Generator size by appliance / window AC
What size generator runs a window air conditioner
Window air conditioner, 12,000 BTU: 3,250 W running plus 3,950 W more for a few seconds as it starts, so a generator needs at least 7,200 W of starting capacity to run it on its own. The smallest of the six portables this site covers that can do it is the Champion 201040 (7,500 W running, 9,375 W starting).
The three tables disagree more on window air conditioners than on any other device. Generac and EWEB both list a 12,000 BTU unit at 3,250 W running and 3,950 W additional starting; Champion lists an unsized window unit at 1,200 W and 2,000 W.
Those are very different answers: the Generac figure needs about a 7,200 W starting capacity, the Champion figure about 3,200 W. The unit's nameplate (volts × amps, or watts) settles which applies to yours.
This page sizes from the Generac row, the cautious one, and shows the Champion row next to it.
The figures and where they come from
| Device | Source | Running W | Additional starting W | Starting need alone |
|---|---|---|---|---|
| Window air conditioner, 12,000 BTU | Generac GP brochure (2023) | 3,250 | 3,950 | 7,200 |
| Window air conditioner, 12,000 BTU | EWEB (2019) | 3,250 | 3,950 | 7,200 |
| Window air conditioner, 12,000 BTU (Champion's row is an unsized window unit) | Champion flyer (2021) | 1,200 | 2,000 | 3,200 |
Highlighted rows are the figures the calculator uses. Every table here prints additional starting watts: the surge on top of running watts.
Which portables can run it
| Generator | Running / peak W | Alone | With fridge, 6 LED bulbs, 2 chargers |
|---|---|---|---|
| Honda EU2200i | 1,800 / 2,200 | No | No |
| Honda EU3000iS | 2,800 / 3,000 | No | No |
| Westinghouse iGen5000 | 3,900 / 5,000 | No | No |
| Honda EU7000iS | 5,500 / 7,000 | No | No |
| Champion 201040 | 7,500 / 9,375 | Yes | Yes |
| Westinghouse WGen9500DF | 9,500 / 12,500 | Yes | Yes |
A generator fits when its running rating covers every running watt and its peak (or "max") rating covers the running total plus the single biggest starting surge. Adding the basics — a refrigerator (700 W running, 2,200 W to start), six 15 W LED bulbs and two phone chargers — raises the running load to 4,080 W and the starting need to 8,030 W. The smallest portable here that covers that is the Champion 201040.
On a standby generator
A home standby unit is rated in kilowatts and has no overload capacity: Generac's rating definition says "No overload capability is available for this rating." Using Generac's own formula — sum of running watts plus the highest single starting burst — the window AC with the basics needs 8 kW. The smallest Guardian model that covers it on natural gas is the 10 kW, rated 9 kW on natural gas and 10 kW on propane, which leaves 1 kW of starting room for everything else. Whole-home lists are on the standby generator pages.
Before running anything, read the carbon monoxide distance rules and how a transfer switch keeps power off the utility lines. Other appliances: refrigerator · sump pump · well pump · furnace · central air · heat pump · space heater · water heater · washing machine · garage door opener · CPAP · oxygen concentrator.