HomeBuildCalcs.com: The Plumb and Level Figures for the Hands-On Homeowner
Meet the Calcs Family →

Generator Size Calculator

Running and starting watts of your essential loads to a recommended generator size.

Your numbers
Your essential loads

Recommended generator size

Enter your numbers above.

Starting watts, not running watts, size the generator

A refrigerator or well pump motor can draw 2 to 3 times its running wattage for a fraction of a second at startup. A generator sized only for your total running watts can stall or trip the moment that motor kicks on, even though it has plenty of capacity once everything is already running.

Add up the running watts of everything you want powered at once — refrigerator, sump pump, a few lights, a furnace blower — that total is your steady-state load. Then find the single largest motor in that list (refrigerators, well pumps, and furnace blowers are common culprits) and add its starting surge watts on top of the running total, since only one motor typically starts at a time.

Generators are rated two ways: running (rated) watts, which they can sustain continuously, and surge (starting) watts, a higher number they can supply briefly. Your generator needs a running rating above your total running watts, and a surge rating above your running total plus that one largest starting surge.

A safety margin of 15-20% on top of both numbers leaves headroom for adding a load later or for a motor that draws slightly more than its label states, and keeps the generator from running flat-out at 100% capacity continuously.

Where these numbers come from

These are planning estimates only. Electrical work must meet the National Electrical Code and your local code, and should be installed or inspected by a licensed electrician. This is not a substitute for a permit or a licensed electrician's judgment.

Frequently asked questions

Where do I find a device's starting watts?

Check the appliance's nameplate or owner's manual; many list both running and starting (or LRA, locked rotor amps) figures for motor-driven appliances. If only running watts are listed, 2-3x running watts is a common estimate for a motor's starting surge.

Do I need to add up every appliance's starting watts?

No — add only the largest single motor's starting surge to your total running watts, since generators and household loads only rarely have every motor starting at the exact same instant.

Should I size for everything in my house, or just essentials?

Most portable and many standby generators are sized for essential loads only — refrigerator, some lighting, a sump or well pump, maybe a furnace blower — not the whole house's simultaneous peak draw, which would require a much larger and more expensive unit.

More Electrical calculators

Electrical Load Calculator Voltage Drop Calculator

All HomeBuildCalcs tools