Quick Answer: A window air conditioner’s wattage runs roughly 400-500W for a 5,000 BTU unit up to 1,500-2,200W for an 18,000 BTU unit, which translates to about 0.8 amps per 1,000 BTU on a standard 115V circuit. The reason this matters isn’t just your electric bill — it’s whether the unit needs its own circuit. Under NEC 210.20(A), a continuously-running load like an AC compressor can’t exceed 80% of a breaker’s rating, so a 12-amp unit on a 15-amp circuit is already at the legal ceiling with zero room for anything else. That’s the real cause behind most “why does my AC keep tripping the breaker” complaints, and it’s also why units above roughly 15,000 BTU move to a 230V circuit instead of straining a standard 115V outlet.
Last updated 2026-08-15: reconfirmed the NEC 210.20(A) 80%-continuous-load rule and the 0.8-amps-per-1,000-BTU estimate against current sourcing — both unchanged.
Most people only think about wattage when the electric bill jumps. The more urgent question is electrical: does the outlet you’re about to plug into have enough headroom, and will the breaker survive the first hot afternoon when the compressor is running flat-out? The answer comes down to two numbers — watts and amps — and one federal wiring rule most buyers have never heard of.
Watts and amps by BTU size
Watts and amps describe the same draw two different ways — watts is the total electrical load, amps is the current through the wire, and the two are linked by voltage (watts = volts × amps). Real-world draw varies by efficiency, so the ranges below reflect that spread rather than a single fixed number.
| Cooling capacity | Running watts | Running amps (115V) | Typical circuit |
|---|---|---|---|
| 5,000 BTU | ~400-500W | ~3.6-5.4A | 115V / 15A shared |
| 6,000 BTU | ~500-600W | ~4.4-6.5A | 115V / 15A shared |
| 8,000 BTU | ~600-1,000W | ~5.8-8.7A | 115V / 15A shared |
| 10,000 BTU | ~850-1,250W | ~7.3-10.9A | 115V / 15A, dedicated recommended |
| 12,000 BTU | ~1,000-1,500W | ~8.7-13A | 115V / 15-20A dedicated |
| 15,000 BTU | ~1,250-1,850W | ~10.9-16.3A | 115V / 20A or 230V dedicated |
| 18,000 BTU | ~1,500-2,200W | ~13-19.6A | 230V dedicated |
Source: BTU-to-amperage ratios compiled from manufacturer nameplate data across window and portable units (learnmetrics.com).
The rule of thumb worth remembering: about 0.8 amps per 1,000 BTU on a 115V circuit, or about 0.44 amps per 1,000 BTU on a 230V circuit. Multiply either by the unit’s rated BTU and you get a close working estimate of running amps — enough to check against a breaker’s rating before you buy, though the unit’s own nameplate spec (printed on the back or in the manual) is always the number to trust once you own it.
Kill A Watt-style plug-in power meter
- Confirms actual draw against the nameplate spec instead of estimating from BTU alone.
- Also catches a failing capacitor early — a unit drawing noticeably more than its rated amps is a warning sign before it trips a breaker.
Don’t want to guess whether your outlet can handle the unit you’re eyeing — Prime delivers a power meter or a new window AC to your door in two days, and you can try it free for 30 days.
The 80% rule that decides whether you need a dedicated circuit
This is the part most buyers skip past. The National Electrical Code, in section 210.20(A), requires that a circuit carrying a continuous load — defined as one expected to run for three hours or more at a stretch, which describes almost any AC compressor on a hot afternoon — be sized so that load never exceeds 80% of the breaker’s rating. A 15-amp breaker, in practice, is only rated for 12 amps of continuous draw; a 20-amp breaker, for 16 amps.
Run the math against the table above and the reason a 12,000 BTU window unit “needs its own circuit” stops being a vague recommendation and becomes an actual number: a 12,000 BTU unit at 8.7-13 amps can already sit right at or past that 12-amp ceiling on a 15-amp circuit by itself, before you’ve plugged in anything else. Add a lamp, a fan, or a phone charger on the same breaker and you’re now asking a 15-amp circuit to carry more than its legal continuous limit — which is exactly the scenario that trips a breaker on the first truly hot day, not a defective unit or a bad breaker.
Below roughly 8,000 BTU, most units draw little enough that sharing a circuit with light loads is usually fine. Above that, a dedicated circuit — one breaker, one outlet, nothing else plugged into it — isn’t overcaution, it’s the code-compliant way to run the unit continuously without nuisance trips.
Why it keeps tripping: startup surge and extension cords
Two more electrical realities compound the 80% math. First, startup surge: every compressor draws a brief spike of two to three times its running amps the instant it kicks on, because an electric motor pulls the most current at the moment it starts spinning from a standstill. A circuit that’s already near its continuous-load ceiling from running-amp math alone can get pushed over the edge by that half-second spike, especially on an older breaker with a lower trip tolerance.
Second, extension cords. Nearly every window AC’s owner’s manual explicitly warns against them, and it isn’t boilerplate — a standard household extension cord’s wire gauge is sized for a lighter, steady load, and pushing an AC’s running amps plus its startup surge through undersized wire for hours generates heat inside the cord’s own insulation. That’s a real fire risk, which is why the fix for an outlet that’s out of reach is a licensed electrician adding a proper outlet, not a cord bridging the distance.
Heavy-duty appliance-rated extension cord
- Look for a cord explicitly rated for air conditioner or major-appliance amperage, not a generic light-duty cord.
- Still not a substitute for a proper dedicated outlet — check your specific unit's manual before using any cord at all.
When you need 230V instead of 115V
Every unit up through roughly 15,000 BTU is built for a standard 115V household outlet. Push past that — the 18,000-25,000+ BTU class covered in industrial and large-room use — and running amps at 115V would climb high enough to demand a 20-amp or larger dedicated circuit with a specialized outlet, so manufacturers instead build these units for a 230V circuit, which roughly halves the amperage for the same wattage (watts = volts × amps, so doubling voltage halves current for the same power). A 230V unit needs its own dedicated circuit and a different outlet shape than a standard 115V plug — not something you retrofit into an existing wall outlet without an electrician.
Our commercial portable air conditioner guide covers the industrial spot coolers where this 230V threshold matters most.
The bottom line
Match the unit’s rated BTU to the amp-draw table above before you buy, not after — about 0.8 amps per 1,000 BTU on a 115V circuit is close enough to check against your breaker’s rating. Anything above roughly 8,000 BTU is a strong candidate for its own dedicated circuit under the NEC’s 80%-continuous-load rule, and anything above 15,000 BTU likely needs 230V entirely. If a unit you already own keeps tripping the breaker, check for a shared circuit first — it’s the free fix — before assuming the unit itself is failing.
Sizing a new unit to a room instead of just a circuit? Start with our BTU sizing guide, or see the full lineup in our best window air conditioner guide. For the running-cost side of wattage — what a less efficient unit actually adds to your electric bill over a season — see our portable AC energy efficiency guide. Worried about a voltage spike frying the compressor rather than just tripping a breaker? Our air conditioner surge protector guide covers the direct-plug device that fits a window or portable unit — a standard power strip isn’t rated for either job. And if a unit keeps shutting off and restarting every few minutes rather than tripping the breaker outright, that’s a separate problem — see our air conditioner short cycling guide.