Tick what you want to keep running in an outage. The calculator adds up running watts and the extra surge motors need to start, then recommends a generator size, the type that fits (inverter, portable or whole-house standby) and roughly how much fuel a day it will burn.
What do you need to keep running?
Start with a preset, then tap items to add or remove them.
Wattages are typical figures for common models and are estimates only. Check the data plate on your own appliances, and have a licensed electrician confirm sizing and install any transfer switch or interlock. Never run a portable generator indoors, in a garage or near windows.
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How to size a generator
Every appliance has two numbers. Running watts is what it draws while it runs. Starting watts is the short surge a motor needs to get going, often two to three times higher. Refrigerators, sump pumps, well pumps, furnace blowers and air conditioners all have motors. A generator has to cover your total running watts and still have room for the biggest motor to start, which is why the calculator adds the single largest surge on top of your running load.
We then add about 20% of headroom, because generators run best and last longest below their maximum, and you will almost certainly plug in something you forgot.
Inverter, portable or standby?
- Inverter generators are quiet and produce clean power for electronics, but most top out around 2,000 to 4,000 watts. Good for a fridge, lights, phones and internet.
- Portable generators in the 5,000 to 12,000 watt range can run most essentials, and some 240-volt loads like a well pump, through a transfer switch or interlock kit installed by an electrician. You will need to refuel them, roughly every 8 to 12 hours.
- Whole-house standby generators are installed permanently, run on natural gas or propane and start on their own within seconds. They are the only practical way to run central air, electric ranges and dryers during a long outage. See our backup power guide for installed costs.
Common sizing mistakes
- Forgetting the well pump. If you are on a well, no pump means no water, and well pumps have a big starting surge.
- Undersizing for central air. A 3-ton AC can need over 10,000 starting watts. A soft-start kit, installed on the AC unit, can cut that surge by roughly half.
- Running everything at once. With load management, a smaller standby unit can cover a big house by shedding the dryer or water heater while the AC starts.