What Size Generator Do I Need? Running vs Starting Watts Explained

Updated October 23, 2026 · The Power Outpost buying guide

Short answer: add up the running watts of everything you want to power at the same time, then add the extra starting watts of the single biggest motor you’ll switch on while the rest are running (a fridge, freezer, well pump or sump pump). That total is the smallest generator that will do the job. Then leave headroom, because a generator shouldn’t sit at its maximum output for long. For basic outage needs (lights, refrigeration, some cooking and pumping water), University of Florida extension experts put an average household at about 3,000 to 5,000 watts.

Running watts vs starting watts

When you compare generators, size to the running number first. The bigger starting number on a listing only covers the short surge.

Starting watts “usually at least twice” running watts, and the 3,000–5,000 W household estimate: University of Florida IFAS Extension, “Portable Generators: Know Watt’s Right And Wrong”, read October 23, 2026.

How to size a generator in 5 steps

1. List what must run at the same time. Start with the essentials: refrigerator, freezer, a few lights, phone chargers, a sump or well pump, a furnace blower. Add comfort items (a microwave, a window AC, a TV) only if you really need them.
2. Find each item’s running watts. Look on the label or nameplate, or in the manual. If it lists only volts and amps, multiply them: watts = volts × amps (for example, 120 V × 5 A = 600 W).
3. Find the starting watts for anything with a motor. Check the manual. If it isn’t listed, plan for at least twice the running watts.
4. Add it up. Total running watts of everything on at once, plus the extra starting watts of the one biggest motor you’ll start while the others run.
5. Add headroom. Don’t plan to run the generator at its maximum. Pick one whose running watts sit comfortably above your total, so motors can cycle on without overloading it.

A worked example

The numbers below are made up to show the math. Use the figures on your own appliances’ labels.

2,400 running watts + 1,600 for the sump pump’s start = 4,000 watts at the moment the pump kicks on. In this example you’d look for a generator rated comfortably above 2,400 running watts that can also handle a 4,000-watt peak, and you’d stagger the start-ups so two motors never start at once.

Start things one at a time

Start the generator with nothing plugged in. Then plug in the most important, highest-wattage appliance first, and add the others one at a time, letting the generator settle after each. Overloading a generator can damage both the generator and your appliances, and the UF/IFAS experts advise against running it at maximum wattage for more than 30 minutes.

Load-up order and the 30-minute limit: same UF/IFAS Extension source, read October 23, 2026.

Portable, inverter or standby?

• Portable generators cover the essentials above with extension cords, or selected circuits through a transfer switch or interlock.
• Inverter generators are usually quieter and give cleaner power for electronics, but most are smaller. See our inverter vs conventional generator guide.
• Standby generators are wired into the house, start on their own when the power goes out, and come in much larger sizes. They need a professional install and often a permit.
• Battery power stations can carry small loads quietly indoors; size those with our power station sizing guide.

Safety, whatever size you buy

Never plug a generator into a wall outlet: that “backfeeds” power onto the utility lines. Use heavy-duty extension cords, or have an electrician install a transfer switch or interlock (see transfer switch vs interlock kit). Run it outdoors, well away from doors, windows and vents, because generator exhaust contains carbon monoxide; our generator placement guide covers distances and CO alarms.

Keep reading

See all generators and home backup gear, or read how to connect a generator to your house safely.

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