The short answer
Which costs less depends on your prices and equipment. Work out the cost of a kWh of heat from each: gas unit rate divided by boiler efficiency, and electricity unit rate divided by the air conditioner's COP.
On this page
In the UK a kWh of electricity has typically cost several times as much as a kWh of gas, but an air conditioner delivers several kWh of heat for each kWh of electricity it uses. The two effects pull in opposite directions, so the result depends on the figures you put in, and small changes to them can tip it either way.
Working out the cost of a kWh of heat
The fair comparison is the cost of heat delivered into the room, not the price of the fuel.
- Gas boiler: gas unit rate ÷ boiler efficiency, with the efficiency written as a decimal (90% is 0.9).
- Air conditioner: electricity unit rate ÷ COP.
Both unit rates are on your bills, in pence per kWh. The other two figures need more care.
Boiler efficiency. Modern condensing boilers are typically rated at around 90%. They work best when the water returning to the boiler is cool enough, below about 55°C, for water vapour in the exhaust gases to condense, so a boiler set up to run hot often falls short of its rating. Older non-condensing boilers are less efficient.
COP. This changes with the weather and falls as it gets colder. For a whole winter, the SCOP on the energy label is a starting point, though your weather, settings and installation will move the real figure. Some manufacturers' technical data also gives COP figures at colder temperatures. COP and EER explains the difference between these figures.
The same method works for an oil or LPG boiler, once you've converted what you pay for the fuel into a price per kWh.
A worked example
Every figure here is an example chosen to show the method. Check your own tariffs and equipment before drawing any conclusion.
- Electricity at an example rate of 25p per kWh
- Gas at an example rate of 6p per kWh
- A boiler at 90% efficiency, or 80% for one that is older or runs hot
Heat from gas costs 6p ÷ 0.9 = 6.7p per kWh with the 90% boiler, and 6p ÷ 0.8 = 7.5p per kWh with the 80% one.
Heat from the air conditioner, at the example electricity rate:
| COP | Electricity per kWh of heat | Cost per kWh of heat |
|---|---|---|
| 2.5 | 0.4 kWh | 10p |
| 3 | 0.33 kWh | 8.3p |
| 3.5 | 0.29 kWh | 7.1p |
| 4 | 0.25 kWh | 6.25p |
| 4.5 | 0.22 kWh | 5.6p |
With these figures, the two cost the same per kWh of heat when the air conditioner's COP is about 3.75 against the 90% boiler, or about 3.3 against the 80% one. In this example, above that COP the heat from the air conditioner costs less per kWh, and below it the heat from the boiler does.
The shortcut is: break-even COP = (electricity rate ÷ gas rate) × boiler efficiency. Change any input and the answer moves. At an example electricity rate of 20p per kWh, perhaps in the cheaper hours of a time-of-use tariff, the break-even COP against the 90% boiler falls to 3; at 30p per kWh it rises to 4.5.
What the sums leave out
- Cold snaps. COP is lowest on the coldest days, when you need the most heat, and the unit's heating output drops too. Defrosting in cold, damp weather also uses energy. A COP measured in mild weather overstates what you'll get in a cold spell.
- Standing charges. If you keep gas for hot water or the rest of the house, its standing charge doesn't change, so leave it out. It only comes into the comparison if you'd stop using gas altogether.
- What each system heats. An air-to-air system heats the rooms with indoor units in them. It doesn't heat your hot water, and the gas bill only changes if the boiler does less, for example because you turn down the radiator in that room.
- How you heat. Heating one room for the hours you use it needs less heat than heating the whole house on a timer. Thermostatic radiator valves can do a similar job on a gas system, so compare like with like.
- Carbon. This is a separate question from cost, covered in air conditioning and carbon emissions.
Other differences
| Point | Gas central heating | Air-to-air heat pump |
|---|---|---|
| Heats | The whole house, and usually hot water | The rooms with indoor units |
| Warmth comes from | Radiators | Warm air from the indoor unit |
| Efficiency | Below 100% | A COP above 1, lower in the cold |
| Cooling in summer | No | Yes |
A wall unit can warm the air in a room fairly quickly, while radiators take a while to heat up from cold. Some people notice the air movement or the fan, and because warm air rises, the louvres need angling downwards when heating. Using air conditioning to heat in winter has practical tips.
Some homes use both, with air conditioning in the rooms that need cooling in summer and gas heating the rest. Running the numbers above for the rooms in question shows which source of heat costs less at your prices.
Related guides
- COP and EER: what efficiency meansCOP (heating) and EER (cooling) compare the heat a unit moves with the electricity it uses, under one set of test conditions. A COP of 3 means 3 kW of heat for every 1 kW of electricity; higher is more efficient.
- How air conditioning heats a roomTo heat, the system runs its refrigerant loop in reverse. The outdoor coil turns cold and collects heat from outside air, even below freezing, while the indoor coil turns hot and warms the air blown into the room.
- Using air conditioning to heat in winterChoose heat mode, point the airflow down and give the room time to warm. Most systems keep heating well below freezing, though less efficiently, and pause now and then to defrost the outdoor unit, which is normal.
- How much does air conditioning cost to run?Multiply the electricity it uses, in kWh, by your unit rate. How much a unit uses depends on the room, the weather, the temperature you set and its efficiency, so estimate it from its average power draw and the hours it runs.