What Size Standby Generator Do I Need? kW Sizing Guide (2026)

Undersizing is the most expensive generator mistake: the unit trips the moment your AC compressor kicks in, and you’re back in the dark with a $12,000 paperweight. Here’s how to get the number right.

The method in 4 steps

1. List what stays on. Fridge, freezer, furnace fan or heat, lights, internet, sump pump, well pump if you have one, and air conditioning. Be honest — “everything” is a valid answer, it just costs more.

2. Add running watts. Typical figures: fridge 200 W, furnace fan 800 W, sump pump 800 W, well pump 1,000 W, electric dryer 5,000 W, electric range 8,000 W, EV charger 7,200 W. AC by tonnage: 2-ton 2,500 W, 3-ton 3,500 W, 4-ton 5,000 W, 5-ton 6,000 W.

3. Add the biggest starting surge. Motors surge when they start — a 3-ton AC needs ~12,000 W for a few seconds. Take the single largest (surge − running) and add it once. You don’t stack surges; they don’t all start at the same instant.

4. Add margin and round up. Add 20–25%, then round up to the next standard size: 14, 18, 20, 22, 24, or 26 kW.

The fuel derate trap

This is where buyers get burned. Several popular models are rated at full kW on propane but derate ~2 kW on natural gas — the Kohler 20RESCL is 20 kW on LP but 18 kW on NG; the Champion 14kW is 14 on LP, 12.5 on NG. If you’re on natural gas, do the entire calculation against the NG rating.

Rules of thumb (then verify)

• 14 kW: essentials + one small AC, up to ~1,800 sq ft, with load management.
• 18–20 kW: most of a 1,500–2,500 sq ft home, one central AC zone.
• 22–26 kW: true whole-house, 2,500+ sq ft, multiple AC zones.

Rules of thumb start the conversation; the kW size calculator finishes it. And once you know your size, the installation cost estimator tells you what the project really costs.