Your air conditioner is also a heater, and this is the arithmetic

Six of the nine air conditioners in our unit database are reversible. They heat as well as cool, and most of their owners bought them in a July heatwave without noticing.

There is a reasonable question in that, now that the heating is going on. A reversible air conditioner in heating mode is an air source heat pump. It does not turn electricity into heat; it moves heat from outside to inside, which is why it can deliver more heat than the electricity it consumes.

So: is it cheaper than the boiler? The answer is arithmetic, and it turns on one number.

The two prices everything hangs on

Ofgem’s price cap for 1 October to 31 December 2026, direct debit, averaged across regions:

Electricity costs 3.3 times what gas costs, per kilowatt hour of energy bought. That gap is the thing a heat pump has to overcome, and the reason a plain electric heater is never the cheaper option.

What a boiler actually delivers

A gas boiler does not turn every kWh of gas into a kWh of heat in your room. A condensing boiler in good order runs somewhere around 90%, and in a system that never gets cold enough at the return to condense properly, rather less.

Divide the gas rate by that efficiency and you get the real cost of heat.

Those are the numbers an electric unit has to beat. Note how narrow the range is: the difference between a poor boiler and an excellent one is about a penny and a half per kWh of heat.

The break-even number

Now the other side. A heat pump’s efficiency is expressed as a coefficient of performance: 3 means three kWh of heat for every one kWh of electricity.

Divide the electricity rate by the coefficient and you get the cost of heat.

Put the two tables next to each other and the whole question collapses into one row.

Against a boiler running at 90%, the break-even coefficient of performance is 2.97. Below roughly 3, the air conditioner costs more to run than the boiler. Above it, less.

For other boiler efficiencies the break-even moves, but not far:

Everything between 2.6 and 3.2, which is a narrower window than most of this debate suggests.

What we cannot tell you

Here is where this page stops, and it stops on purpose.

None of the six reversible units in our database publishes a seasonal heating efficiency figure that we have verified with the manufacturer. Not one. So we are not going to tell you whether your unit clears 2.97, because we do not know, and a number invented to complete a table is worse than a gap in one.

What we can tell you is where to look. The figure is usually printed on the energy label or in the technical specification as SCOP, for seasonal coefficient of performance, in heating mode. It is a different number from the cooling one, and a unit can be good at one and ordinary at the other.

If you find it, the comparison above takes ten seconds.

Three things that move the answer against the unit

Cold makes it worse, exactly when you need it. The unit is extracting heat from outside air. The colder that air, the less there is to extract and the harder the compressor works. Efficiency falls as the outdoor temperature falls, so the break-even is hardest to clear on a frosty night in January and easiest to clear on a mild evening in October.

It heats one room. This is the honest advantage and the honest limit. If you want one room warm for three hours in the evening, a unit in that room against a boiler heating the whole house is not a close contest, and the comparison above understates the case for the unit. If you want the house warm, the unit is not in the competition at all.

A portable is not a split. A portable unit with a hose is working against the air it pushes out of the window, in heating mode as in cooling. The three Mitsubishi units in our database are fixed split systems with an outdoor unit; the reversible portables are not the same machine, whatever the brochure implies. The same distinction we drew about what a portable air conditioner costs to run applies here.

Working out your own number

Two steps, both quick.

Find your own electricity and gas unit rates. They are on your bill in pence per kilowatt hour, and if you are not on a standard variable tariff they will not match the capped figures above. Redo the two tables with your numbers; the arithmetic is a division.

Then use the running cost calculator for the electricity side. It takes the rated power and your rate and gives you a cost per hour, which is the input to everything above.

If the heating question has turned into a whether-to-buy question, what the October 2026 price cap changes covers the tariff side, and when to put the heating on covers the part that has nothing to do with equipment.

The short version

Electricity costs 3.3 times what gas costs. A boiler at 90% delivers heat at 8.86p per kWh. An air conditioner in heating mode needs a coefficient of performance of about 3 to match that.

Six of our nine units can do the job at all. Whether yours clears the bar depends on a figure none of their manufacturers has published in a form we can verify, and we are not going to make one up to finish the sentence.