Generator Size Calculator

List your appliances with running and starting watts to get the total load, the surge demand and a recommended generator size.

Generator size calculator

Appliances, one per line as name, running watts, starting watts

Total running watts: —

Surge watts: —

Largest starting surge: —

Recommended generator size: —

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The surge load adds the largest starting surge to the other appliances, because motors rarely start at exactly the same moment.

List each appliance on its own line as name, running watts and starting watts.
Leave a line out if an appliance has no starting surge of its own.
Read the total running load, the surge demand and the recommended generator size.

Common questions about the free online generator size calculator.

Add up the running watts of everything you need, then add the largest starting surge to get the peak demand the generator must survive.

Running watts are the steady load, while starting watts are the brief surge a motor draws for a second or two when it starts up.

A margin of about 25 percent above the running load and 10 percent above the surge is a common and practical guide.

No. Usually only one large motor starts at a time, so the surge calculation adds just the biggest starting surge to the rest.

Motors with compressors are the worst, such as air conditioners, fridges and well pumps, which can draw three times their running watts.

Whole house backup needs a much larger unit, so most homes use a transfer switch and size the generator for the essential circuits only.

Rounding up is sensible. A slightly larger unit runs cooler and quieter, and it leaves room for an appliance you add later.

Generator ratings assume a power factor close to one. Check that the voltage and frequency match your appliances before you buy.

No. The load list is calculated in your browser, so the appliances you type are never sent to a server or stored.

Yes. It is free to use, needs no account, and you can plan as many backup load lists as you like.

Last updated: October 10, 2026