Heat Pump vs Electric Heater: The Maths Might Surprise You

Every autumn, the same thing happens. The temperature drops, the heating debate starts, and someone drags a plug-in oil radiator out of the cupboard. It's familiar, it's simple, and it's costing you a fortune.

If you've already got a split-system air conditioner - or you're thinking about getting one - you might not realise it's also one of the most efficient heaters you can buy. Not just "quite good." Dramatically, mathematically, can't-argue-with-the-numbers better than any resistive electric heater.

Let me show you why.

The physics in 30 seconds

A plug-in electric heater - fan heater, oil radiator, convector, whatever - converts electricity into heat at a ratio of 1:1. Stick in 1 kW of electricity, get 1 kW of heat. That's the best it can ever do. It's not inefficient exactly; it's just that turning electricity straight into heat is an inherently expensive way to stay warm.

An air-to-air heat pump (which is what your split-system air conditioner is when you flip it to heating mode) doesn't generate heat. It moves it. Even when it's 2°C outside, there's thermal energy in the air. The heat pump extracts that energy and dumps it inside your house. The electricity powers the compressor and fans, but the actual heat comes from outdoors.

This is why heat pumps have a COP - Coefficient of Performance - greater than 1. A COP of 3.5 means for every 1 kW of electricity you put in, you get 3.5 kW of heat out. You're not breaking the laws of thermodynamics; you're just being clever about where the heat comes from.

Let's do the maths

Say you want to heat a bedroom for 4 hours on a cold evening.

Plug-in oil radiator (2kW): - Power consumption: 2 kW - Duration: 4 hours - Electricity used: 8 kWh - At 24.5p/kWh (Ofgem cap, October 2024): £1.96

Split-system heat pump (2kW heat output, COP 3.5): - Heat delivered: 2 kW - Electricity consumed: 2 ÷ 3.5 = 0.57 kW - Duration: 4 hours - Electricity used: 2.28 kWh - At 24.5p/kWh: £0.56

That's the same room, the same warmth, and the heat pump costs less than a third. Over a winter - say October through March, running 4 hours a day - the oil radiator costs roughly £356. The heat pump: £102. You're saving over £250 a year heating one room.

Scale that to two or three rooms with a multi-split, and the savings pay for the system within a couple of winters.

But what about really cold days?

Fair question. Heat pump efficiency drops as the outdoor temperature falls. At 7°C outside, your COP might be 4.0 or higher. At 0°C, it drops to maybe 2.5. At -5°C, perhaps 2.0.

But here's the thing: even at a COP of 2.0 - the worst-case scenario on the coldest night of a British winter - the heat pump is still twice as efficient as the oil radiator. The radiator is stuck at 1.0 no matter what. You can't beat "moves heat" with "makes heat" unless the outdoor temperature drops to about -15°C, which happens in the UK roughly never.

Modern inverter units are rated to operate down to -15°C or even -20°C. They slow down a bit, but they keep working.

Why don't more people know this?

Partly because we think of air conditioning as a cooling product. The heating function feels like a bonus feature, like the torch on your phone. But for the millions of UK homes that heat rooms with electric radiators - either because they're off the gas grid, or because they rent, or because they just need a top-up in one room - it's the main event.

There's also a cultural thing. We're suspicious of anything that seems too good to be true. "You mean I can heat my room for a third of the price AND cool it in summer? Pull the other one." But it's not magic. It's thermodynamics. The same principle that makes your fridge work.

What about gas central heating?

Gas is still cheaper per kWh than electricity (roughly 6p vs 24.5p at current rates). But a gas boiler only converts fuel to heat at about 90% efficiency, while a heat pump delivers 3–4x the heat per unit of electricity. When you do the maths:

• Gas boiler: 6p ÷ 0.9 efficiency = 6.7p per kWh of heat
• Heat pump: 24.5p ÷ 3.5 COP = 7p per kWh of heat

They're almost identical. And that's before you factor in the standing charge for a gas supply, annual boiler servicing, and the direction of travel on gas prices. Heat pumps are already competitive with gas for individual rooms, and the gap is closing every year.

The comfort factor

There's one more thing the oil radiator can't match: speed and precision.

A heat pump gets the room to temperature in minutes and holds it there with an inverter compressor that ramps up and down smoothly. Set it to 21°C and it stays at 21°C. An oil radiator blasts out maximum heat until the room overshoots, then coasts until it gets cold again, then blasts again. It's a rollercoaster.

Heat pumps also dehumidify slightly as they heat, which makes the warmth feel more comfortable and reduces condensation on windows - a genuine problem in well-sealed UK homes in winter.

The bottom line

If you're heating any room with a plug-in electric heater, you're spending roughly three times more than you need to. A split-system air conditioner in heating mode delivers the same warmth for a fraction of the running cost, heats faster, holds temperature better, and gives you cooling in summer as a bonus.

The upfront cost is higher than a £40 oil radiator, obviously. But you'll make that back within a year or two of typical use - and then it's just savings, every winter, for the 10–15 year life of the unit.

Something to think about before you plug that radiator in again this November.