Less than most people think. A modern split air conditioner cooling a bedroom typically costs somewhere between 10p and 20p an hour to run — closer to the price of leaving a couple of old lightbulbs on than running a tumble dryer. But "typically" is doing some work there, so here's the actual maths, and you can plug in your own numbers.
The simple maths
Running cost is just: electricity the unit draws (kW) × your electricity price (p/kWh) × hours running. At the time of writing, most standard UK tariffs sit somewhere around 24–28p per kWh — check your own bill, because that number matters more than anything else on this page.
The trap people fall into is reading the unit's size as its consumption. A "2.5kW" air conditioner delivers 2.5kW of cooling — but because it's a heat pump moving heat rather than generating it, it only draws roughly a third of that in electricity, often less. So a 2.5kW bedroom unit pulls perhaps 0.7–0.8kW flat out.
And it's rarely flat out. Modern splits are inverter-driven: they sprint to bring the room down to temperature, then cruise at a fraction of full power to hold it there. Over a warm evening, average draw on a bedroom unit is often 0.3–0.5kW. That's the number your bill actually sees.
What that means per hour
Using a mid-range 26p/kWh, here's roughly what different setups cost per hour once they're up to temperature. These are honest ranges, not lab figures — hotter days and poorly insulated rooms push you toward the top end.
| Setup | Typical draw | Rough cost per hour |
|---|---|---|
| Bedroom split (2.5kW cooling) | 0.3–0.8kW | 8–21p |
| Living room split (3.5kW cooling) | 0.4–1.1kW | 10–29p |
| Multi-split, 3 rooms all running | 1.5–2.5kW | 39–65p |
| Portable unit | 1.0–1.3kW | 26–34p |
Worth pausing on that last row: a portable unit often costs more per hour than a proper split while delivering noticeably less cooling, because single-hose portables leak much of their own work back into the room. We've broken that down properly in our guide to split vs portable air conditioning.
What a whole summer actually costs
British summers are not Texan summers. Realistically you might want cooling on perhaps 40–60 days a year — a few hours on warm evenings, longer stretches during the two or three genuinely hot spells.
Worked example: a bedroom unit running 8 hours a night for 50 nights, mostly cruising at around 0.4kW. That's 160kWh, or roughly £40–45 for the entire summer at 26p/kWh. Even doubling the hours and running it harder, you'd struggle to spend £100 cooling one room.
A whole-home multi-split used generously through a hot summer lands somewhere around £150–300. Not nothing — but a long way from the "eye-watering bills" reputation air conditioning still carries in the UK, which mostly comes from inefficient portables and American-scale usage.
The heating twist: it can undercut your gas boiler
Here's the part that changes the economics. A modern split isn't just a cooler — it's a reversible air-to-air heat pump, and in heating mode it's remarkably efficient. Manufacturers quote seasonal heating efficiency (SCOP) of around 4 for a decent unit, with the best models higher — meaning every 1kWh of electricity becomes roughly 4kWh of heat. Real-world figures usually land a little below the brochure, but the principle holds.
Run that through the maths: at 26p/kWh electricity and an efficiency of 4, you're paying about 6–7p per kWh of delivered heat. On recent standard tariffs, mains gas has sat at roughly 6–7p per kWh too — before you account for boiler losses. Per unit of heat, they're roughly level, and the heat pump often edges ahead once those losses are counted.
But the real win is targeting. Your boiler heats the whole system to warm one room; a split heats just the room you're in. For a home office in January or a bedroom on shoulder-season evenings, heating with your air conditioning is often the cheapest way to warm that one room. Cooling in July becomes almost a bonus feature.
Cutting the bill further with solar and a battery
There's a neat overlap here: air conditioning works hardest on exactly the days solar panels generate most. On a hot, sunny afternoon, a solar array can run your cooling essentially for free — the catch is the evening, when you want cool bedrooms and the panels have clocked off.
That's where a battery earns its keep, shifting midday surplus into the evening and typically lifting the share of your own solar you actually use from around half to something like 80%. We've covered the full picture in will solar panels run my air conditioning? and, if you're sizing seriously, what battery size you'd need to run air conditioning.
The bottom line
Cooling one room costs pennies per hour and tens of pounds per summer. Whole-home cooling in a hot year runs to a couple of hundred. And in winter the same unit heats individual rooms at a cost that goes toe-to-toe with mains gas. The running costs are rarely the reason to hesitate — the installation quote is the number that matters, and that depends on your home.
If you want a real figure rather than a range, get a free quote and we'll match you with one vetted installer — a single specialist who'll size the system properly, not a list of firms competing for your phone number.