How many watts does an air conditioner use? A window unit draws 500–1,500 watts while the compressor is on, a portable 900–1,500W, and a central system 3,000–5,500W. Startup briefly pulls two to three times those figures. For sizing a battery, the number you actually want is kilowatt-hours per cooling day: 3–8 kWh for a room unit.
Cooling is the largest electrical load in most homes and the one people size wrongly most often, because the number on the carton is heat moved out of the room rather than power drawn through the plug.
The three numbers, and which one you need
An air conditioner carries three separate electrical figures. They are measured in different units, they answer different questions, and nearly every sizing error is one of them being used in place of another.
| Number | Typical value, window unit | What it decides |
|---|---|---|
| Running watts | 500–1,500W | Circuit capacity and continuous inverter output |
| Startup surge | 1,500–4,000W | Whether the compressor starts at all |
| Energy per cooling day | 3–8 kWh | Battery capacity, and your bill |
Split the decisions and the confusion goes away. Inverter output comes from the top two rows: the continuous rating has to cover running watts with room to spare, and the surge rating has to cover the spike. Battery capacity comes from the third row alone, because watt-hours accumulate over hours while the spike lasts a second. Buy the inverter for the worst instant and the battery for the whole afternoon.
The appliance tells you none of this. The nameplate gives amps, and sometimes watts, both of them running figures. If you are budgeting a whole outage rather than one appliance, our breakdown of how many watts it takes to run a house places cooling against everything else you want on at the same moment.
Why BTU is not a wattage
A 12,000 BTU air conditioner does not draw 12,000 watts, and it does not draw 3,517W either, although that is the correct thermal conversion at 3.412 BTU per hour per watt. An air conditioner is a heat pump: it moves three or four times more heat than the electricity it consumes. The ratio between the two is the efficiency rating, and that is what converts one figure into the other.
- EER, the energy efficiency ratio, is BTU per hour divided by watts at a fixed test condition. A current window unit sits around 11–15.
- CEER is the same ratio with standby power folded in, and it is the figure on the yellow EnergyGuide label for room units.
- SEER2 is a seasonal average for central systems and mini-splits, so you cannot divide by it to get an instantaneous draw.
For a room unit the arithmetic is watts = BTU/h ÷ EER. A 12,000 BTU unit at EER 11 draws roughly 1,090W; the same capacity in a good inverter mini-split, at an effective ratio near 20, settles closer to 600W. Identical comfort, close to half the electricity. Our BTU to watts calculator handles the thermal conversion, but for electrical sizing it is the EER division you want.
Never size an inverter from the thermal conversion. Turning 12,000 BTU/h into 3,517 watts describes heat leaving the room, not power entering the plug. Do that and you will buy roughly three times the inverter you need. It is the single most common mistake on this topic.
Duty cycle: an air conditioner is not a fridge
A refrigerator compressor runs about a third of the time, which is why a fridge averages 40–80W despite a 150W compressor. Cooling does not behave that way, and borrowing the fridge’s logic is how people underestimate an air conditioner by two or three times.
On a mild evening a correctly sized unit might run at a 40–60% duty cycle. On the afternoon you actually bought it for, it runs at 70–100%. The conditions that make you switch it on are the same conditions that hold the compressor on. What moves the number:
- The temperature gap. A 95°F day against a 72°F setpoint is roughly twice the work of an 85°F day against the same setpoint.
- Sun on the room. A west-facing window in late afternoon adds several hundred watts of heat gain by itself. Closing the blinds before the sun reaches the glass is free capacity.
- Sizing, in both directions. A unit too small never cycles off, so its duty cycle is 100%. A unit far too large short-cycles, cools without pulling moisture out, and leaves the room cold and clammy at similar cost.
- The room envelope. Poor insulation, an unsealed portable exhaust kit or a gappy window frame is a load the compressor pays for every minute it runs.
Size on the hot-day duty cycle, not the seasonal average. For any battery, inverter or generator decision, assume the compressor is running 80–100% of the time. You are planning for a July afternoon, not a pleasant evening in May.
How many watts does an air conditioner use, by type and size
The energy columns below assume 8 hours of use per day at a 70% duty cycle across a 100-day cooling season. Scale them to your own summer: a Phoenix household running sixteen hours a day for six months should roughly triple the yearly figure, and someone who uses a unit for three weeks in August should divide it by four.
| Type and size | Running watts | Per cooling day | Per year |
|---|---|---|---|
| Window, 5,000 BTU | 400–600W | 2.2–3.4 kWh | 220–340 kWh |
| Window, 8,000 BTU | 600–900W | 3.4–5.0 kWh | 340–500 kWh |
| Window, 12,000 BTU | 1,000–1,400W | 5.6–7.8 kWh | 560–780 kWh |
| Portable, 10,000–14,000 BTU | 900–1,500W | 5.0–8.4 kWh | 500–840 kWh |
| Inverter mini-split, 9,000–12,000 BTU | 300–1,000W | 2.5–5.0 kWh | 250–500 kWh |
| RV rooftop, 13,500 BTU | 1,200–1,600W | 5.0–9.0 kWh | — |
| RV rooftop, 15,000 BTU | 1,400–1,900W | 6.0–10.5 kWh | — |
| Central, 2 ton (24,000 BTU) | 2,000–3,000W | 11–17 kWh | 1,100–1,700 kWh |
| Central, 3–5 ton | 3,000–5,500W | 17–31 kWh | 1,700–3,100 kWh |
The ranges are wide because efficiency spreads the draw by 30% or more within any one BTU rating: a unit bought in 2008 sits at the top of its band, a current high-EER model at the bottom.
Three rows deserve a second look. The inverter mini-split is the only line that behaves better under sustained load, because its compressor modulates rather than switching between full power and off, holding a room at 300–500W for hours. The portable rows look like window units on paper and are worse in the room: a single-hose design blows conditioned air out of the window and pulls hot outside air in through every gap to replace it. The central rows include the indoor blower, 400–800W on its own, which runs whenever the system calls.
Startup surge, and what a soft starter does
A single-speed compressor is a motor starting under pressure, and it draws a heavy spike for a second or so before settling. Budget two to three times running watts for room units, more for an older one or a tired start capacitor. This is a genuine surge, not a safety margin: a resistive load such as a space heater has none at all, a compressor emphatically does, and that is why a small inverter copes with one and not the other.
| Unit | Running | Startup surge | With a soft starter |
|---|---|---|---|
| Window, 5,000 BTU | 400–600W | 1,000–1,800W | Rarely fitted |
| Window, 12,000 BTU | 1,000–1,400W | 2,500–4,000W | Rarely fitted |
| Portable, 12,000 BTU | 1,100–1,500W | 2,500–4,000W | Rarely fitted |
| RV rooftop, 13,500 BTU | 1,200–1,600W | 2,800–3,500W | Around 1,300–1,800W |
| Inverter mini-split | 300–1,000W | Negligible | Not needed |
| Central, 3 ton | 3,000–4,000W | 8,000–20,000W | Roughly halved |
Unlike a domestic fridge, an air conditioner is exactly the appliance soft starters were designed for. One ramps the compressor up over about a second instead of slamming it on, and on RV rooftop units it commonly takes roughly two-thirds off the spike — often the difference between a rooftop unit being unstartable on a mid-size inverter and being an ordinary load on it. Fitting one is usually cheaper than buying the next inverter up.
You can replace these ranges with your own worst case. Room units print amps on the nameplate: multiply by 120V for running watts, and our watts to amps calculator works the sum in either direction when you are checking a circuit. On a central system the outdoor data plate lists LRA, locked rotor amps; multiply that by supply voltage for the figure an inverter would have to survive.
A marginal inverter fails late, not immediately. The unit starts, runs happily for an hour, then trips when the compressor restarts against warm refrigerant pressure. If the surge rating and the measured spike are within a few hundred watts of each other, count that as a failure rather than a pass.
How long a power station really runs one
Take rated capacity, subtract 13% for inverter and conversion losses, then divide by the running draw. Because a compressor barely cycles off on a hot afternoon, the table treats a 5,000 BTU window unit as a steady 500W and a 13,500 BTU rooftop unit as a steady 1,300W, with nothing else plugged in.
| Capacity | Usable | 5,000 BTU window (~500W) | 13,500 BTU rooftop (~1,300W) |
|---|---|---|---|
| 300Wh | ~260Wh | Inverter too small to start it | Inverter too small to start it |
| 500Wh | ~435Wh | Usually will not start it | Inverter too small to start it |
| 1,000Wh | ~870Wh | ~1 hr 45 min | ~40 min, soft starter needed |
| 2,000Wh | ~1,740Wh | ~3 hr 30 min | ~1 hr 20 min |
| 3,600Wh | ~3,130Wh | ~6 hr 15 min | ~2 hr 25 min |
On the numbers. Capacity and output are manufacturer specifications. Expect 80–90% of rated capacity in real use after inverter losses, and less in cold weather.
Read the small rows honestly. A 300Wh or 500Wh power station usually ships with a 300–600W inverter, which cannot start a compressor of any size: those are not short runtimes, they are refusals. Output and surge decide whether the unit switches on, capacity only decides how long it stays on.
Central air conditioning is missing from the table deliberately. Three tons pulls 3,000–4,000W running and spikes into five figures, so a portable power station is simply the wrong tool; that is a fixed battery and transfer switch question, and our home battery backup hub is where it starts. For RV and van banks quoted in amp-hours, convert first with the amp-hours to watt-hours calculator: 100Ah of lithium at 12.8V is about 1,280Wh, or roughly 50 minutes of rooftop cooling after losses. Running one overnight realistically means 400Ah or more, plus an inverter rated well clear of the surge.
Does cooling belong on battery backup at all?
Frequently, no. Cooling is a poor match for stored energy: high continuous draw, a difficult start, and demand that lasts hours rather than minutes. A 2,000Wh unit that carries a refrigerator for well over a day carries a small window unit for three and a half hours, and sizing a battery for a full night of cooling produces a number most households do not want to spend. Three cases where it does make sense:
- One room, part of the day. A 5,000–8,000 BTU window unit in a bedroom, run from a 2,000–3,600Wh station to get through the worst of an afternoon or to fall asleep.
- Alongside solar, in daylight. Cooling demand peaks when the sun is strongest, which makes it the one large load whose timing suits panels. Roughly 1,200–1,600W of panel offsets a small window unit while the sun is on it.
- An inverter mini-split. No meaningful surge and a modulating draw as low as 300–400W make it the easiest cooling to run from storage.
The alternative deserves saying plainly: a box or ceiling fan uses 30–100W. The same 2,000Wh battery that gives you three and a half hours of air conditioning gives you more than a day of air movement, and moving air is most of what makes a hot room bearable. If the goal is getting through a summer outage rather than holding 72°F, buy the fan first and keep the fridge cold second. If a room unit really is the right call, the power station finder filters by continuous output and surge rating together, which is the pair that matters here.
How to measure your own unit in one day
Every figure above is an estimate. Your unit, in your room, in your weather, is knowable for about $25 and one hot day.
- Buy a plug-in energy monitor, the kind that sits between the plug and the socket and totals kilowatt-hours. Check its current rating: a 12,000 BTU unit pulls around 10–12A, and cheap monitors are rated to 15A at best.
- Plug the air conditioner in through it, reset the counter, and run a normal day: same setpoint, same hours, same blinds. Do not tidy up your habits, because the habits are part of the measurement.
- Read the kilowatt-hour total the next day. Watch the live wattage through one compressor start too, though most monitors sample too slowly to catch the true peak.
That single number replaces this entire article for your situation, and it costs less than the gap between two battery sizes. The watts to kWh calculator moves you between an hourly draw and a daily total.
This does not work on central air conditioning. A central system is 240V and hardwired, so there is no plug to meter. The realistic options are a clamp meter on the circuit, which is electrician territory, a whole-home monitor at the panel, or comparing a hot month’s bill against a mild month’s and attributing the difference.
What air conditioning costs to run per year
At a US average residential rate of 17 cents per kilowatt-hour, the yearly column converts directly into money. Substitute your own rate from your bill, because state prices vary by more than a factor of three and the average flatters some readers badly.
- Window, 5,000 BTU, 280 kWh a year: about $48
- Window, 8,000 BTU, 420 kWh: about $71
- Window, 12,000 BTU, 670 kWh: about $114
- Portable, 12,000 BTU, 700 kWh: about $119
- Central, 3 ton, 1,700–3,100 kWh: $290 to $525
Two things move those totals more than the equipment does. The first is the setpoint: a degree on the thermostat is commonly quoted as around 3% off the cooling bill, a rule of thumb rather than a measurement, but it points the right way and compounds across a season. The second is running hours — a window unit left on around the clock rather than eight hours a day costs roughly three times the figures above, and a timer or a smart plug closes most of that gap for the price of lunch.
Frequently asked questions
How many watts does a window air conditioner use?
Between 400W and 1,500W while the compressor runs, depending on size. A 5,000 BTU unit draws 400–600W, an 8,000 BTU unit 600–900W and a 12,000 BTU unit 1,000–1,400W. Startup briefly pulls two to three times those numbers. Over a typical eight-hour cooling day, expect 2–8 kilowatt-hours.
How many watts does a 12,000 BTU air conditioner use?
Roughly 1,000–1,400 watts for a window or portable unit, and closer to 600W for an inverter mini-split of the same capacity. Divide the BTU rating by the unit’s EER: 12,000 divided by an EER of 11 is about 1,090W. The 3,517W thermal equivalent of 12,000 BTU is heat removed, not electricity drawn.
Will a 2,000W power station run an air conditioner?
A 2,000W continuous output rating runs most window and portable units, provided the surge rating clears 3,000–4,000W. It will not run central air conditioning. Note that 2,000W of output and 2,000Wh of capacity are different specifications: a 2,000Wh battery holds a 5,000 BTU window unit for about three and a half hours.
How long will a 5,000 BTU air conditioner run on a battery?
About 1 hour 45 minutes on 1,000Wh, three and a half hours on 2,000Wh, and around six hours on 3,600Wh, allowing 13% for inverter losses and assuming the compressor runs continuously on a hot day. Anything under 1,000Wh generally lacks the inverter output to start the compressor at all.
What size inverter do I need for an RV air conditioner?
A 13,500 BTU rooftop unit runs at 1,200–1,600W and surges to 2,800–3,500W, so budget 2,000W continuous with a surge rating above 3,500W. A soft starter typically cuts the spike by around two-thirds, which often brings the unit within reach of an inverter that could not otherwise start it and costs less than upsizing.
How much does it cost to run an air conditioner for 8 hours?
Around 40 cents for a 5,000 BTU window unit and about $1.20 for a 12,000 BTU one, at 17 cents per kilowatt-hour with the compressor on roughly 70% of the time. A 3 ton central system over the same eight hours is closer to $2 to $4. Use your own rate, since regional prices vary widely.
Next step. Measure one hot day, convert it to kilowatt-hours, and size the battery from that. Then check the running and surge figures against the inverter, not the capacity: output decides whether the compressor starts, and capacity only decides how long it keeps going.