Generator Size Calculator

Select the appliances you need to power during an outage. This calculator accounts for both running watts and starting watts (surge) to recommend the right generator size for your home.

Check the appliances you want to power. Adjust wattage if your appliance differs.

	Appliance	Running W	Starting W

Starting vs Running Watts Explained

Motor-driven appliances (refrigerators, AC, pumps) require a brief surge of power at startup — 2 to 3 times their running wattage. Your generator must handle this surge, even though it only lasts a fraction of a second.

Required Size = max(Total Running × 1.25, Total Running + Highest Surge)

Frequently Asked Questions

How do I calculate what size generator I need?

Add up the running watts of all appliances you want to power simultaneously. Then find the appliance with the highest starting watt surge (usually a motor: refrigerator, AC, pump). Your generator must handle total running watts plus that highest starting surge. Add 25% safety margin. Result: recommended generator size in watts.

What is the most important appliance to include?

For most homeowners, the refrigerator, sump pump, and heating/cooling system are critical. A well pump is essential if you are on a private well. Medical equipment (oxygen concentrators, CPAP) should be included if applicable. For whole-home backup, include the HVAC system — this typically requires a 10,000W+ standby generator.

Portable vs standby generator — which should I choose?

Portable generators (1,000–12,000W) are less expensive ($500–3,000), run on gasoline, and must be manually started and refueled. They suit occasional use and partial home coverage. Standby generators (7,000–22,000W+) run on natural gas or propane, start automatically within seconds of an outage, and can power an entire home. They cost $3,000–10,000+ installed.

How much fuel does a generator use?

A 5,000W gasoline generator typically consumes 0.5–0.75 gallons per hour at 50% load. Running 8 hours uses 4–6 gallons. A 10,000W standby generator running on natural gas uses approximately 150–200 cubic feet per hour. Always ensure you have adequate fuel reserves before an anticipated storm or outage.