Generator Size Calculator
Estimate required generator running and peak watts from continuous loads, startup demand, and safety margin.
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How this calculator works
Recommended running capacity = running load × (1 + margin). Recommended peak capacity = (running load + largest simultaneous startup demand) × (1 + margin), rounded up.
Worked example
A 4,200 W running load plus 1,800 W startup demand with 20% margin suggests about 5,100 running watts and 7,200 peak watts.
Sources & assumptions
The method adds the largest expected startup demand to continuous load, applies a margin, and rounds up. Verify nameplate loads, simultaneous starts, voltage, fuel derating, transfer equipment, and electrical requirements with qualified guidance.
Frequently asked questions
Why not add every appliance’s starting watts?
Motors do not always start at the same moment. A common planning method adds the largest expected simultaneous startup demand, but load sequencing matters.
Can this size a home standby system?
It is an estimate only. Fuel type, voltage, phase, transfer equipment, load management, altitude, temperature, and electrical code require professional review.
Generator sizing and safety guide
Build a load list, compare running and startup watts, and review critical carbon-monoxide guidance in How to size a generator.