Continuous and startup power planning

Generator Size Calculator

Select appliances that may run together, enter their running and starting watts, and estimate continuous and surge output targets.

Appliance loads

Starting watts means total watts during startup, not extra watts above running. If a chart lists additional surge watts, add those to running watts before entering the total. For a device without a surge, enter its running watts in both fields. Values are per unit; quantity counts identical units.

Uncheck appliances excluded from the same running scenario. Excluded appliances do not contribute any demand. Add up to 50 rows; test separate selections for different usage plans.

The 20% starting margin is an editable planning assumption, not a code requirement or guarantee. Both output targets include the same margin. One-at-a-time mode assumes starts never overlap, even for appliances that cycle automatically or restart after an outage.

Use appliance manufacturer specifications or measured startup demand. Example wattages are illustrative, not specifications for your equipment. Inputs stay in your browser and are not saved.

How generator sizing is estimated

Continuous demand is the sum of running watts × quantity for selected appliances. With all units starting together, surge demand is the sum of total starting watts × quantity. With only one unit starting, surge demand is total running demand plus the largest single-unit difference between starting and running watts. This avoids counting that unit's running power twice. Required outputs equal the respective demand × (1 + margin / 100), rounded upward to whole watts.

Fuel choice does not change the load. Generator output can differ between gasoline, propane and natural gas, and ratings are model-specific. Compare against the manufacturer's ratings for your chosen fuel and site conditions. No fuel consumption, runtime, automatic load shedding, motor-start duration, apparent power (VA), power factor or circuit loading calculation is performed.

Professional assessment and safe operation

Have a licensed electrician assess any building connection, transfer equipment and whole-house requirements. This tool provides no wiring or installation instructions. Follow the generator manufacturer's manual. Never operate a fuel-burning generator indoors or in a garage; CPSC advises outdoor operation at least 20 feet from homes and buildings, with exhaust directed away from openings.

Sources reviewed October 5, 2026: Honda running and starting wattage guidance; Champion example of fuel-specific output ratings; CPSC generator carbon-monoxide guidance. Confirm whether a manufacturer's wattage chart means total or additional starting watts before using it here.

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