OutageCalc

About

How we calculate

OutageCalc is a set of free calculators for people choosing backup power: a generator for the house, a power station for camping, or enough battery to keep a CPAP or fridge going through an outage. The math is simple and we show all of it here, so you can check our numbers.

Running and starting watts

We add up the running watts of every device you pick, as if they were all on at once. Then we add the extra starting watts of the one device with the biggest surge. Motors and compressors start one at a time in practice, so adding every surge together would oversize your generator.

Battery size for power stations

Energy is each device's average draw × hours per day × quantity, scaled to your outage length. Only about 85% of a power station's rated watt-hours reach your devices, because the inverter and battery management use the rest. We divide by 0.85 and round up to a common size (300, 500, 1,000, 1,500, 2,000, 3,000 Wh and so on).

We recommend continuous output of at least 125% of your running watts and a surge rating that covers your starting peak.

Generators

For portable generators we add a 10% margin to running watts. Standby generators are sized at 110% of your starting peak, rounded up to common sizes (10, 14, 18, 22, 26 kW and up). Large loads like central AC can often run on a smaller generator with a soft-start kit; ask your installer.

Appliance data

Our wattage figures are typical values for modern appliances. Older or larger appliances can use much more. The label on your device, or a plug-in power meter, always gives the best number.

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