Generator Sizing Calculator — What Size Standby Generator?

What size standby generator do you need? Check the loads you want to back up and this calculator totals the running watts, adds motor starting surge, applies a 20% headroom margin, and recommends a standby generator class — 14 kW, 18 kW, 22 kW, 26 kW, or liquid-cooled 30 kW+.

Select the loads you want to power during an outage
Running Total
0 W
With Surge + 20% Margin
0 W
Recommended Class
—
Check at least one load to get a recommendation.

Running watts vs. starting watts

Motors and compressors (air conditioners, well pumps, refrigerators, furnace blowers) draw two to three times their running wattage for a few seconds at startup. A standby generator must cover the highest single starting surge on top of everything already running. This calculator adds the largest surge in your selection to the running total, then applies a 20% headroom margin so the generator is never running flat-out.

Air-cooled standby units top out around 26 kW — enough for most homes up to roughly 3,500 sq ft with one central AC. Larger homes, multiple AC systems, or all-electric loads typically step up to liquid-cooled 30–60 kW units. Wattages shown are typical values; check your equipment nameplates for exact numbers.