Generator Sizing Calculator

This calculator gives the minimum generator output for a set of loads you want to run at once. You enter the total running watts, the starting (surge) watts of the single biggest motor load and that load's own running watts, plus a safety margin. The result is the continuous rating to look for on a portable, inverter or standby generator.

Homeowners use it for power-cut planning — fridge, freezer, sump pump, furnace blower, lights — and contractors use it for tools on job sites. Motors in compressors, pumps and air conditioners draw several times their running power for a second or two when they start, and that brief surge is usually what decides the generator size.

The method assumes you start the largest motor last, after the other loads are running, and that only one big motor starts at a time. If several motors may start together, add their surges. For loads on a three-phase supply, the three-phase power calculator converts amps to kW.

This is the sizing method generator manufacturers use in their wattage worksheets: running total plus the largest additional starting load. Compare the result with the generator's running (rated) watts, not its peak or starting watts. Output falls with altitude and high ambient temperature; manufacturers' manuals give the derating, commonly a few percent per 1,000 ft of elevation. For a standby system connected to house wiring, NFPA 70 (NEC) Article 702 applies and a transfer switch is required.

Ranges are typical household figures. Always check the nameplate: running watts ≈ volts × amps, and motor nameplates may list locked-rotor amps (LRA) for starting current. Resistive loads like heaters have no meaningful surge.

• Comparing with peak watts. Generators advertise a higher starting or peak figure; your result must be within the running rating.
• Adding every surge. Surges last a second or two, so only the biggest one usually needs headroom — unless two motors (say a well pump and AC) can kick in together.
• Forgetting 240 V loads. Well pumps, central AC and electric ranges often need a generator with a 240 V outlet or a standby unit, not just more watts.
• Ignoring fuel and run time. A generator running at 50–75% of its rating is usually most fuel-efficient; one sized exactly to the peak spends most of its time lightly loaded or overloaded.
• Sensitive electronics. Inverter generators produce cleaner power for computers and modern appliances with electronic controls.

Portable generators produce carbon monoxide: the US Consumer Product Safety Commission advises running them only outdoors and far from doors, windows and vents, never in a garage, and fitting battery CO alarms indoors. Never back-feed a house through an outlet with a double-ended cord — it can energize utility lines and kill line workers. Use a transfer switch or interlock installed by a licensed electrician. For a battery-and-solar alternative, compare with the solar panel calculator and battery runtime calculator.