How to Size a Whole-Home Generator (Sizing Calculator + Guide)
By LumoWatt Editorial Team · Published 2026-04-01 · Updated 2026-08-27
Generator sizing & cost planner
This is an educational estimate, not an electrical load calculation, engineering plan, quote, or installation recommendation. Final sizing and installation must be completed by qualified professionals. Use this to get in the right ballpark before you request quotes.
Optional, for local pricing
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 to 14 kW): refrigerator, well pump, a few lights and outlets, one AC zone.
- Most of the home (16 to 20 kW): the above plus most lighting circuits and a larger AC system.
- Whole home (22 to 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 to 3 quotes from verified local installers for a real load calculation.
See our generators by home size guide for a home-size-based starting point, and once you have a rough size in mind, our whole house generator cost guide covers what installation actually costs. Ready to move forward? Browse license-checked installers in your area.
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.