Generator Size Calculator

What size generator do I need?

Refrigerator or freezer
Sump pump
Well pump
Furnace blower, 1/3 HP
Window AC, 12,000 BTU
Microwave, 1,000 W
Space heater
W

Your generator should handle

4,700starting watts

Typical wattages from a utility sizing guide, plus the largest starting surge. Check real nameplates, and use a transfer switch installed by a licensed electrician. Never run a generator indoors.

Generator load2,500 W runningRunning: 2,500 WLargest surge: 2,200 W
Total running watts2,500 W
Largest starting surge2,200 W
Amps at 240 V running10.4

Pick the loads you want running during an outage and see the running and starting watts you need.

How it works

Adds the running watts of your selected items, then adds the single largest additional starting watts, based on typical values in a utility sizing guide.

Before you order

  • Stagger motor starts so only one big surge hits at a time.
  • Well pumps and air conditioners are usually what decide the size.
  • Fuel and carbon monoxide safety matter as much as size. Keep a portable generator outdoors, away from windows.

Common questions

Do I need a transfer switch?

To power home circuits you need a transfer switch or interlock, installed by a licensed electrician. Never backfeed through a dryer outlet.

Why does starting watts matter?

Motors draw extra power to start. A generator that cannot supply it will stall even if running watts look fine.