Generator Size Calculator
Size a backup generator for a house from total running watts plus the surge of the largest motor, with headroom — running and starting watts and recommended kW.
How it works
surge = largest motor watts × (start multiple − 1) · peak = running watts + surge · recommended = each × 1.25 headroom
A generator has two ratings: continuous (running) watts and a higher short-lived starting (surge) watts. The running figure must cover everything on at once; the surge must cover the inrush of your single largest motor starting while the rest already runs, since motors draw several times their running current for a fraction of a second. Adding 25% headroom keeps the generator off its limit, where fuel efficiency and voltage regulation suffer.
Worked example
A home runs 6,000 W of loads, with a 1,500 W AC compressor as the biggest motor at a 3× start: surge = 1,500 × 2 = 3,000 W, so peak = 6,000 + 3,000 = 9,000 W. With 25% headroom that points to roughly a 7.5 kW running / 11.3 kW peak generator — a common 10 kW–12 kW standby unit.
Frequently asked questions
Do I add up every motor’s surge?
No — only the single largest motor’s surge, because motors rarely start at the exact same instant. You add that one surge on top of everything already running, which is the worst realistic case.
Running watts or starting watts — which sizes the generator?
Both. The running (rated) watts must exceed your steady load, and the starting (peak/surge) watts must exceed running load plus the largest motor’s inrush. A generator can fail either test, so check both.
What about the whole house at once?
For a portable unit you typically back up essentials (fridge, furnace fan, well pump, a few circuits). A whole-house standby with a transfer switch is sized from the full load calculation, which the electrical-load tool estimates.
Related calculators
Educational estimates only — electrical work must follow the National Electrical Code (NEC) and local amendments, and be done or inspected by a licensed electrician.