Generator Size Calculator — Running Watts & Surge

How to use

1. Tick every appliance that must run together during an outage.
2. Add custom rows for anything else — read its nameplate running watts.
3. Motor surge (pumps and AC ×3, fridge ×2) is applied automatically.
4. Read running watts, peak surge and the recommended class with 20% headroom.

About this tool

The right generator question is not what you can afford but what actually runs at the same time. Tick the appliances you need in an outage — refrigerator, sump pump, furnace fan, AC, lights, well pump and more — and this calculator sums running watts, then models motor-start surge automatically: three times running watts for pumps and AC compressors, twice for the fridge, with custom rows for anything with a nameplate. The result is total running watts, the realistic peak with the biggest motor starting, and a recommended size class with the 20% headroom professionals build in. The most important output is printed in red on the page: never run a generator indoors or in a garage — carbon monoxide kills. All sizing math stays on your device.

FAQ

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