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Generator Size Calculator

Select the appliances you need to power during an outage. The calculator adds up running watts and accounts for starting watt surges to recommend the minimum generator size you need.

Key Takeaways

  • Add up the running watts of all appliances you want to power simultaneously, then add the highest starting watt surge (usually a refrigerator at 2,200W).
  • A standard refrigerator draws 800 to 1,200 running watts and roughly 2,200 starting watts.
  • Add a 20% safety margin to your calculated total to account for voltage drops and aging equipment.
  • Starting watts (surge) are always higher than running watts because electric motors need extra power for 2 to 3 seconds to start.

Select Your Appliances

Your Generator Size Recommendation

Select appliances above to calculate your generator size.

How does the generator size calculator work?

The calculator adds the running watts of every selected appliance to find your continuous power demand, then identifies the highest single starting watt surge among those appliances to determine your peak demand. The recommended generator size is your peak starting watts rounded up to the nearest 500W, plus a 20% safety margin for voltage drops and aging equipment.

Every electric motor needs a brief surge of extra power to start. A refrigerator, for example, draws 800 to 1,200 watts while running but needs about 2,200 watts for 2 to 3 seconds to start its compressor. Your generator must be able to deliver that surge. The calculator accounts for this by tracking the highest starting watt value among your selected appliances and adding it to the running watts of everything else.

What size generator do I need for my house?

For essential home backup (refrigerator, furnace blower, lights, and a few outlets), you need 5,000 to 7,500 running watts. For a larger home with a well pump or window AC, you need 7,500 to 10,000 running watts. For whole-house backup including a central AC, you need 10,000 to 24,000 watts. Use the calculator above for a precise figure based on your specific appliances.

Frequently Asked Questions

How do I calculate what size generator I need?
To calculate generator size, add up the running watts of all appliances you want to power simultaneously, then add the highest starting watt surge (typically a refrigerator at 2,200W). The generator must have at least that many starting watts. Add a 20% safety margin. Use our calculator above for a precise figure based on your specific appliances.
What is the difference between running watts and starting watts?
Running watts (also called rated watts) are the continuous power an appliance needs to operate. Starting watts (also called surge watts) are the extra power needed for 2 to 3 seconds to start a motor. A refrigerator needs 800 to 1,200 running watts but 2,200 starting watts. Your generator must handle the highest starting surge of any appliance you run.
Will a 5,000W generator run my whole house?
A 5,000W generator can run essential loads: refrigerator, furnace blower, lights, and a few outlets. It cannot run a central AC, electric water heater, or electric range simultaneously. For whole-house backup including AC, you need 10,000 to 24,000W. Use our calculator to check your specific load.
How many watts does a refrigerator use?
A standard refrigerator draws 800 to 1,200 running watts and roughly 2,200 starting watts. This is the single most important load to account for when sizing a generator, because the 2,200W starting surge is often the highest surge in a typical home.

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