Generator size calculator
Enter how many of each thing you want to keep running during an outage. The calculator adds up the load and allows for the surge when a motor starts.
Look for a generator of
3,500 running watts
- Starting watts at least
- 4,400 watts
- Your running load
- 2,850 watts
- Biggest startup surge
- Furnace blower (+1,550 W)
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How it works
Generators have two ratings. Running watts is what they can supply all day. Starting watts is the brief extra boost they can give when a motor kicks on, since fridges, pumps and furnace fans draw two to three times their running power for a second or two at startup.
The calculator adds the running watts of everything you chose, using typical figures for each appliance. It adds 20% headroom so the generator isn’t running flat out, and rounds up to the next 500 watts. For starting watts, it adds the extra surge of the single biggest motor, since they rarely all start at the same moment.
Example
With refrigerators or freezers 1, furnace blowers 1, sump pumps 1, well pumps 0, window air conditioners 0, microwaves 0, space heaters (1,500 W) 0, electric water heaters 0, lED light bulbs 10, tV, internet and phone chargers: yes, the answer is 3,500 running watts.
Tips before you buy
- Check the labels. Your own appliances may draw more or less than these typical figures, and the label or manual gives the real number.
- Never run a generator indoors, in a garage or near windows. Carbon monoxide from the exhaust can kill within minutes. Keep it at least 20 feet from the house and use a battery CO alarm inside.
- Connecting a generator to your home’s wiring needs a transfer switch or interlock installed by an electrician. Backfeeding through an outlet can kill utility workers.
- Store fuel safely and run the generator for a few minutes every month or two so it starts when you need it.
Common questions
What size generator do I need for a refrigerator, furnace and sump pump?
About 3,500 running watts and 4,400 starting watts, including a few LED lights and a TV, using typical appliance figures.
What size generator will run a whole house?
Running central air, an electric range and an electric water heater usually takes a home standby generator of 20,000 watts or more. A 5,000 to 7,500 watt portable covers the essentials: fridge, furnace fan, sump pump, lights and outlets.
What’s the difference between running and starting watts?
Running watts is what the generator supplies continuously. Starting watts is a brief extra burst for motors that draw more power for a second or two as they start.