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.
List running and starting loads separately
Motor-driven appliances can require a short starting surge above their normal running watts. Record the nameplate or manufacturer data for each intended load and identify which motors may start at the same time. Do not simply add every possible surge if operating procedures prevent simultaneous starts.
Fuel type, temperature, altitude, extension-cord length, and generator power quality can affect usable output. Follow the generator manufacturer’s continuous and surge ratings.
Safety requirements are not optional inputs
Portable generators must be operated outdoors at a safe distance from openings and never in a garage, house, or enclosed area because carbon monoxide can be fatal. Connection to building wiring requires approved transfer equipment and qualified installation; never backfeed through an outlet.
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.