Short answer: A modern inverter split system uses roughly a third to a fifth of its cooling or heating output in electricity. A 2.5 kW unit cooling a bedroom often draws around 0.5–0.8 kW at full output and much less once the room is at temperature, so running costs are usually pence per hour rather than pounds.
The simple formula
Cost per hour = electrical input (kW) × your unit price (pence per kWh). The electrical input is the rated output divided by the efficiency (EER for cooling, COP for heating). A 3.5 kW unit with an EER of 3.5 uses about 1 kW at full output. Inverter units then slow down once the room is at temperature, so average use is lower.
A worked example
Take a 3.5 kW living-room unit with an EER of 3.5, running at an average of 50% output for 6 hours on a hot day. That is about 0.5 kW × 6 h = 3 kWh. Multiply by your tariff to get the day's cost. Check your bill for your exact pence-per-kWh figure, as tariffs change.
Heating with the same unit
In heating mode a heat pump typically delivers three or more units of heat per unit of electricity in mild weather, falling in very cold weather. Compared with panel heaters, oil-filled radiators or storage heaters, which deliver one unit of heat per unit, that is a large saving.
How to keep costs down
Set a sensible temperature (around 24–25 °C for cooling is comfortable for most people), close curtains on sunny windows, keep doors shut, clean filters regularly, and let the inverter run steadily rather than switching on and off. Servicing keeps coils clean, which keeps efficiency up.
Related services
Sources
More guides
- How much does air conditioning cost to install in the UK?
- What size air conditioner do I need?
- Fixed split or portable air conditioning?
- Do you need planning permission for air conditioning?
- The £2,500 grant for air-to-air heat pumps explained
- F-gas rules: what homeowners need to know
- R32, R410A and R22: refrigerants explained
- Is air conditioning the same as a heat pump?
- What happens on air conditioning installation day
- Air conditioning noise and your neighbours
- Looking after your air conditioning: an owner's checklist
- Can you fit air conditioning in a listed building?
- Air conditioning in a flat: leases, freeholders and planning
- Keeping a home cool in a heatwave
- Multi-split or separate single systems?
Reviewed October 2026. Rules and prices change; check the date and ask us if anything is unclear.