Running an air conditioning system in a UK home typically costs between 8p and 30p per hour per room, depending on how hard the unit is working. That range is wide because a modern inverter system does not run flat out — it surges to pull the room down to temperature, then throttles back to a trickle to hold it. Understanding that behaviour is the difference between a system that costs pennies a night and one that quietly eats electricity.
Key takeaways
- Cost per hour = unit power draw (kW) × your electricity rate (p/kWh).
- Inverter systems use most of their energy in the first 30–60 minutes.
- Holding a temperature is far cheaper than repeatedly cooling from scratch.
- The same unit in heating mode usually beats electric heaters by a factor of three or more.
- Blinds, shading and door discipline change running cost more than any setting on the remote.
The only formula you need
Cost per hour equals the electrical power the unit is drawing, in kilowatts, multiplied by what you pay per kilowatt-hour. The trap is that people use the cooling capacity instead of the power input. A '3.5kW' unit does not consume 3.5kW — that is the heat it moves. Its actual electrical input at full load is usually around 1kW, because a heat pump moves heat rather than creating it.
Find the input figure on the data plate or the manufacturer's spec sheet, listed as 'power input' or 'rated input'. You will normally see a minimum, rated and maximum figure. The minimum is what matters most, because that is where a well-sized system spends the majority of its time.
| Unit size (cooling) | Typical input at full load | Typical input when holding | Cost/hr holding @26p |
|---|---|---|---|
| 2.5 kW | 0.6 – 0.8 kW | 0.15 – 0.3 kW | 4p – 8p |
| 3.5 kW | 0.9 – 1.1 kW | 0.2 – 0.4 kW | 5p – 10p |
| 5.0 kW | 1.4 – 1.6 kW | 0.3 – 0.6 kW | 8p – 16p |
| 7.1 kW | 2.0 – 2.3 kW | 0.5 – 0.9 kW | 13p – 23p |
Indicative inverter split system figures. Check your own model's data plate for exact values.
Worked example one: a bedroom overnight
A 2.5kW unit in a double bedroom, switched on at 10pm when the room is 26°C and set to 21°C. The first 30 minutes run near full load at around 0.7kW. The remaining seven and a half hours modulate down to roughly 0.2kW as the room stabilises and the outside temperature falls.
- Pull-down: 0.5 h × 0.7 kW = 0.35 kWh
- Holding: 7.5 h × 0.2 kW = 1.5 kWh
- Total: 1.85 kWh ≈ 48p per night at 26p/kWh
Across a genuinely hot fortnight, that is roughly £6.70. Most households find that comfortably acceptable for reliable sleep, especially in a west-facing bedroom that has been baking since mid-afternoon.
Worked example two: a home office
A 3.5kW unit in a converted garage office, running 8am to 5pm on a hot weekday. Garage conversions typically have weaker insulation and a large door-side thermal bridge, so the unit works harder throughout the day rather than settling to a low trickle.
- Pull-down: 1 h × 1.0 kW = 1.0 kWh
- Working day: 8 h × 0.45 kW average = 3.6 kWh
- Total: 4.6 kWh ≈ £1.20 per day
Twenty hot working days across the summer come to around £24. Set against the alternative — an unusable room from June to August — that is generally the easiest cost to justify in the house.
Worked example three: an open-plan rear extension
This is the expensive one, and the reason honest sizing matters. A 7.1kW unit serving a kitchen-diner with three metres of south-facing bifolds, cooking heat, and a family in and out of the garden all afternoon will run near full load for hours because heat keeps arriving.
- Peak afternoon: 4 h × 1.9 kW = 7.6 kWh
- Evening hold: 4 h × 0.8 kW = 3.2 kWh
- Total: 10.8 kWh ≈ £2.80 per day
External shading changes this dramatically. Fitting external blinds or a brise soleil on that glazing can cut the solar gain by more than half, which takes the unit off full load and often halves the daily figure. Internal blinds help far less, because the heat is already inside the glass by then.
Winter: the part people forget
Air conditioning in heating mode is the cheapest electric heat you can buy. A unit with a seasonal coefficient of performance of 3.8 delivers 3.8kW of heat per 1kW of electricity, so a room that needs 2kW of heat costs about 0.53kW of input — roughly 14p per hour, against about 52p per hour for a 2kW fan heater.
For a home office, a granny annexe or a chilly extension, that often means you stop firing the whole gas system to warm one room. Several Derbyshire clients use their systems more in October and March than in July.
Seven ways to cut running cost that actually work
- 1Set 23–24°C, not 18°C. Every degree lower increases run time noticeably, and the room will not cool any faster for a lower setpoint.
- 2Shade the glass externally. Solar gain is the load; blocking it before the glass is worth more than any efficiency setting.
- 3Close internal doors. Cooling a bedroom is cheap; accidentally cooling a landing and stairwell is not.
- 4Use the timer to pre-cool. Cooling a bedroom for 45 minutes before bedtime while the outside air is dropping is cheaper than a hard pull-down at midnight.
- 5Clean the filters monthly in summer. A clogged filter forces longer run times for the same result.
- 6Keep the outdoor unit clear. Grass cuttings, leaves and a fence built too close to the coil all reduce heat rejection and raise consumption.
- 7Book the annual service. Low refrigerant charge from a slow leak makes the system run constantly while never quite reaching temperature.
What about solar panels?
Air conditioning and solar PV pair unusually well, because peak cooling demand and peak generation happen at the same time of day. A household with a 4kW array will often run summer cooling largely on self-generated electricity, which changes the running cost conversation entirely. If you have panels and a battery, pre-cooling in the early afternoon and coasting into the evening is the pattern to aim for.
Reading your own numbers
If you want a real figure rather than an estimate, put a plug-in energy monitor on the outdoor unit supply for a week, or check your smart meter's half-hourly data on a hot day against a day when the system was off. That comparison is more useful than any manufacturer figure, because it includes your property, your habits and your tariff.
If the numbers look much higher than the ranges above, that is worth investigating: the usual culprits are an oversized or undersized unit, low refrigerant charge, a badly sited outdoor unit, or dirty coils. All four are fixable.
Frequently asked questions
Is it cheaper to leave air conditioning on all day?+
For a well-insulated room you are occupying all day, holding a steady temperature is usually cheaper than repeated pull-downs. For a room used only in the evening, timed operation wins.
How much does air conditioning add to a summer electricity bill?+
For a typical Derbyshire home cooling one or two rooms during hot spells, £20 to £60 across the whole summer is common. Whole-house cooling in a heavily glazed property can be several times that.
Does the fan-only mode cost much?+
Very little — usually 20 to 50 watts, comparable to a pedestal fan. It is a sensible option when the outside air has cooled but the room still feels stuffy.
Do bigger units always cost more to run?+
Not necessarily. A correctly sized larger unit running at part load can be more efficient than a small unit running flat out. Oversizing is the problem, not size itself.