Size my backup

Runtime calculator

Switch on what you want to keep running during an outage. You’ll see the load, the startup surge and how many hours each battery size lasts.

What needs power

How long do you need power?
Kitchen
Basement and utility
Heating
Everyday
Cooling and air
Medical

Edit appliance

How we calculate

Each appliance uses its watts for the hours a day you give it. Fridges and freezers use the yearly kWh on their EnergyGuide label instead, because they cycle on their own. Runtime is usable battery capacity divided by that energy spread over the day.

Running load adds up everything switched on, whatever its hours, since any of it can run at the same moment. The peak is that total plus the biggest single startup surge. Motors seldom start at the same moment, and EWEB’s generator sizing sheet uses the same method.

Where the numbers come from

Most running and starting watts come from a generator sizing sheet published by EWEB, the public utility in Eugene, Oregon. The window AC, dehumidifier and TV use the median of current ENERGY STAR models. The table below lists the source for each appliance, and each name links to a page with every source we found. The hours a day are our guesses, so change them to match your home. If your appliance has a rating label, use its numbers too.

Your edits stay in this browser. A shared link carries them in the part of the address after the #, which isn’t sent to our server.

Medical devices

If someone in your home depends on a CPAP or other powered medical equipment, ask the supplier which backup batteries the device supports. Ask your utility, too: many keep a list of customers who rely on medical equipment.

The numbers behind the calculator

ApplianceRunningStartingTypical useSource
Refrigerator700 W2,900 W457 kWh a yearEWEB, Generator Sizing ConsiderationsWatts from EWEB. Yearly use is the median of 2,987 full-size ENERGY STAR models.
Chest freezer700 W2,900 W213 kWh a yearEWEB, Generator Sizing ConsiderationsWatts from EWEB. Yearly use is the ENERGY STAR rating of a 10.1 cu ft chest freezer.
Microwave1,000 W1,000 W1 h a dayOur guessEWEB, Generator Sizing Considerations
Coffee maker1,000 W1,000 W1 h a dayOur guessEWEB, Generator Sizing Considerations
Toaster850 W850 W1 h a dayOur guessEWEB, Generator Sizing Considerations
Sump pump, 1/3 hp800 W2,100 W24 h a dayOur guessEWEB, Generator Sizing Considerations
Sump pump, 1/2 hp1,050 W3,250 W24 h a dayOur guessEWEB, Generator Sizing Considerations
Well pump, 1/3 hp1,000 W3,000 W4 h a dayOur guessEWEB, Generator Sizing Considerations
Furnace blower, 1/3 hp700 W2,100 W4 h a dayOur guessEWEB, Generator Sizing Considerations
Space heater1,500 W1,500 W8 h a dayOur guessEWEB, Generator Sizing Considerations
Wi-Fi and modem20 W20 W24 h a dayOur guessXfinity, Internet equipment energy usageMedian of 19 modem-router gateways Xfinity has measured.
LED bulbs, 460 W60 W6 h a dayOur guessEWEB, Generator Sizing Considerations
Phone chargers, 240 W40 W4 h a dayOur guessEWEB, Generator Sizing Considerations
Laptop60 W60 W4 h a dayOur guessEWEB, Generator Sizing Considerations
TV, 55 inch98 W98 W4 h a dayOur guessENERGY STAR certified televisions datasetMedian on-mode power of ENERGY STAR TVs from 50 to 59 inches.
Radio100 W100 W8 h a dayOur guessEWEB, Generator Sizing Considerations
Window AC, 8,000 Btu533 W533 W8 h a dayOur guessENERGY STAR certified room air conditioners datasetMedian of current ENERGY STAR models, cooling capacity divided by CEER. We haven't found a published startup surge for these variable-speed units, so check your manual.
Dehumidifier, 50 pint488 W488 W12 h a dayOur guessENERGY STAR certified dehumidifiers datasetMedian of ENERGY STAR models rated 40 pints a day or more, capacity divided by IEF.
CPAP machine56 W56 W8 h a dayOur guessResMed AirSense 11 power supply specificationTypical draw of a ResMed AirSense 11. A heated humidifier and tube can push it toward the 73 W peak.