If you're trying to keep one room warm without heating the whole house — a home office, a bedroom, an extension the radiators never quite reach — the cheapest answer is probably a machine most people file under "summer". A split air conditioning unit is a reversible air-to-air heat pump. Flick it to heating mode and it becomes, per unit of heat, one of the cheapest ways to warm a single room in a UK home. Here's the maths, honestly done, including where it loses.
Why "100% efficient" is the trap
Plug-in electric heaters — fan heaters, oil-filled radiators, panel heaters, those expensive "eco" ones — are all marketed as 100% efficient. That's true, and it's also the ceiling. One kilowatt-hour of electricity in, one kilowatt-hour of heat out. At the sort of electricity prices most UK households are on as we write — somewhere in the mid-to-high 20s of pence per kWh, depending on your tariff — that's 25p or so for every unit of heat, whatever the badge on the box says.
A heat pump doesn't make heat from electricity — it moves heat from the outside air into the room, using electricity to run the compressor. That's how it breaks the 100% ceiling. A modern split unit has a seasonal efficiency (SCOP) of roughly 3.5 to 5, meaning each kilowatt-hour of electricity delivers around 3.5–5 kilowatt-hours of heat. Same electricity price, roughly a fifth to a third of the running cost.
The honest maths for one room
Say your home office needs about 2kW of heat, five hours a day. That's 10kWh of heat per day. Taking 26p/kWh for electricity as a worked example (your tariff will differ — check it), here's roughly what that heat costs by method:
| Heating method | Efficiency | Cost per day (10kWh heat) | Per winter month |
|---|---|---|---|
| Plug-in electric heater | 1.0 | ~£2.60 | ~£78 |
| Gas central heating (one room's share) | ~0.85–0.9 boiler | ~70–85p | ~£21–26 |
| Air-to-air heat pump (split AC) | SCOP ~3.5–5 | ~50–75p | ~£15–22 |
Two honest caveats. The gas figure assumes gas at a handful of pence per kWh — roughly a quarter the price of electricity, which is broadly where UK tariffs have sat for a while — and a decent condensing boiler. Your numbers will vary, so run them with your own tariff. And SCOP is a seasonal average: on a mild autumn day the unit runs nearer the top of its range; on a freezing January morning, nearer the bottom. We've used the range rather than the flattering end.
The comparison against plug-in heaters, though, isn't close. If you're heating a room with a fan heater or panel heater every day through winter, a split unit typically cuts that specific bill by roughly three-quarters. Over a four-month winter, that can be a couple of hundred pounds on one room alone.
When gas central heating still wins
Here's the part sales pages skip. Per unit of heat, mains gas is cheap — cheap enough that a heat pump at SCOP 3.5–5 only roughly matches or slightly beats it on running cost. So:
- Heating the whole house? Gas central heating usually stays the sensible default. Replacing it room-by-room with split units rarely stacks up on cost alone.
- Heating one room while the boiler heats everything? This is where gas quietly loses. To warm your office with radiators, the boiler typically fires up the whole system. A split unit heats just the room you're in, and nothing else.
- No mains gas at all? If you're on electric-only heating, an air-to-air unit is hard to argue against — you're cutting the cost of every unit of heat by roughly two-thirds or more from day one.
If you're weighing a whole-house solution, the comparison you actually want is air-to-air versus air-to-water heat pumps — they solve different problems.
What it costs to install — and the grant catch
As a rough guide, a single-room split unit tends to land somewhere in the £1,500–£3,000 range installed, depending on the unit, the wall run and who's doing the work — our guide to air conditioning costs breaks that down properly. One thing to know upfront: although a split AC unit genuinely is a heat pump, it's an air-to-air heat pump, and the government's Boiler Upgrade Scheme grant only covers air-to-water systems (the kind that feed radiators and hot water). So there's no grant here. The economics have to stand on their own — which, for a single hard-to-heat room, they generally do.
The both-seasons argument
This is what tips it for most people. The unit that heats your office in January is the same unit that cools it in August — and it's the only appliance in a UK home that does both well. Judged as a heater alone, it's a considered purchase. Judged as a heater and the air conditioning you were half-thinking about anyway, the sums look very different: one install, two jobs done.
And if you have solar panels, or you're considering them, the numbers improve again: surplus generation on bright winter days offsets some of the heating, and a battery lets summer's midday sun run the cooling in the evening. More on that in will solar run my air conditioning?
The bottom line
For one room: air-to-air beats plug-in heaters by a mile, and typically edges gas — especially when the alternative is firing up the whole central heating system for a single office. For the whole house on mains gas: keep the boiler, at least for now. And whatever a brochure says, the honest number is roughly 3.5–5 units of heat per unit of electricity — not a miracle, just good engineering.
If you'd like a proper quote for a split unit — sized for your room, installed by someone we've actually vetted — tell us about your home. We match you with one installer per trade, not a list of five who'll all ring at once.