Generator Size Calculator
Find out what size generator you need for a power outage. Enter how many of each appliance you want to run at the same time to get the running watts, the starting watts and a recommended size.
- Running watts needed
- 3,170 W
- Starting watts needed
- 4,670 W
- Largest starting surge
- Refrigerator or freezer (+1,500 W)
What you will need
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How this calculator works
Running watts are added for everything you want to power at once. Motors (refrigerators, pumps, furnace fans, air conditioners) briefly draw extra starting watts; the generator must also cover the largest single starting surge, because motors rarely all start at the same moment.
Wattages come from Honda Power Equipment’s generator wattage guide, with the space heater, flat-screen TV and water heater from Virginia Cooperative Extension. The recommendation adds a 10% margin and rounds up to the next 500 W.
Always check the nameplate of your own appliances, especially well pumps and air conditioners, which vary widely. Connecting a generator to house wiring requires a transfer switch or interlock installed by a licensed electrician.
Example: refrigerator, sump pump, furnace fan, TV, computer and 200 W of lights
- Running: 700 + 1,000 + 1,000 + 120 + 150 + 200 = 3,170 W.
- Largest surge: refrigerator, 2,200 − 700 = +1,500 W → 4,670 starting watts.
- With a 10% margin, rounded up: a generator rated for about 5,500 starting watts.
Quick reference
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator or freezer | 700 | 2,200 |
| Sump pump, 1/2 HP | 1,000 | 2,100 |
| Furnace fan, 1/3 HP | 1,000 | 1,800 |
| Central AC, 24,000 BTU | 3,800 | 4,950 |
| Clothes washer | 1,150 | 2,300 |
| Microwave (625 W) | 625 | 800 |
| Coffee maker | 1,750 | 1,750 |
Tips before you buy
- Never run a portable generator indoors, in a garage or near windows — carbon monoxide is deadly. Install CO alarms.
- Inverter generators are quieter and safer for electronics; conventional generators give more watts per dollar.
- Stagger startups: switch on the largest motor first, then the rest one at a time.
Guides
Frequently asked questions
What size generator do I need to run a house?
Keeping the essentials running — refrigerator, sump pump, furnace fan and some lights — usually takes 5,000 to 7,500 starting watts. Adding a central air conditioner pushes that to around 10,000 W.
What is the difference between running and starting watts?
Running watts are what an appliance needs continuously. Starting (surge) watts are the brief extra power a motor needs to start, often two to three times the running watts.
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Last updated October 26, 2026