Overnight cost is kilowatt-hours times the unit rate on your bill. The cooling kilowatt on the brochure is not the electrical kilowatt. A bedroom split that is holding temperature, not freezing a glass room from scratch, might draw well under 1 kW electrical for long stretches, so a night of about eight hours can land around a handful of kilowatt-hours. At a domestic unit rate in the mid twenties to mid thirties pence, that illustration is often low pounds, not a shocking sum. Your tariff, the setpoint, the glass and a portable heater-like unit change it. Survey and a clean filter matter more than the badge.
A bedroom held from about eleven until seven
Read the meter or the in-home display if you want your own number. The night you care about is the one where the room is already shut and the unit is holding, not starting from a heatwave panic at midnight.
A handful of units times your pence rate
As an illustration only: if the outdoor and indoor together draw about 0.6 to 0.9 kW electrical while they hold a normal bedroom, eight hours is roughly 5 to 7 kWh. Multiply by the pence per kWh you actually pay. Standing charge is on the bill anyway. It is not caused by the night's cooling. If your rate is nearer the mid twenties pence, that handful of units is around a pound or two. If you are on a high peak rate, it is more. There is no national air conditioning tariff to look up.
- Cooling kW is heat moved. Electrical kW is what you pay for.
- A holding load is lower than a flat-out start in a heatwave.
- Multiply kWh by the rate on your bill.
- A dirty filter and a lower setpoint raise the hours at full draw.
Glass, a late start and a unit that never catches up
A loft bedroom, a wide west window and a setpoint in the high teens will run the compressor harder than a shaded room at a moderate setpoint. Starting at midnight in a room that is already 30 degrees is a long pull. Shading by day and cooling a little earlier can use fewer units than a panic blast at bedtime. An undersized unit that never catches up draws its maximum for the whole night and still fails you. An oversized unit that short-cycles is noisier and less gentle, though the units may not be huge. The survey's kilowatt choice is a running-cost decision. Energy Saving Trust's air-to-air heat pump guidance is the right frame: these are heat pumps, and efficiency depends on conditions.
A time-of-use rate that helps the night
A time-of-use tariff that is cheap at night helps a bedroom and hurts a late-afternoon living room. There is still no national air conditioning tariff to look up. Multiply whatever the meter shows by the pence on your own bill.
Stop treating a July night as a January figure
In heating mode the same machine can use more electricity in a cold snap than it does cooling a mild summer night, because the load is larger. Do not take a July illustration and reuse it for January. Ask for efficiency figures, then still expect weather to win. Timers that stop the unit when you leave for work matter more than chasing a slightly higher brochure efficiency between two good brands. Hours, setpoint, shade and a clean coil usually move the bill more than the badge between current mainstream splits. The installer who sets that schedule, and who fitted a legal outdoor unit with clean airflow, has done more for the bill than the logo. GOV.UK F-gas certification is what keeps the refrigerant side of that install lawful.
Frequently asked questions
Is it a few pounds a night or tens of pounds?
For a normal bedroom split holding a moderate setpoint, a 2026 illustration is often a handful of kWh, which is low pounds at common domestic unit rates. A glass room run flat out, or a high tariff, costs more. Check your rate.
Does 2.5 kW cooling mean 2.5 kW on the meter?
No. That is cooling capacity. Electrical input is lower when the system is working efficiently, often a fraction of the cooling figure, and it rises in extreme heat.
Core buying guides
Start with these high-intent guides, then dig into the rest of the library.
Related guides
Sources and further reading
How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.