Window AC units have one of the most demanding starting surges of any household appliance. Here's the wattage math by BTU size — and which generator to buy so you stay cool when the grid goes down.
Air conditioning is often the reason people buy a generator in the first place — and also the reason many generators trip under load. The culprit is the compressor motor's starting surge, which can be 2–3× the unit's running wattage. Get this number wrong and your generator shuts down the moment the AC cycles on.
💡 Running more than just the AC? Use our free generator size calculator to add your AC plus refrigerator, lights, and any other loads for a precise total recommendation.
Window AC units are rated in BTUs (British Thermal Units). The BTU rating directly determines watt draw. Use this table to find your unit's approximate wattage requirements:
| Window AC Size | Running Watts | Starting Watts (Surge) | Min. Generator Size |
|---|---|---|---|
| 5,000 BTU | 450–500 W | 1,200–1,500 W | 2,000 W |
| 6,000 BTU | 550–600 W | 1,500–1,800 W | 2,000 W |
| 8,000 BTU | 700–800 W | 1,800–2,200 W | 3,000 W |
| 10,000 BTU | 900–1,200 W | 2,200–3,000 W | 3,500 W |
| 12,000 BTU | 1,100–1,400 W | 2,700–3,500 W | 5,000 W |
| 14,000 BTU | 1,250–1,500 W | 3,000–4,000 W | 5,000 W |
| 18,000 BTU | 1,700–2,000 W | 4,200–5,000 W | 7,500 W |
| 24,000 BTU | 2,200–2,800 W | 5,500–7,000 W | 10,000 W |
Important: The "Min. Generator Size" column is the generator starting (surge) wattage you need for the AC unit alone. Add 1,500–2,000W of headroom to also run a refrigerator, lights, and chargers.
A 10,000 BTU window AC running at 1,100 watts sounds manageable — but when it starts (which happens every 10–20 minutes as the thermostat cycles), the compressor motor draws up to 3,000 watts for 2–3 seconds. During that surge, your generator must supply all its normal loads plus the 1,900W additional starting demand.
If the generator's peak/surge rating is below the combined demand at the moment of startup, it trips the overload circuit or shuts down entirely. This is the most common generator complaint: "it runs fine, then shuts off randomly" — almost always caused by a motor starting surge that exceeds the generator's peak capacity.
The fix: match generator surge wattage (not running wattage) to your AC's starting demand plus your other running loads. See our complete sizing guide for the full math walkthrough.
Running watts: AC (1,100W) + fridge (175W) + lights (100W) + TV (125W) = 1,500W running.
Largest surge: AC starting adds ~2,200W.
Peak demand: 1,500 + 2,200 = 3,700W peak.
Recommended: 5,000W generator (typically ~6,250W surge). This leaves meaningful headroom for chargers and a microwave burst.
Two 8,000 BTU units (750W each running) + fridge + lights:
Running: 1,500 + 175 + 100 = 1,775W running.
Largest surge (one AC starting): 2,000W.
Peak: 1,775 + 2,000 = 3,775W peak.
Recommended: 5,000–6,500W generator with at least 6,250W surge. A dual-fuel 6,500W model is ideal here.
Running the AC alone with a small fan and lights: AC (1,300W) + fan (75W) + lights (50W) = 1,425W running.
Surge: ~3,500W starting burst.
Peak: 1,425 + 3,500 = 4,925W peak.
Recommended: 5,000W generator at minimum. Using a pure sine wave inverter generator in this size range gives the cleanest power for the AC's electronics.
A 2,000–3,000W inverter generator is quiet, fuel-efficient, and provides clean power for the AC and your electronics. Perfect for a bedroom AC unit during an outage or for camping with portable AC.
→ Browse 2,000W Quiet Inverter Generators on Amazon
A 5,000W generator with at least 6,000W surge handles the most common window AC size plus a full essential load. Choose electric-start for convenience and a model with multiple 20A outlets for flexible hookup.
→ Browse 5,000W Generators with Electric Start on Amazon
Large window units and multiple-AC setups need a 7,500W generator with 9,000W+ surge. Dual-fuel models provide flexibility during extended outages when gasoline availability is uncertain.
Yes — but only a small one. A 5,000–6,000 BTU window AC that draws around 500W running and ~1,500W starting is within range of a 2,000W generator (which typically surges to 2,200–2,400W). Larger units (8,000 BTU+) will usually overload a 2,000W generator during the starting cycle. Check your AC's spec label for the exact starting amp draw.
Possibly — it depends on the specific generator's surge capacity. A 3,500W generator typically surges to 4,000–4,375W. A 10,000 BTU window AC needs up to 3,000W to start. If the AC startup surge is within the generator's peak capacity and you're not running much else simultaneously, it can work. But the margin is thin, and we'd recommend a 5,000W generator for reliable operation.
Yes, with two caveats. First, use a properly-sized generator (surge wattage meets or exceeds starting demand). Second, use a quality extension cord — 12 AWG minimum, rated for outdoor use, and no longer than necessary (voltage drop increases with cord length). An inverter generator's pure sine wave output is gentler on the AC's electronics than a conventional generator's modified sine wave.
Running watts range from about 450W for a 5,000 BTU unit to 2,800W for a 24,000 BTU unit. Check the EnergyGuide label or the nameplate on your AC for the exact wattage and amperage. Multiply amps × volts to get watts (e.g., 8.5A × 120V = 1,020W).
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