing gas boiler and a seasonal performance factor (SCOP) of 3 for an air-to-water heat pump, average values that vary widely from one home to the next.

A conventional electric heater costs 2,3 to 2,6 times more than gas for the same heat, in all three regions. The heat pump beats gas everywhere, with a margin of only 1,9 cents in Brussels, against 3,8 cents in Wallonia, where gas is the most expensive in the country. Our comparison of gas versus electric heating runs this reasoning over a full year.

How many kWh do your appliances use in an hour?

Orders of magnitude, to be checked against the rating plate of your own appliance.

The last row always surprises people.

Does a 24 kW boiler use 24 kWh an hour?

No, and it is the most widespread confusion among households heated by gas.

The 24 kW is the maximum output the burner can deliver, sized for the coldest day of winter and often for instant hot water, which demands a lot of power for a few minutes. The rest of the time, a modern boiler modulates its flame, idles or stops as soon as the thermostat is satisfied. Take the CREG standard profile of 17 000 kWh of gas a year and spread it over a heating season of about 212 days, from October to April: the average power drawn comes to around 3,3 kW, roughly 14 % of the rated output. Put another way, those 17 000 kWh are equivalent to about 708 hours at full power over the whole year. An oversized boiler therefore does not raise the bill through its label alone, but it starts and stops more often, which wears the equipment.

This calculation is our own, it ignores domestic hot water spread over twelve months and replaces no diagnosis. Sizing a boiler is a job for a certified heating engineer, as is its periodic servicing, which is mandatory in all three regions.

And the cubic metre of gas: how do you convert it to kWh?

Your gas meter measures a volume, not energy. The supplier converts your m³ into kWh using a factor the CREG sets as a reference at 11,6 kWh per m³ of high-calorific gas, and which the CWaPE, the Walloon regulator, places between 10 and 12 kWh per m³ depending on the calorific value of the gas delivered. The 1 465 m³ in the table above therefore correspond to 17 000 kWh. The details, low-calorific gas included, are in our article on converting gas m³ to kWh.

When do you pay for kW rather than kWh?

With your supplier, never. On the grid side of the bill, it depends on the region.

In Flanders, the capacity tariff bills the average of your monthly power peaks, measured per quarter hour by the digital meter. Fluvius, the Flemish DSO (distribution system operator, the regional grid company), details which appliances cause a high quarter-hour value. The rate stood at around 56 € per kW per year in 2026, with a minimum billed peak of 2,5 kW, figures gathered on this site in early September 2026. Switching on the oven, the kettle and a portable heater together means 6,5 kW drawn in the same quarter hour.

In Wallonia, the IMPACT tariff, optional since 1 January 2026, bills not power but the time of consumption. In Brussels, the network bill stays proportional to kWh. The connection type, single-phase or three-phase, makes no difference there either, as our article on single-phase versus three-phase explains.

Five mix-ups that skew an energy calculation

• Writing "kW/h". The unit does not work that way: it is a kW multiplied by an hour, not divided.
• Taking rated power for consumption. A 24 kW boiler or a 2 500 W oven almost never runs flat out for a whole hour.
• Comparing an "energy only" price with an all-in price. The first ranged from 10,6 to 26,2 c€ for electricity in the offers listed by Callmepower in October 2026, the second is above 32 c€.
• Comparing a kWh of gas and a kWh of electricity without efficiency. The heat produced is not the same depending on the appliance.
• Forgetting that gas is read in m³. Before any cost calculation, you have to go through the conversion factor on your bill.

Once you know your annual consumption in kWh, compare offers on total annual cost in the energy supplier ranking, and place your own price within our tracking of the electricity price per kWh in Belgium.

Sources. CREG, average commercial electricity rates for Q4 2026 by region, reported by Securex on 4 September 2026 (accessed 12 October 2026); Selectra, all-in average prices per kWh for October 2026, updated 16 September 2026, profile of 3 500 kWh of electricity and 17 000 kWh of gas (accessed 12 October 2026); Callmepower, range of energy-component prices in October 2026, tables dated 11 October 2026 (accessed 12 October 2026); CREG, "Conversion du m³ de gaz en kWh"; CWaPE, "À combien de kWh correspond 1 m³ de gaz ?"; Fluvius, page on high quarter-hour values under the capacity tariff. Appliance power ratings: common orders of magnitude, to be checked on the rating plate. Costs per appliance, price per kWh of heat and average boiler power: our own calculations, with assumptions stated in the article. Variable offers are re-indexed monthly and the CREG rate changes every quarter.