How to Size a Backup Generator for Your House

The most common generator mistake is guessing. People buy whatever's on sale, then find out in the first outage that it can't start the sump pump and the fridge at the same time. Sizing it takes about 20 minutes with a pencil.

01Running watts vs. starting watts

• Running (rated) watts: what an appliance uses while it runs.
• Starting (surge) watts: the extra burst a motor needs for a second or two to start. Fridges, freezers, sump pumps, well pumps and furnace blowers can need two to three times their running watts to start.

Generators list both: "running watts" (or rated) and "starting watts" (or peak). Size by running watts, and make sure the starting watts cover the biggest motor starting while everything else runs.

02Step 1: List what must run

Start with your must-run list from our power station vs. generator guide: sump pump, refrigerator, freezer, furnace blower, well pump, medical devices, lights, phone charging, internet.

03Step 2: Find the watts

Look at the label on each appliance or its manual, which lists watts, or volts and amps (watts = volts × amps). Typical ranges, only as a rough guide:

• Refrigerator: several hundred running watts, with a bigger starting surge.
• Sump pump (1/3 to 1/2 hp): several hundred to around a thousand running watts, with a large surge.
• Gas furnace blower: several hundred watts.
• Well pump: often the biggest load in a rural house, with a big surge; check its rating.
• Lights, router, phone chargers: small loads that add up.

Always use your actual labels.

04Step 3: Add it up

1. Add the running watts of everything you want on at the same time.
2. Find the single biggest starting surge (often the sump or well pump) and add the difference between its starting and running watts.
3. Add a margin of 10–20%.

That's your minimum generator size.

05Step 4: Manage the load

You don't have to run everything at once. Cycle the freezer, run the dryer never, and stagger big motors. A smaller generator managed smartly beats a bigger one you can't afford to fuel.

06Inverter or conventional?

Inverter generators make cleaner power for electronics, are quieter and more fuel efficient, and cost more per watt. Conventional open-frame generators give more watts for the money. For a sump pump and fridge, either works; for sensitive electronics, inverters are nicer.

07Fuel matters too

Bigger generators burn more fuel. Check the run time at half load, and plan how much fuel you'll store and rotate.

08Connect it safely

Extension cords for a few appliances, or a transfer switch or interlock installed by a licensed electrician to run house circuits. Never backfeed through an outlet. See transfer switch vs. interlock kit and generator safety.

Related: Midwest Power Outages: How Long They Really Last (NIPSCO's August Storm)