Generator Size Calculator

Add up running and starting watts for the appliances you need during an outage and get a recommended generator size with a safety buffer.

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Results will appear here once you enter your values.

Running watts vs starting watts

Running (rated) watts are what an appliance draws once it is operating.

Starting (surge) watts are the brief spike, usually one to three seconds, that an induction motor needs to get moving.

Resistive loads such as lights, toasters, space heaters and water heaters have no surge.

Compressor and pump motors do: a refrigerator that runs at about 700 W can draw 2,200 W at start-up, and a 3-ton central air conditioner can need three times its running load unless it has a soft-start kit.

The sizing method

Manufacturer wattage worksheets (Generac, Honda, Champion) use the same approach: add the running watts of everything that will run at once, then add the largest single extra surge (starting minus running watts) among the motor loads, because motors rarely start at the same instant.

Example: refrigerator 700/2,200 W, 1/2 HP sump pump 1,050/2,150 W, furnace blower 875/2,350 W, LED lights 100 W and electronics 150 W total 2,875 running watts.

The biggest extra surge is the refrigerator's 1,500 W, so the peak is 4,375 W.

With a 20% buffer you need a generator rated about 3,500 running and 5,300 starting watts.

Why add a 20-25% buffer

Portable generators are designed to run continuously at no more than about 80% of rated output; sustained full load raises fuel use, overheats the alternator and causes voltage sag that stresses motors and electronics.

A buffer also covers nameplate figures that understate real draw, aging motors that surge harder, and the extra item someone plugs in during a long outage.

If you plan to add a well pump or central AC later, size for it now.

Watch the 240-volt loads

Well pumps, central AC, electric ranges, dryers and water heaters usually run on 240 V.

A generator's total wattage is split across two 120 V legs and a 240 V outlet, so check that the model has a 240 V outlet (L14-30 or 14-50) and enough current on it.

Inverter generators under about 4,000 W often lack 240 V output entirely.

Connect it safely

Carbon monoxide from portable generators kills people every year, according to the U.S.

Consumer Product Safety Commission.

Run the generator outdoors only, at least 20 feet from the house with the exhaust pointed away from doors, windows and vents, never in a garage even with the door open, and install battery-powered CO alarms.

Never back-feed the house by plugging into an outlet; that can electrocute utility line workers.

Use a transfer switch or breaker interlock installed by a licensed electrician.

Frequently Asked Questions

Common questions about the Generator Size Calculator

For essentials (refrigerator, sump pump, furnace blower, lights and electronics) most homes need 3,500-5,000 running watts. Adding a well pump or window AC pushes that to 6,000-8,000 W, and whole-home coverage including central air and electric appliances typically takes a 15-22 kW standby generator.

Sources & References

Generator Sizing Guide

Appliance running and starting wattage references and the running-plus-largest-surge method.
→Source: Generac

Generator Wattage Calculator

Manufacturer running and starting watt figures for household appliances.
→Source: American Honda Power Equipment

Portable Generators Safety

Carbon monoxide hazards, 20-foot placement guidance and CO alarm recommendations.
→Source: U.S. Consumer Product Safety Commission

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