Generator Size Calculator
Calculate the recommended generator size from running watts, motor surge watts, and a margin.
Estimates for planning only — verify against local codes and a qualified electrician before purchasing or installing a generator.
How it works
- 1Enter the total running watts of everything you need to power at once.
- 2Enter the starting (surge) watts of the largest motor — often 2–3× its running watts.
- 3Set a margin (default 20%); the result is the larger of running × margin and running + surge.
Use cases
- Sizing a portable generator for a jobsite with tools and lighting.
- Choosing a home standby generator for AC, fridge, and essential circuits.
- Planning backup power for a sump pump or well pump with high startup loads.
Frequently asked questions
Why cover both running and surge watts?
Motors draw a large inrush at startup (2–3× running watts). The generator must supply the running load plus that surge, so the size is the maximum of (running × margin) and (running + surge).
What sizing margin should I use?
A 20–25% margin is standard. A 3000 W running load at 20% needs at least 3600 W continuous, leaving headroom for losses and growth.
How do I find an appliance’s surge watts?
Check the nameplate for starting watts or LRA. If only running watts are listed, multiply motors by 2–3 as a conservative estimate; heaters and lights have little surge.
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