How to Size a Whole-Home Generator (Sizing Calculator + Guide)
By LumoWatt Editorial Team · Reviewed by Licensed electrical contractor (reviewer bio pending) · Updated 2026-06-10
Why sizing matters
An undersized generator will trip under load and shorten its own lifespan; an oversized one costs more up front and burns more fuel than necessary. Sizing depends on which circuits you want backed up, not just square footage.
The three sizing tiers
- Essential circuits only (10–14 kW): refrigerator, well pump, a few lights and outlets, one AC zone.
- Most of the home (16–20 kW): the above plus most lighting circuits and a larger AC system.
- Whole home (22–26 kW+): everything, including multiple AC zones and an electric range/dryer.
How installers calculate it
A certified installer will do a load calculation based on your electrical panel's circuit list, not a square-footage rule of thumb — treat any quote that skips this step as a red flag.
Use the calculator above for a starting estimate, then get 2–3 quotes from verified local installers for a real load calculation.
Frequently asked questions
- Can I just size based on my square footage?
- No — square footage is a poor proxy for electrical load. Two identically-sized homes can need very different generator capacity depending on appliances, HVAC zones, and how many circuits you want backed up.
- What happens if I undersize my generator?
- The unit will trip its breaker or shut down under load, and repeated overload cycling shortens its lifespan.
Ready to install? Find installers in your city.