The short answer
A typical 2.5 kW split draws roughly 0.6–1.0 kWh per hour, so at about 25p/kWh it costs around 15–25p per hour to run. Use it eight hours and that is roughly £1.20–£2.00 a day. The actual figure depends on the unit’s efficiency, how hard it works against the outside temperature, your unit rate and how long you run it — not just its nameplate capacity.
Running cost is the figure people most often guess wrong. The key is to separate the cooling capacity (the kW of heat it can move) from the electrical power it draws (the kWh it consumes), because a modern inverter unit moves far more heat than the electricity it uses. This page shows the arithmetic plainly so you can estimate your own bill rather than rely on a headline number.
Running cost at a glance
- Power drawn (2.5 kW split) ~0.6–1.0 kWh/hour
- Cost per hour ~15–25p at ~25p/kWh
- Eight-hour day ~£1.20–£2.00
- Biggest lever Efficiency and run time
- Unit rate source Ofgem
The honest maths
Running cost is just power drawn multiplied by hours multiplied by your unit rate. A typical 2.5 kW split draws roughly 0.6–1.0 kWh of electricity per hour when cooling. At a unit rate of about 25p/kWh that works out as follows:
- 0.6 kWh × 25p = 15p per hour
- 1.0 kWh × 25p = 25p per hour
- Eight hours at 0.6–1.0 kWh = 4.8–8.0 kWh × 25p = £1.20–£2.00 per day
Note the distinction that trips most people up: 2.5 kW is the cooling capacity (the heat moved out of the room), while 0.6–1.0 kWh is the electricity the unit actually consumes. A good inverter unit moves several times more heat than the power it draws, which is why air conditioning is cheaper to run than its capacity figure might suggest. Always work from the power input on the data sheet, not the capacity, when estimating cost.
What changes your bill
Real running cost moves with several factors. The hotter it is outside, the harder the unit works to reject heat, so it draws more power. A well-insulated, shaded room holds its temperature and lets the unit cycle down to a gentle tick-over. A higher energy-rating model uses less electricity for the same cooling. And of course your unit rate — which Ofgem-regulated tariffs set — scales the whole figure, so the same usage costs more on a dearer tariff. None of these change the method; they only change the numbers you put into it.
| Run time per day | Energy (at 0.8 kWh/hr) | Cost at 25p/kWh |
|---|---|---|
| 2 hours | 1.6 kWh | ~40p |
| 4 hours | 3.2 kWh | ~80p |
| 8 hours | 6.4 kWh | ~£1.60 |
| 12 hours | 9.6 kWh | ~£2.40 |
Cooling versus heating
Many splits are reverse-cycle and heat as well as cool. In heating mode an air-to-air unit acts as a heat pump and can be efficient, but the power drawn shifts with the outside temperature: colder air means more work and a higher draw. The same per-hour logic applies in either mode — multiply the power drawn by hours by your rate. Because a reverse-cycle unit can heat efficiently in milder weather, some households find it cheaper for autumn and spring heating than they expected. See heating and cooling air con for how reverse-cycle works.
- Cooling on a hot day — higher draw than a mild day.
- Heating on a cold day — higher draw than a mild day.
- Setting a sensible target temperature reduces both.
Keeping the cost down
The Energy Saving Trust’s general efficiency advice applies directly: do not over-cool, keep doors and windows shut while running, shade the room from direct sun, clean the filters so airflow stays free, and choose an efficient model in the first place. Each of those lowers either the power drawn or the hours run, and since cost is power × hours × rate, that feeds straight into a lower bill. To go further, read the cheapest way to run air con or check your appliance against electricity usage.
A worked example for one home
To make this concrete, take a bedroom with a 2.5 kW split run for six hours on a warm evening. At a steady 0.8 kWh per hour that is 4.8 kWh; at a 25p unit rate the evening costs about £1.20. Do that on twenty evenings across a summer and the seasonal cost is roughly £24 — a modest sum for cool sleep through the hottest weeks. Now imagine the same room cooled by a less-efficient portable drawing 1.2 kWh per hour: the same six hours becomes 7.2 kWh, or about £1.80 an evening, and the seasonal figure climbs to around £36. The lesson is that the per-hour draw, set by the unit’s efficiency and how hard it is working, drives the whole bill far more than the headline capacity does. Estimate with your own numbers and you will rarely be surprised by the meter.
These figures are typical 2026 estimates, not a guarantee for your home. Your actual cost depends on your unit, your tariff and your usage.
Estimate your own running cost
Take your unit’s power draw, multiply by the hours you run it and by your Ofgem-tariff unit rate — that is your real daily cost.
Frequently asked questions
How much does it cost to run air con for an hour?
A typical 2.5 kW split costs about 15–25p per hour: it draws roughly 0.6–1.0 kWh and a unit rate of about 25p/kWh applies.
Is it cheaper to leave air con on all day or switch it on and off?
For most homes, cooling a room only when you need it and letting an inverter unit hold a sensible temperature is cheaper than running it cold all day. Avoid over-cooling, which simply burns more electricity.
Does the capacity in kW tell me the running cost?
No. The kW capacity is the heat it can move; the running cost depends on the electricity it draws (kWh) times your unit rate. A good inverter moves far more heat than the power it uses.
Why is my bill higher on very hot days?
On hotter days the unit works harder to reject heat, so it draws more power per hour. The same happens in heating mode on very cold days.
Sources & further reading
- Ofgem — typical domestic electricity unit rates
- Energy Saving Trust — running cost and efficiency guidance
- Manufacturer technical data (e.g. Daikin, Mitsubishi Electric) — power input specifications
- gov.uk — energy efficiency information for households
This guide is general information, not a site-specific survey or a substitute for a quote from an F-Gas-certified installer. Installation, servicing and refrigerant handling are legally restricted to F-Gas-certified engineers.