GUIDE
How to Size a Generator for Your Home
Most people buy the wrong size because they guess from the price. A better way takes about ten minutes and a pen.
Two numbers matter
Running watts are what your appliances use while they are on. Starting watts are the extra burst some appliances pull for a second or two when a motor kicks in. Refrigerators, sump pumps, well pumps, air conditioners and furnace blowers all have a starting surge.
Your generator has to cover the running watts of everything you plan to run, plus the biggest single starting surge.
Step 1: List what you need to run
Think essentials first. Most households in a storm want a refrigerator and freezer, a few lights, internet and phones, a sump pump if they have one, and a way to keep cool or warm.
Step 2: Find the watts
Look for the label on each appliance, often on the back or inside the door. It lists watts, or volts and amps. Multiply volts by amps to get watts. If you cannot find it, use the typical numbers in our wattage calculator and treat them as estimates.
Step 3: Add it up
Add the running watts of everything. Then find the appliance with the largest extra starting surge, which is its starting watts minus its running watts. Add that to your running total.
Example, with typical numbers:
| Item | Running watts | Starting watts | Extra at start |
|---|---|---|---|
| Refrigerator | 200 | 800 | 600 |
| Sump pump (1/3 hp) | 800 | 1,300 | 500 |
| Gas furnace blower | 600 | 1,800 | 1,200 |
| Lights (about 10 LED) | 100 | 100 | 0 |
| Wi-Fi router and modem | 30 | 30 | 0 |
| Total running | 1,730 |
Largest extra start: 1,200 (the furnace blower). Peak need: 1,730 + 1,200 = 2,930 watts.
Step 4: Add headroom
Add about 20 percent so the generator is not at its limit all day. In the example, that is about 3,600 watts of peak capacity and about 2,100 running watts. Your numbers will differ, so use your own list.
Inverter or conventional
An inverter generator produces cleaner power for electronics and is usually quieter and more fuel efficient. A conventional open-frame generator is often cheaper per watt and suits heavy loads. For phones, laptops and a router, inverter is the safer choice.
Fuel
Gasoline, propane and dual-fuel units are common. Gasoline goes stale in storage, propane keeps well, and dual-fuel gives you a choice. Check how many hours a tank gives you at half load, not at the lightest load.
Safety comes first
CPSC's guidance: operate portable generators outside only, at least 20 feet from the house, and point the exhaust away from the home. Ready.gov gives the same distance from windows. Never run one indoors, in a garage or on a porch, even with the door open, and keep it away from doors, windows and vents. Look for models with a carbon monoxide shutoff, and CPSC's May 2026 release says models certified to the latest PGMA G300-2023 and UL 2201-2023 standards include carbon monoxide safety technology. Install battery-powered carbon monoxide alarms in the house.
What to do next
- Run your list through the wattage calculator.
- Read how to connect a generator to your home safely, since extension cords have limits and a hardwired circuit needs a transfer switch or interlock installed by a licensed electrician.
- Decide if a generator or a power station fits your outages. See our guide on that choice.
Sources
- CPSC Carbon Monoxide Information Center: https://www.cpsc.gov/Safety-Education/Safety-Education-Centers/Carbon-Monoxide-Information-Center
- CPSC, What to Know About Generators and CO (PDF): https://www.cpsc.gov/s3fs-public/468_WhattoKnowGenerators_2022_v5.pdf
- Ready.gov, Power Outages: https://www.ready.gov/power-outages
- CPSC generator warning, May 27, 2026: https://www.cpsc.gov/Newsroom/News-Releases/2026/CPSC-Warns-of-Generator-Carbon-Monoxide-and-Fire-Hazards-Ahead-of-Hurricane-Season
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