Window air conditioners are rated in BTU, sold on BTU, and argued about in BTU, but your electricity bill has never heard of a BTU. What the meter sees is watts. A 8,000 BTU unit that draws 700 W and a 12,000 BTU unit that draws 1,100 W will both cool a room; one of them will do it for half again as much money per hour.
A typical window air conditioner draws around 900 W while the compressor is running, which is [hed_cost appliance=”window-air-conditioner” period=”hour”] per hour at the US average rate of [hed_var key=”us_rate_cents” format=”cents”] per kWh. Eight hours a night comes to [hed_cost appliance=”window-air-conditioner” period=”day”], and a full 30-day month of that pattern is [hed_cost appliance=”window-air-conditioner” period=”month”]. Small 5,000 BTU units land closer to 500 W; large 12,000 BTU units push past 1,400 W.
BTU is what it does. Watts is what it costs.
BTU per hour is cooling output. Watts is electrical input. The ratio between them is the efficiency rating, published as CEER on the yellow EnergyGuide label. Divide BTU by CEER and you get the running wattage, which is the number that costs money.
A 10,000 BTU unit with a CEER of 12 draws about 833 W. The same 10,000 BTU with a CEER of 9 draws about 1,111 W. Over a summer that difference is real money, and it is the only spec on the box that predicts your bill. Bigger BTU does not mean better; an oversized unit cools the room fast, switches off, and leaves the humidity behind, which is why the room feels clammy and you end up setting it colder.
[hed_wattage_matrix appliance=”window-air-conditioner”]The compressor is not running as much as you think
An air conditioner almost never runs continuously. The compressor cycles: it runs until the room hits the thermostat setting, shuts off, and the fan often keeps going. On a mild evening a correctly sized unit might run 30 percent of the time. During a heat wave, with the outdoor temperature above 95, it may approach 100 percent and simply never catch up.
This is why August bills shock people who did nothing different. Same unit, same thermostat setting, far more compressor hours. The table below shows what different amounts of actual compressor time cost.
[hed_hours_matrix appliance=”window-air-conditioner”]What your rate does to the same unit
Cooling is a summer-peak load, and summer peak is exactly when time-of-use tariffs charge the most. If your utility bills by time of day, the rate you pay for 3pm cooling can be double the overnight rate. Our time-of-use bill calculator is the better tool if that describes your plan.
[hed_rate_matrix appliance=”window-air-conditioner”]Window AC running cost by state
Same 900 W unit, eight hours a day, every state. Note how the Southeast and South Central states, which need the most cooling hours, mostly enjoy the lowest rates. New England has the opposite arrangement and generally needs fewer hours.
[hed_state_costs appliance=”window-air-conditioner”]Six ways to cut the cooling bill
Things that meaningfully reduce the number:
- Raise the setpoint before you do anything else. Each degree warmer cuts compressor runtime, and runtime is the whole cost. Going from 70 to 74 often cuts cooling energy by a fifth or more.
- Seal the window kit properly. Those accordion side panels leak badly. Foam board and weatherstrip tape around the unit costs a few dollars and directly reduces run hours.
- Clean the filter monthly during the season. A clogged filter starves airflow, which extends every cycle.
- Use a fan with it. A [hed_cost appliance=”ceiling-fan” period=”hour”]-per-hour ceiling fan lets most people sit comfortably two or three degrees warmer, and the fan draws roughly one fifteenth of what the compressor does.
- Shade the window. Direct sun on the room is load the compressor has to remove. Blinds closed during the day is free.
- Do not run it in an empty room on a timer to have it cool when you arrive. Pre-cooling a room for an hour costs more than the ten minutes it takes to cool once you are in it.
Window unit versus central air
A window unit uses roughly a quarter of what central air uses, because it is cooling one room instead of an entire house and has no duct losses. If you only occupy one or two rooms for most of the day, running window units and leaving the central system off is usually cheaper, sometimes dramatically.
That flips once you are running three or four of them. At that point a properly sized central system or a mini-split is more efficient per unit of cooling, and a mini-split has the additional advantage of working as a heat pump in winter. The cooling cost calculator handles that comparison with your own numbers.
How this compares to everything else you plug in
Cost per hour of runtime for everything people use to stay cool. The gap between a ceiling fan and a compressor is the reason fans are worth running alongside the AC rather than instead of it.
[hed_compare_table appliances=”window-air-conditioner,portable-air-conditioner,central-air-conditioner,mini-split,ceiling-fan,box-fan”]Questions people actually ask
[hed_faq]How much does it cost to run a window AC for 8 hours?A 900 W window unit running eight hours costs about [hed_cost appliance=”window-air-conditioner” period=”day”] at the US average rate. That assumes the compressor runs the full eight hours, which on a mild night it will not.
How many watts does a window air conditioner use?Between about 500 W for a small 5,000 BTU unit and 1,440 W for a 12,000 BTU unit. Around 900 W is typical for the 8,000 to 10,000 BTU sizes most people buy. Divide the BTU rating by the CEER number on the EnergyGuide label to get the running watts for your specific model.
Is it cheaper to run a window AC or central air?For one or two rooms, a window unit is cheaper, often by a factor of three or four. Central air moves and conditions air for the whole house and loses some of it to ducts. Once you are cooling most of the house, central air becomes the more efficient way to do it.
Does turning the AC off when I leave save money?Yes, for absences longer than an hour or so. The house warms up while you are gone, but the energy to re-cool it is less than the energy to hold it cool for hours in an empty room. The exception is very humid climates, where letting humidity build back up has its own cost.
Do window ACs use a lot of electricity on fan-only mode?No. The fan alone is usually 60 to 120 W, which is around a tenth of the compressor draw. Fan-only costs roughly the same as running a box fan.
Does a higher BTU unit cost more to run?Per hour, yes, because it draws more watts. But an undersized unit runs continuously and can end up using more over a day than a correctly sized unit that cycles. Match the BTU to the room area rather than buying the biggest one that fits the window.[/hed_faq]
Related running costs
[hed_related appliances=”portable-air-conditioner,central-air-conditioner,ceiling-fan,dehumidifier,mini-split”]Running it on solar
A window air conditioner uses [hed_daily_energy appliance=”window-air-conditioner”] a day, which needs about [hed_panels_needed appliance=”window-air-conditioner”] standard 400 W panels. Full array, battery and inverter sizing is on how many solar panels to run a window air conditioner.