What Size Generator Do You Need to Run a Window AC Unit?

Running a window air conditioner on a generator can make a summer power outage bearable, but choosing the wrong size leads to stalled compressors, tripped breakers or wasted fuel. The right generator depends on the AC’s starting surge, what else you want to run at the same time, and whether you prefer a quiet inverter or a larger conventional unit. This guide gives clear sizing recommendations, fuel math and setup tips.

Quick sizing chart

Step-by-step sizing method

1. Find running watts: BTU ÷ EER/CEER (for example, 10,000 ÷ 11 ≈ 910W).
2. Estimate starting watts: about 2–3× running watts for fixed-speed compressors (1,800–2,800W).
3. Add other loads: refrigerator running 150–250W, lights and fans 100–300W, router 20W.
4. Calculate peak: other running loads + AC starting watts. If the fridge compressor could start at the same time, add its surge too.
5. Add 20% headroom for heat, altitude and aging equipment.

Worked example

A 10,000 BTU AC (950W running, ~2,500W starting), a fridge (200W running), fans and lights (200W) and a router (20W). Peak = 2,500 + 200 + 200 + 20 = 2,920W. With 20% headroom, aim for about 3,500 starting watts and 3,000 running watts. A 3,500W-class inverter handles this comfortably.

Inverter or conventional generator?

Inverter generators are quieter and more fuel-efficient, ideal for bedrooms and campgrounds where you will run the AC overnight. Their surge capacity can be tighter than conventional units of the same rating, so choose at least one size up or add a soft-start kit. Conventional generators provide more surge for the money but are louder, often 68–74 dBA.

Fuel and runtime

Soft-start kits

A soft-start module installed on the AC compressor reduces inrush current by 50–70%, letting a smaller generator start the AC. Many RV owners run 13,500–15,000 BTU units from 2,200–3,000W inverters this way. Installation typically takes 30–60 minutes and may affect the AC warranty, so check terms first.

Setup tips for safe operation

• Place the generator 20+ feet from windows and doors with exhaust pointed away.
• Use a 12-gauge or thicker extension cord for AC loads; avoid long thin cords that cause voltage drop.
• Start the generator and let it warm up before turning on the AC.
• Stagger startup: start the AC first, then the refrigerator.
• Install a battery-powered CO alarm in the room.

Common mistakes to avoid

• Sizing by running watts only. The AC may run at 900W but still stall a 1,500W generator on startup.
• Using a long, thin cord. A 100-foot 16-gauge cord can drop voltage enough to overheat the compressor. Use 12-gauge for runs up to about 50 feet.
• Starting everything at once. Let the AC start first, then add the fridge and smaller loads.
• Ignoring altitude and heat. Generators lose roughly 3–4% of output per 1,000 feet of elevation and some capacity in high heat.
• Running a 230V AC on a 120V-only generator. Check the AC’s plug and voltage before buying.

Avoiding these mistakes often lets a correctly sized generator run a window AC for years without trouble.

FAQ

Can a 2,000W generator run a window AC?

Yes for a 5,000–6,000 BTU unit, especially an inverter model or one with a soft starter. Larger units usually need 3,000W or more.

How long will a generator run a window AC on one tank?

A small inverter running a 6,000 BTU AC may last 6–9 hours per tank; a mid-size generator with a 10,000–12,000 BTU unit typically lasts 5–10 hours depending on tank size and duty cycle.

Bottom line

Size your generator for the AC’s starting surge plus everything else running at the same time, then add 20% headroom. For most 8,000–10,000 BTU window units, a 3,000–3,500W inverter is the sweet spot; soft starters and inverter ACs let you go smaller.