As hot spells get more common, more people are reaching for air conditioning, and it is one of the heaviest electrical loads a home can run. The good news is that the cost is easy to work out, because a cooling unit is just an appliance with a wattage like any other. The figure usually surprises people, in both directions: a portable unit run flat out through a heatwave adds a real sum to the bill, while a few sensible habits cut that sharply.
The short answer. A portable air conditioner typically draws 1 to 1.5 kilowatts, so at the standard electricity rate of about 26p a unit it costs roughly 26p to 39p an hour, or around 2 to 3 pounds for an eight-hour day. A more efficient fixed split system averages less. A fan, by contrast, costs about a penny an hour. The compressor in an air conditioner is what makes the difference, drawing thirty to fifty times what a fan does.
Why air conditioning costs so much more than a fan
The gulf comes down to what each device actually does. A fan does not cool the air at all; it moves it, and moving air cools your skin by helping sweat evaporate, which takes very little power. Air conditioning genuinely removes heat from the room and pumps it outside, using a compressor and refrigerant in the same way a fridge does, and shifting heat like that is energy-hungry. A fan draws tens of watts; an air conditioner draws a kilowatt or more. That is the whole story of the bill, and it is why the keeping cool without air conditioning guide treats real cooling as a last resort rather than a first move.
What it costs per hour and per day
The sum is the same one the running cost calculator does for any appliance: the power in kilowatts multiplied by the hours, multiplied by your price per unit. The table below uses the standard price-cap rate of about 26p per kWh, current in mid 2026, across the common types of cooling. Find your unit's wattage on its label or in its manual and read across.
| Cooling device | Typical power | Cost per hour | Cost per 8-hour day |
|---|---|---|---|
| Pedestal or tower fan | 50 W | ~1.3p | ~10p |
| Evaporative air cooler | 60 to 100 W | ~2p to 2.6p | ~13p to 21p |
| Fixed split system (inverter), average | 600 to 800 W | ~16p to 21p | ~£1.25 to £1.67 |
| Portable air conditioner | 1,000 to 1,500 W | ~26p to 39p | ~£2.09 to £3.13 |
At an example 26p per kWh (Ofgem price cap, July to September 2026, 26.11p). A split system cycles its compressor off once the room reaches temperature, so its average draw is below its peak; a cheap portable unit tends to run harder for longer. Run a 1.2 kW portable unit eight hours a day for a month and that is roughly 60 to 75 pounds on top of your usual bill.
Portable versus fixed: not the same machine
The two common types of home air conditioning behave very differently on cost. A portable unit on wheels is cheap to buy and needs no installation, but it is the least efficient option: it sits in the room it is cooling, dumps its heat through a hose out of a window, and a single-hose model draws warm air back into the room as it works, so it has to run harder. A fixed split system, with a quiet indoor unit and a compressor mounted outside, costs more to buy and must be installed, but it is far more efficient, cools more effectively, and its inverter compressor throttles back once the room is comfortable rather than running flat out. If you cool often, the running-cost saving of a fixed system can outweigh its higher purchase price over time, much as it does with any efficient appliance.
The venting hose matters more than people think
With a portable unit, the hose is the difference between cooling and just making noise. The hose carries the heat the unit has removed out of the window, and it must vent outside; if it does not, the unit is simply moving heat around the same room and adding its own motor heat on top, so the room never really cools and you pay for the privilege. The window gap around the hose needs sealing too, or hot outside air pours straight back in. Single-hose units have a built-in handicap, because the air they blow outside has to be replaced by air drawn in from elsewhere in the house, often warm. Twin-hose units avoid that and are noticeably more effective for the energy. Whatever the type, a unit venting properly through a sealed window does the job for far less running time than one fighting itself.
Read the energy rating before you buy
Air conditioners carry an energy label like other appliances, and for something that may run for hours on the hottest days it is worth reading. The figure to look at is the efficiency, often shown as a seasonal rating, which tells you how much cooling you get per unit of electricity. A more efficient unit delivers the same cool room for less power, and over several summers that gap repeats every time you switch it on. As with fridges and washing machines, the cheapest unit to buy is rarely the cheapest to run, and for a heavy seasonal load the running cost is the number that matters.
How to keep the cost down
The discipline that controls any heavy load applies here. Cool only the room you are in, not the whole house, and shut its door and windows so you are not cooling the outdoors. Do all the free things first: close curtains and blinds against the sun by day, open up at night to flush the heat out, and switch off indoor heat sources, all covered in the keeping cool guide, so the air conditioner has less work to do and runs for less time. Set the target temperature modestly, in the mid twenties rather than as cold as it will go, because every degree cooler costs more. Use the timer so it is not running in an empty room, and switch it off when you leave. Done this way, occasional targeted cooling on the worst days costs a fraction of running a unit flat out all summer.
The bottom line
Air conditioning costs real money because it removes heat rather than just moving air, drawing a kilowatt or more against a fan's few tens of watts. A portable unit runs to roughly 26p to 39p an hour at the standard rate, a fixed split system rather less, and a fan about a penny. Read the wattage off your unit, run it through the calculator, and you will know your own figure. Then keep it down the same way you would any big load: cool one room, do the free measures first, set a sensible temperature, and reserve the compressor for the days when shading and a fan are genuinely not enough.