Guide · SEAI prices, 1 July 2026
Combi boiler running costs
A combi costs what the gas costs. SEAI's unit prices, what an hour, a shower and a year come to, and why the house matters more than the boiler.
A gas combi costs what the gas costs. On SEAI's price comparison for 1 July 2026, natural gas was 13.0 cent per kWh including VAT for a typical household band, so a 24 kW combi running flat out for an hour burns about €3.12 of gas, and a 30 kW one about €3.90. In practice a combi modulates and cycles, so an hour of heating costs less than that.
Over a year, the CRU's average gas consumption of 11,000 kWh comes to about €1,430 at that unit rate, before standing charges. The boiler's efficiency, the controls and the house decide where you land against that average.
The unit prices behind every running cost
SEAI publishes a quarterly comparison of what each fuel costs per kilowatt-hour delivered, including VAT. These are the 1 July 2026 figures (SEAI Domestic Fuels comparison, read 29 October 2026):
| Fuel | Band | Delivered cost, cent per kWh | Useful heat at 90% efficiency |
|---|---|---|---|
| Natural gas | D1: under 5,556 kWh a year | 16.41c | 18.24c |
| Natural gas | D2: 5,556 to 55,556 kWh a year | 13.0c | 14.45c |
| Electricity | DD: 5,000 to 15,000 kWh a year | 34.38c | 38.2c |
| Electricity | Night rate | 15.42c | – |
Source: SEAI, Domestic Fuels: Comparison of Energy Costs, 1 July 2026. Gas and electricity prices are VAT-inclusive prices from the EU price survey for July to December 2025, updated every six months, and exclude standing charges. The 90% column is SEAI's useful-energy cost at that seasonal efficiency; SEAI gives a condensing boiler a typical seasonal efficiency of 85–97%.
The same SEAI comparison prices kerosene. For an oil-heated house weighing a switch to a gas combi, our sister site sets the two side by side: oil boiler replacement in Dublin.
Cost per hour
A combi's output in kW is the most gas it can burn in an hour, converted to heat. Multiply by the unit price for the cost of an hour at full output:
| Combi output | Gas per hour at full output | Cost per hour at 13.0c (band D2) | Cost per hour at 16.41c (band D1) |
|---|---|---|---|
| 18 kW | about 18 kWh | €2.34 | €2.95 |
| 24 kW | about 24 kWh | €3.12 | €3.94 |
| 30 kW | about 30 kWh | €3.90 | €4.92 |
| 35 kW | about 35 kWh | €4.55 | €5.74 |
Two corrections make the real figure lower. A modern combi modulates, dropping its output to what the house needs once the radiators are warm, so a 30 kW boiler rarely runs at 30 kW for long. And the heating side is thermostatically controlled: once the rooms are at temperature the boiler cycles on and off. An hour of heating in a well-insulated house might burn a third of the full-output figure. The hot water side is different: while a shower runs, the combi burns close to full output to heat the water on the way through.
Cost per shower, cost per bath
Heating water takes about 1.16 kWh for every 1,000 litres raised by 1 °C. Warming 10 litres a minute by 35 °C, the rise Vaillant quotes its flow rates at, needs about 24 kW while the tap runs. A ten-minute shower at that rate is about 4 kWh of heat: €0.58 of gas at 90% efficiency and 13.0 cent per kWh. A 100-litre bath at the same rise is about 4 kWh too. A combi has no cylinder losing heat overnight, which is its one standing advantage over a stored system.
Cost per year
The CRU's average annual consumption, used by every Irish supplier for the Estimated Annual Bill, is 11,000 kWh of gas and 4,200 kWh of electricity (Electric Ireland, citing the CRU, read 29 October 2026). At 13.0 cent that's about €1,430 of gas a year before standing charges, heating and hot water together, for the average gas home. A small apartment on a combi can be well under half of that; a big draughty house can be double.
What moves your figure, in order: how well the house holds its heat, the thermostat setting, how many rooms are heated and for how long, the boiler's efficiency, and the hot water you use. The boiler is fourth on that list. A new condensing combi at 85–97% against an old non-condensing boiler is a real saving; a new combi against a 10-year-old condensing one is a small one.
Does a combi cost more to run than a system boiler?
No, on the heating side: the same gas heats the same radiators. On hot water, a combi avoids the standing loss from a cylinder, but a cylinder heats water on cheaper night-rate electricity if it has an immersion and a night meter, and it can take solar water heating. For most households the difference is small either way. The choice is about flow and fit, not running cost: see combi vs system boiler.
Gas combi or electric combi to run?
This one isn't close. At 34.38 cent per kWh for electricity on the typical band against 13.0 cent for gas, an electric combi costs roughly two and a half times as much per unit of heat, and SEAI's efficiency figures don't close the gap: 100% for electric resistance heating against 85–97% for a condensing gas boiler. Night rate at 15.42 cent only helps a stored system. See electric combi boilers.
Cutting the running cost of a combi
- Zone the heating and put a thermostat on each zone: the shape of SEAI's €700 heating controls grant, which requires a second zone on a combi
- Set the boiler's flow temperature low enough to condense: a condensing boiler only reaches the top of its 85–97% range when the return water is cool
- Fit thermostatic radiator valves in rooms without the thermostat
- Service it every year: a dirty burner wastes gas
- Insulate before you upsize: the house, not the boiler, sets the bill
Questions people ask
How much does a combi boiler cost to run per hour?
At full output, about the boiler's kW rating in kWh of gas: a 24 kW combi burns about €3.12 an hour at 13.0 cent per kWh, a 30 kW one about €3.90. Once the house is warm it modulates and cycles, so a real hour costs less.
How much does gas cost per kWh in Ireland?
On SEAI's 1 July 2026 comparison, 16.41 cent per kWh on the smallest domestic band and 13.0 cent on the next band, including VAT and before standing charges.
What does the average house spend on gas a year?
The CRU's average consumption is 11,000 kWh a year. At 13.0 cent that's about €1,430 before standing charges, for heating and hot water together.
Does a combi boiler cost more to run than a system boiler?
Not on heating. On hot water a combi has no cylinder losses, while a cylinder can use night-rate electricity or solar. For most homes the difference is small.
How much does a shower cost on a combi?
About 4 kWh of heat for a ten-minute shower at 10 litres a minute and a 35 °C rise: roughly €0.58 of gas at 90% efficiency and 13.0 cent per kWh.
Is an electric combi cheaper to run?
No. Electricity on the typical band was 34.38 cent per kWh against 13.0 cent for gas on SEAI's July 2026 comparison, roughly two and a half times the cost per unit of heat.
Sources (1)
Prices and facts on this page were checked against these published pages. We are not connected to these businesses.
- Electric Ireland, citing the CRU: electricireland.ie
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