A modern A++ inverter split rated at 2.5kW cooling costs roughly £0.10–£0.20 an hour to run at the current Ofgem unit rate. Across a typical UK summer (6 hours/day × 90 days), that's £55–£110 per indoor unit.
Cost per hour, day and year
At 27p/kWh, a 2.5kW cooling unit drawing 0.6kW under partial load costs about 16p/hour. Six hours a day across summer is £85. Heat-mode use in shoulder seasons adds another £40–£80 per year.
What drives running cost
SEER rating (higher = cheaper to run), correct sizing, thermostat setpoint (every 1°C lower costs ~8% more), insulation quality, and use of timers vs leaving on all day.
How to cut running cost
Set cooling to 24°C not 21°C, use night and economy modes, close blinds in direct sun, and service the unit annually to keep efficiency from drifting.
See what installers in your area would actually charge.
Get matched quotesFrequently asked questions
Is AC cheaper than a portable unit?+
Yes; fixed splits are 2–3x more efficient than portable AC and quieter too. A portable can cost £200+ per summer to run vs £55–£110 for a split.
Should I leave AC on all day?+
For a well-insulated home, leaving it on at a higher setpoint (e.g. 26°C) often costs less than cooling from hot every evening. For leaky homes, scheduled use wins.