The short answer
Not as expensive as most people expect: a typical 2.5 kW split costs about 15–25p per hour to run, or roughly £1.20–£2.00 for an eight-hour day. Because a modern inverter unit moves several times more heat than the electricity it draws, it is more efficient than its capacity figure suggests. Cost only becomes high if the unit is oversized, run constantly, or set to an extreme temperature.
“Is air conditioning expensive to run?” depends entirely on the unit, the run time and the temperature you set. The fear usually comes from confusing the cooling capacity (kW of heat moved) with the electricity consumed (kWh). This page gives the honest answer, shows the arithmetic, and explains the handful of habits that turn a reasonable bill into an expensive one.
Running cost reality at a glance
- Typical cost per hour ~15–25p (2.5 kW split)
- Eight-hour day ~£1.20–£2.00
- Main cost drivers Run time, set temperature, efficiency
- Efficiency edge Inverter moves more heat than power used
- Worst value Oversized or constantly-run units
The short, honest answer
For a typical 2.5 kW split, running cost is about 15–25p per hour, because the unit draws roughly 0.6–1.0 kWh and the unit rate is around 25p/kWh. Run it eight hours and you are looking at roughly £1.20–£2.00 a day. That is not, by most household standards, expensive — it is comparable to running a couple of other major appliances for the same period. It becomes expensive only when the system is the wrong size, left on around the clock, or set to chase an extreme temperature it can never comfortably reach.
- Right-sized unit, sensible use — modest cost.
- Oversized unit or constant running — cost climbs.
- Extreme set temperature — the unit never cycles down.
Why it is more efficient than it looks
The reason air conditioning surprises people is the gap between capacity and consumption. A 2.5 kW unit can move 2.5 kW of heat out of a room while drawing well under 1 kW of electrical power. Modern inverter compressors achieve this by varying their speed instead of switching fully on and off, so once the room is cool they tick over gently rather than slamming between full power and off. That is why the per-hour cost stays low for steady-state cooling, and why a quality inverter unit is cheaper to run than an older fixed-speed design. See inverter air conditioning explained.
| Usage pattern | Energy (0.8 kWh/hr) | Cost at 25p/kWh |
|---|---|---|
| A couple of hot evenings a week | Low total | A few pounds a week |
| 4 hours a day in summer | 3.2 kWh/day | ~80p/day |
| 8 hours a day, hot spell | 6.4 kWh/day | ~£1.60/day |
What makes it genuinely expensive
Cost spikes have predictable causes. An oversized unit short-cycles and never settles into its efficient range, so it draws more than it should. A portable, being less efficient than a fixed split, costs more per hour for the same cooling. Poor insulation or direct sun means the unit fights a losing battle and runs at full power for longer. And setting the thermostat very low forces it to run flat out continuously rather than coasting. In other words, “expensive” air conditioning is almost always a sizing, hardware or habit problem, not an inherent property of the technology.
- Oversizing — correct sizing avoids it (see what size do I need).
- Portables — convenient but less efficient.
- Leaky, sun-baked rooms — the unit can never coast.
How to keep it cheap
The Energy Saving Trust’s efficiency principles apply directly: size the unit correctly, set a moderate temperature, keep windows and doors shut while it runs, shade the room, and clean the filters so airflow stays free. Each of those reduces either the power drawn or the time the unit needs to run at full tilt. Used selectively — reserved for the hottest spells and paired with a fan on milder days — a split system is an affordable comfort, not a bill shock. For the full method see the cheapest way to run air con or the underlying figures in running cost.
Putting it in perspective
It is worth comparing air conditioning with the appliances households already run without a second thought. A 2.5 kW split at 15–25p per hour is in the same territory as a number of everyday electrical loads, and unlike many of them it is delivering something you actively value on a hot day: a comfortable, sleepable room. The fear of a runaway bill almost always traces back to one of three avoidable mistakes — buying an oversized or inefficient unit, leaving it running when no one needs it, or setting it far colder than comfort requires. Address those and the cost is predictable and modest. Air conditioning is not inherently a luxury that punishes your meter; run with a little discipline, it is an affordable comfort, and a reverse-cycle model also earns its keep as efficient heating in the shoulder seasons.
These are typical 2026 figures for guidance only. Your real cost depends on your unit, your tariff and how you use it.
Run it well, keep it cheap
Size the unit correctly, set a moderate temperature and maintain the filters — that is how a reasonable bill stays reasonable.
Frequently asked questions
Is air conditioning more expensive than a fan?
Yes, per hour a fan is far cheaper because it only moves air. But a fan does not lower the room temperature; air con does. The comparison depends on whether you need real cooling or just a breeze.
Does running air con all night cost a lot?
An overnight run of, say, eight hours at 0.6–1.0 kWh per hour is roughly 4.8–8.0 kWh, or about £1.20–£2.00 at a 25p unit rate. Setting a moderate sleep temperature keeps it at the lower end.
Is heating with air con expensive?
In heating mode a reverse-cycle unit works as a heat pump and can be efficient, though it draws more power on very cold days. The per-hour maths is the same: power drawn times hours times your unit rate.
Why do some people report huge air con bills?
Usually because the unit is oversized, left running constantly, set very low, or a less-efficient portable. Correct sizing and moderate use avoid those costs.
Sources & further reading
- Energy Saving Trust — running cost and efficiency guidance
- Ofgem — typical domestic electricity unit rates
- Manufacturer technical data (e.g. Daikin, Mitsubishi Electric) — power input specifications
- gov.uk — household energy efficiency information
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.