What Size Generator Do I Need for My House?

Northlight Publishing · Updated September 2026

Buying a generator after the power goes out is how people end up with the wrong size at the wrong price. Sizing takes about 20 minutes with the right method.

Step 1: decide what must run

List only what you truly need during an outage: refrigerator, freezer, a few lights, phone charging, internet, a furnace blower or a window AC, a sump or well pump, and any medical equipment. Whole-home coverage is a different project entirely.

Step 2: add running watts

Every appliance has a running (rated) wattage, usually on its label. A refrigerator might run around 150 to 800 watts, a furnace blower several hundred, and lights and electronics much less. Add up everything that could run at the same time.

Step 3: account for starting surge

Motors draw a burst of extra power for a second or two when they start. Refrigerators, well pumps, and air conditioners can need two to three times their running watts to start. Add the single largest starting surge to your running total, because appliances rarely start at the exact same moment.

Step 4: match the machine

A portable inverter generator in the 2,000 to 3,500 watt range covers essentials for many homes. Well pumps and central air push you higher. Battery power stations are silent and safe indoors, and handle fridges, lights, and electronics well, but struggle with high-draw loads for long periods.

The safety rules that matter more than size

Never run a fuel generator indoors, in a garage, or near windows: carbon monoxide kills people every storm season. Never plug a generator into a wall outlet, which can backfeed the grid and electrocute line workers. Use a transfer switch or interlock installed by a licensed electrician, or heavy-gauge extension cords run directly to appliances.

The Home Backup Power Handbook walks through all of this in plain English, with a fill-in sizing worksheet, an appliance wattage table, a comparison of every backup option, and full carbon monoxide and fuel storage guidance.