How we calculate

Every number on OutageReady comes from a few simple formulas. Here they are, along with the standards we follow.

The core runtime formula

Runtime in hours equals the battery capacity in watt-hours, multiplied by 0.85, divided by the average watts your appliances draw.

The 0.85 factor accounts for energy lost in the inverter, which converts the battery's DC power into 120 V AC household power, plus a small battery reserve most units keep. Real efficiency ranges from roughly 80 to 90 percent depending on the unit and the load.

Average watts and duty cycles

Many appliances do not draw power continuously. A refrigerator compressor cycles on and off; a sump pump runs only when water rises. We turn running watts into an average by multiplying by the hours per day the appliance actually runs. Our calculators let you change those hours.

Startup surge

Motors and compressors pull two to three times their running watts for a moment when they start. We check that the largest single startup, added to everything else running, fits within a power station's surge rating or a generator's starting watts.

Generator headroom

We add 20 percent to your total running watts when suggesting a generator size, because generators run less efficiently near their limit and appliance ratings are averages.

Where our appliance values come from

Values in the appliance wattage chart are typical figures for US appliances, based on manufacturer ratings, ENERGY STAR data and common generator sizing references. Real appliances vary, so we always show how to check your own.

Editorial standards

Every article is reviewed by Aleix Viñals before publishing. We use software tools, including AI, to help draft and check content; a person checks every number against the formulas on this page.
Safety and food-safety guidance is based on official sources such as the U.S. Consumer Product Safety Commission and the USDA, which we link in each article.
We do not publish electrical installation instructions. Anything connected to your home's wiring should be done by a licensed electrician.
When we test equipment ourselves, we say so and show the measurements.

Advertising and independence

The site is funded by display ads and may use affiliate links. Ads are always labeled, and no advertiser or brand can pay to change a calculation, a recommendation or a ranking.

Corrections

If you spot an error, tell us. We correct confirmed errors promptly and update the date on the page.