Using your air conditioning for heating: cost per hour, the right settings, and what to expect

Yes, your air conditioning can heat your home, and it is the cheapest form of electric heating you can buy. Every modern wall-mounted split sold in the UK is a reverse-cycle unit (an air-to-air heat pump), and in heating mode it delivers 3 to 5 kWh of heat for every kWh of electricity it uses. On the Ofgem price cap for 1 July to 30 September 2026 (26.11p/kWh), a 2.5kW unit running flat out costs around 14p to 16p an hour and a 3.5kW unit 20p to 23p. A 2kW fan heater costs 52p an hour for less heat.

That is the headline. The rest is the practical part: how heat mode behaves, the settings that matter, what to expect in a cold snap, and which portables genuinely heat. If you are still deciding whether to buy, start with our air-to-air heat pumps guide. This page assumes you have a unit, or are about to.

Which air conditioning units heat

Any wall split with a heating energy label or a SCOP figure on its spec sheet is a heat pump. SCOP (seasonal coefficient of performance) is the average heat output per unit of electricity over a whole heating season, and current R32 units sit between 4.0 and 5.2. Retailers list the Daikin Perfera FTXM35A at SCOP 5.1 to 5.2 with an A+++ heating label, Mitsubishi Electric's MSZ-AP25 at SCOP 4.8 (A++), and Panasonic's Etherea Z25 at SCOP 5.2 (A+++). Cooling-only splits exist but are rare in UK homes.

Portables are the exception, and it matters:

• The Midea PortaSplit is a genuine two-part heat pump. Midea quotes 3.5kW of heating, SCOP 4.0, an A+ heating label, and a heating range down to minus 10°C.

• Single-hose "heater" portables such as the MeacoCool CHR range do reverse the refrigerant cycle, but the hose has to stay out of the window in heat mode and they fade as outdoor air approaches freezing. Haier's support page says portables "tend to be somewhat ineffective in any climate where the outdoor air temperature falls near or below freezing".

• Many cheap "4-in-1" portables heat with a resistive element. If there is no SCOP and no heating energy label in the specification, assume the heat mode is a fan heater and will cost fan-heater money.

How heat mode works

An air conditioner moves heat rather than making it. In heating mode a reversing valve flips the refrigerant circuit: the outdoor coil pulls heat from the outside air (there is plenty in it at 0°C) and the indoor coil releases it into the room. The compressor's electricity only pays to move the heat, which is why one kWh in becomes three, four or five kWh out. Three things follow from that, and they catch people out in the first week.

Warm air takes a few minutes. The indoor fan waits until the coil is warm so you do not get a cold draught. Daikin says wait 1 to 4 minutes; Mitsubishi's manual says the unit "is preparing to blow out warm air. Please wait."

The unit is high on the wall and blows downwards. Warm air rises, so the indoor unit pushes it down along the wall to mix the room. Daikin's Perfera sets its flaps "downward in heating operation mode" automatically; if you fixed the vanes horizontal for summer, drop them.

It will pause to defrost. When the outdoor coil runs below freezing it frosts up, and the unit briefly reverses to melt it. Daikin's user guide is blunt: "During defrosting operation, hot air is NOT blown from the indoor unit." Expect the indoor fan to stop or slow for 3 to 12 minutes, a gurgling sound, and steam or a puddle at the outdoor unit. All normal, and more frequent in cold, damp weather.

Air conditioning heating cost per hour: the worked examples

The sum is simple. Electricity drawn = heat output divided by COP. Cost = electricity drawn multiplied by the unit rate. We use the Ofgem price cap for 1 July to 30 September 2026, direct debit, 26.11p/kWh, and assume the unit runs at full rated output for the whole hour, which is the worst case. A 5-hour evening and a 30-evening month follow from that.

Rates correct as of September 2026 at 26.11p/kWh (Ofgem cap, 1 July to 30 September 2026). October's price cap moves electricity to 26.32p/kWh, which adds a fraction of a penny an hour and changes nothing here.

Per kWh of heat delivered, a heat pump at SCOP 4.0 costs 6.5p and at SCOP 4.6 costs 5.7p, against 26.1p for any resistive heater. Fan heaters, oil-filled radiators, panel heaters and infrared panels are all 100% efficient, as the Centre for Sustainable Energy puts it, "because all the electricity used is converted into heat", and that is their ceiling. The 3.5kW split costs more per hour than the 2.5kW only because it delivers more heat: 75% more than the fan heater for less than half the money.

Per kWh of heat, a wall split at SCOP 4 costs about 6.5p. A fan heater costs 26p. Nothing you can do to a fan heater closes that gap.

Your bill will be lower than the table. An inverter unit runs hard for the first 20 to 30 minutes, then throttles back once the room is at temperature, often drawing a few hundred watts to hold it. For your own room and hours use the running cost calculator; for summer and winter bills together see are air conditioners expensive to run.

The settings that matter

Most of the efficiency is built into the unit. The rest is how you drive it, and the habits people bring from fan heaters are mostly wrong for a heat pump.

Set 19°C to 21°C and leave it. Daikin's guidance for heating is 20°C to 24°C; most UK households find 20°C or 21°C comfortable with the warm air moving. The Energy Saving Trust suggests you "try turning it down in one degree increments" until you find the lowest setting you are happy with.

Use the unit's own thermostat, not a plug-in timer. The controller reads the room and modulates the compressor. Cutting power at the plug forces a cold restart, a warm-up, and often a defrost every time. Use the schedule on the remote or app.

Do not switch it off and on constantly. Reheating a cold room from scratch is the expensive part. The Energy Saving Trust: "Leaving the heat pump on for longer periods can help too. This doesn't increase your overall running costs because the heat pump can run more efficiently, offsetting the longer running period." For a room used morning and evening, set back to 16°C or 17°C in between rather than off. For a room used once a day, switching off is fine; give it 20 to 30 minutes' notice.

Fan on auto, not max. Auto lets the unit push hard while the room warms and drop to a whisper once it is there. Locked on high you get noise and a draught for no more heat; locked on low the coil runs hotter than it needs to and efficiency slips.

Vanes pointing down. Warm air has to reach the floor. Auto or comfort airflow does this on most units; if you set a fixed angle, use the lowest position.

Eco and quiet modes are fine at night. They cap the compressor and fan, costing a little peak output you rarely need once the room is warm. Powerful or heat boost modes are for the first ten minutes on a cold morning, not all evening.

Close the door. A 2.5kW split is sized for one room, and heating the hall through an open door is where most "it never gets warm" complaints come from. There is more on schedules, set-backs and tariffs in the most efficient way to run an air-to-air heat pump.

What happens when it gets cold

Three things change as the outdoor temperature drops, and none of them makes the unit not worth running.

Output falls. The nominal heating figure is measured at 7°C outside. Panasonic quotes the Etherea Z25 at 3.4kW nominal but 2.8kW at minus 7°C; Mitsubishi lists the MSZ-AP25 at 3.2kW nominal and 2.62kW under its UK test condition. A unit sized tightly for the room can feel short of puff on the coldest few nights of the year.

Efficiency falls with it. SCOP is a seasonal average. At 7°C the units above run at a COP of 4.1 to 4.9; around 0°C something nearer 3 is realistic, and in a hard frost with regular defrosts perhaps 2.5. Redo the sum for a 2.5kW unit and you get roughly 22p an hour at COP 3 and 26p at COP 2.5, still half the fan heater's cost for the same heat. The heat pump on a bad day beats the resistive heater on its best day.

Defrost gets more frequent. Damp British cold at 0°C to 5°C is the worst case for frosting, worse than dry cold at minus 10°C. Expect a pause every hour or so in that weather. Do not stand a fan heater in front of the unit to help: the thermostat reads the warm air and backs off.

Every current unit keeps heating well below anything the UK normally throws at it: Panasonic rates the Etherea to minus 15°C and Daikin the Perfera pair to minus 20°C (one retailer lists minus 21°C), with Daikin's multi-split outdoor units at minus 15°C. The Energy Saving Trust's summary is that heat pumps "can perform down to -15°C or lower, but there may come a point when the heat pump's output isn't enough for you". If that happens, it is a sizing problem. The numbers are in do air-to-air heat pumps work in cold weather.

Noise, dry air and filters

Noise at night. On the lowest fan speed the indoor unit is far quieter than a fan heater: Daikin quotes the Perfera at 19 to 20 dB(A) on silent and Mitsubishi the MSZ-AP25 at 19 dB(A) on its lowest heating speed, rising to 39 to 45 dB(A) flat out. What people notice is the change in sound at a defrost (fan stopping, a gurgle, a click from the reversing valve) rather than the running noise; night mode deals with most of it. The outdoor unit is louder in heating than cooling, around 48 dB(A) for the MSZ-AP25, worth knowing if it sits under a bedroom window.

Dry air. In heating mode the indoor coil is warm, so it does not pull moisture out of the air as it does in summer; warming air simply lowers its relative humidity, as any heater does. Dry eyes or static get the same fix as with radiators: a humidifier or a slightly lower set point.

Filters. Dust cuts output. Mitsubishi's manual says clean the filter every two weeks in use; Daikin warns that dirty filters mean "poor heating/cooling". It is a two-minute job with a vacuum. Keep the outdoor unit clear of leaves and make sure defrost water can drain rather than freezing on a path.

Is it cheaper than what you have now?

Against any direct electric heating, yes, by a factor of three to four; if you are on storage or panel heaters, a wall split in the living room is usually the biggest single cut you can make to a winter bill. Against a gas boiler it is close: at 26.11p electricity and SCOP 4.0 you pay about 6.5p per kWh of heat, and gas at 7.33p/kWh through a 90% efficient boiler is about 8.1p. A small win for the heat pump in the room it serves, but a split heats one room and does not do hot water, so most gas households run it in the room they live in and let the boiler do the rest.

Installed prices, the £2,500 air-to-air grant (real, but barely paid out yet), tariffs and sizing are covered in our heating guides, and the rest of the winter care advice sits in running and maintenance.

Prices and rates correct as of September 2026. The energy price cap changes quarterly, so check current rates before budgeting.