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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.
- CEER is the same ratio with standby power folded in, and it is the figure on the yellow EnergyGuide label for room units. The federal minimum for a cooling-only window unit under 14,000 BTU was 10.9–11.0 for units made from June 2014, and is 13.1–16.0 for units made from May 26, 2026, depending on size (table below).
- 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, one built to the 2014 federal minimum in the lower half. Window units made since May 26, 2026 face a much higher minimum, set out below.
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. Two of these rows have guides of their own: portable air conditioner wattage covers why single-hose units cost more to run than their rating suggests, and heat pump wattage covers mini-splits by size.
What the efficiency label means for a unit bought now
The table above fits single-speed window units built to the 2014 rules or earlier. New ones face a higher bar: the Department of Energy’s minimum CEER, set in 10 CFR 430.32, rose for units manufactured from May 26, 2026. For an ordinary cooling-only window unit:
| Window unit, cooling only | Minimum CEER, made June 2014 to May 25, 2026 | Minimum CEER, made from May 26, 2026 | Yearly use at the minimum, DOE basis |
|---|---|---|---|
| Under 6,000 BTU (5,000 shown) | 11.0 | 13.1 | 341 → 286 kWh |
| 6,000–7,999 BTU (6,000 shown) | 11.0 | 13.7 | 409 → 328 kWh |
| 8,000–13,999 BTU (8,000 shown) | 10.9 | 16.0 | 550 → 375 kWh |
| 8,000–13,999 BTU (12,000 shown) | 10.9 | 16.0 | 826 → 563 kWh |
The last column is DOE’s own yardstick and the figure ENERGY STAR lists: capacity ÷ CEER over 750 hours of cooling a year, as DOE’s test procedure sets it. On that basis the new floor saves a 12,000 BTU unit about 263 kWh a year, roughly $47 at 18 cents.
Do not turn a new unit’s CEER into inverter watts. For a single-speed unit, BTU ÷ CEER lands close to the running draw: about 1,100W for 12,000 BTU at 10.9, inside the table row above. DOE rates a variable-speed (inverter) unit differently. Its CEER starts from its full-speed efficiency at 95°F, then is adjusted for how it compares, at three cooler test points from 82°F to 92°F, with a single-speed unit that DOE charges for cycling on and off there. A unit that does well at those points rates higher than its full-speed efficiency, so 12,000 ÷ 16.0 = 750W can understate what it draws flat out on the hottest afternoon. Size the inverter from the nameplate amps or watts.
ENERGY STAR’s certified list (575 models on October 15, 2026, every one with a variable-speed compressor) shows where the market sits. Cooling-only window units list CEER 15.0–16.0 at both 8,000 BTU (375–400 kWh a year) and 12,000 BTU (563–600 kWh). Frigidaire’s FHWW125WF1, for example, lists 16.0 and 562.5 kWh, its FHWW125WE1 15.0 and 600 kWh. Heat/cool models sit in a separate DOE class with lower minimums and list 13.2–15.1 at 8,000 BTU. A 15.0 rating is below the new 16.0 floor, and the rule follows the manufacture date, so check the CEER on the label rather than assuming a unit on the shelf meets it.
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.
In amps: how many amps does an air conditioner use?
A window unit draws roughly 3–12 amps on a 120V outlet, a portable unit 7.5–12.5 amps, and a central system 12.5–23 amps on a 240V circuit. Those are the running watts from the table above divided by the supply voltage, because amps = watts ÷ volts. The startup column is the same sum applied to the surge figures.
| Type and size | Running watts | Voltage | Running amps | Startup amps (a second or so) |
|---|---|---|---|---|
| Window, 5,000 BTU | 400–600W | 120V | 3.3–5.0A | 8–15A |
| Window, 8,000 BTU | 600–900W | 120V | 5.0–7.5A | 10–22.5A |
| Window, 12,000 BTU | 1,000–1,400W | 120V | 8.3–11.7A | 21–33A |
| Portable, 12,000 BTU | 1,100–1,500W | 120V | 9.2–12.5A | 21–33A |
| Inverter mini-split, 9,000–12,000 BTU | 300–1,000W | 120V or 230V | 2.5–8.3A at 120V, 1.3–4.3A at 230V | Negligible |
| RV rooftop, 13,500 BTU | 1,200–1,600W | 120V | 10.0–13.3A | 23–29A, or 11–15A with a soft starter |
| Central, 3 ton | 3,000–4,000W | 240V | 12.5–16.7A | 33–83A |
Startup amps are the surge rows from the previous section divided by 120V (240V for the central system), and the 8,000 BTU row uses the two-to-three-times rule for room units. The two 12,000 BTU rows are the ones to check against your wiring: at up to 11.7A and 12.5A running they reach the 12A a 15A circuit can carry continuously, as the circuits section below works out.
Why the same watts read as fewer amps at 240V
Doubling the voltage halves the current for the same power: 3,600W is 30A at 120V and 15A at 240V. That is why a central system pulling several thousand watts still shows only 12.5–23A. Two details change the numbers on a real nameplate:
- 115V against 120V. Some nameplates rate the unit at 115V, which adds about 4% to the amps: 1,200W is 10.0A at 120V and 10.4A at 115V. The same applies at 230V against 240V.
- Power factor. Watts ÷ volts is a close estimate for a compressor, not an exact one. A motor load draws somewhat more amps than that division gives, so when the nameplate lists a higher figure than this page, use the nameplate.
To go from BTU to amps in one chain, divide by the efficiency rating to get watts, then by the voltage. A 12,000 BTU unit at EER 11 draws roughly 1,090W, which is 1,090 ÷ 120 = 9.1A. The same capacity at EER 14 draws 12,000 ÷ 14 = 857W, or 7.1A.
Circuits and the 80% rule
The National Electrical Code limits a continuous load to 80% of a circuit’s rating, and an air conditioner on a hot afternoon is a continuous load. A 15A circuit therefore carries 12A (1,440W at 120V) and a 20A circuit 16A (1,920W). Plug-in window and wall units also have a stricter rule of their own: under NEC 440.62 a room air conditioner’s nameplate amps may reach 80% of the circuit rating (12A on a 15A circuit) only when it is the only load on that circuit, and no more than 50% (7.5A on 15A, 10A on 20A) when the circuit also feeds lights or other outlets. A 12,000 BTU window unit at 8.3–11.7A therefore needs a 15A circuit to itself, or a 20A circuit (also to itself if its nameplate is above 10A). Which code edition applies depends on what your state or town has adopted, so have a licensed electrician check the circuit before you add a large unit. Central systems are sized differently: the installer picks the dedicated circuit from the minimum circuit ampacity (MCA) printed on the outdoor unit’s data plate, not from running amps. That plate also lists RLA, the running load amps, next to the LRA mentioned earlier. The central rows above include the indoor blower, which can be on a separate circuit, so an outdoor unit’s own plate may read lower than the table.
Plug it straight into the wall
GE’s owner’s manual for its AHD06 and AHD08 window units labels extension cords a fire risk that could cause serious injury or death, and says not to use one, nor a surge protector, a multi-outlet adaptor or an adapter plug; the unit should have its own grounded outlet. The plug on the cord is a current interrupter: GE says to test it periodically by pressing TEST, then RESET, while it is plugged in, and to stop using the unit and call a qualified service technician if TEST does not trip it or RESET will not stay engaged. GE also says to wait at least 3 minutes after switching the unit off before switching it back on, so the compressor does not overload.
On a power station, plug the cord straight into one of its AC outlets, not a power strip. A generator is different. CPSC says to run one outdoors only, at least 20 feet from the home with the exhaust directed away from windows, doors and vents, never in a garage even with the door open, and to keep battery-operated or battery-backup CO alarms on every level. That distance collides with the no-extension-cord rule, so settle with a licensed electrician before the outage how a generator will feed a window unit. Never backfeed the house by plugging a generator into a wall outlet, which CPSC warns is an electrocution risk to utility workers.
Power stations: amps, volts and the 240V question
A power station’s spec sheet is in watts, but the conversion is the same. 1,800W continuous is 15A at 120V and 2,400W is 20A. For a 12,000 BTU window unit, the 21–33A startup range means a surge rating of 2,500–4,000W.
Voltage is the other thing to read. Many portable power stations are 120V only, which suits window, portable and RV units but cannot feed a 240V compressor. Some units do offer 240V. The Jackery Explorer 5000 Plus is one: it is listed at 7,200W continuous, 14,400W surge and 5,040Wh, and some other brands sell 240V connect boxes or kits for particular models, so check the exact model on the spec sheet. At 240V the amps halve: a 3,500W load is 29A at 120V but 14.6A at 240V.
That makes voltage a solvable problem for central air and leaves capacity and startup as the real barriers. At 3,500W, the middle of the 3 ton running range, 5,040Wh gives 5,040 × 0.87 ÷ 3,500 = 1.25 hours, about an hour and a quarter, and a 3 ton compressor’s 8,000–20,000W startup range runs past that unit’s 14,400W surge rating at the top end.
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.
That advice has a limit. CDC says to use fans only while indoor temperatures are below 90°F, because above that a fan can raise body temperature. Past that point you need air conditioning or a place that has it; dial 2-1-1 or contact your local health department to find one. CDC lists people 65 and older, young children, pregnant women and people with heart disease or asthma among those at greater risk, so plan around the 90°F line if your household includes any of them.
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 the US average residential rate of about 18 cents per kilowatt-hour (EIA, January–July 2026), 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, from about 12 cents in North Dakota to about 46 cents in Hawaii over the same months, and the average flatters some readers badly.
- Window, 5,000 BTU, 280 kWh a year: about $50
- Window, 8,000 BTU, 420 kWh: about $76
- Window, 12,000 BTU, 670 kWh: about $121
- Portable, 12,000 BTU, 700 kWh: about $126
- Central, 3–5 ton, 1,700–3,100 kWh: $306 to $558
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 an older single-speed window or portable unit, and closer to 600W for an inverter mini-split of the same capacity. A cooling-only 12,000 BTU window unit made since May 26, 2026 must reach CEER 16.0, or 750W on paper; its nameplate gives the full-speed draw. 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?
About 40 to 60 cents for a 5,000 BTU window unit and $1.00 to $1.40 for a 12,000 BTU one, at about 18 cents per kilowatt-hour with the compressor on roughly 70% of the time. A 3 ton central system over the same eight hours is about $3 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.