Sizing Backup Power: The Arithmetic

Why sizing is the whole decision

Every buying guide in this category starts with products. That is backwards. The product is a consequence of a number, and the number comes from your house and your utility, not from a review.

Get the number too low and you have bought something that shuts off when the fridge starts. Get it too high and you have spent two or three times what your actual outages justify, on a machine that is heavier than you can move and burns more fuel than you can store. Both failures are common and both are avoidable in about fifteen minutes of arithmetic.

The three quantities

1. Running watts. What your chosen appliances draw continuously, added up. A fridge at 200W, a furnace fan at 600W, six LED bulbs at 10W each, a router and a laptop at 100W — that is about 960W. This is the number that decides whether the machine can carry your house at all.

2. Starting watts. The largest single surge in that group, not the sum of all of them, because motors rarely start at exactly the same instant. If the furnace fan is the biggest surge at 1,600W, your peak requirement is roughly the running total minus that fan's running draw, plus its starting draw. Around 1,960W in the example above.

3. Watt-hours. Running watts multiplied by hours. Only relevant to batteries — a generator's runtime is a fuel question, not a capacity one. 960W for eight hours is 7,680Wh, which is well beyond any portable battery and tells you immediately that an overnight whole-critical-load outage is generator territory.

The mistake almost everyone makes

People size for everything at once. In an actual outage you do not run the fridge, the furnace, the well pump, the microwave and the space heater simultaneously — you run the fridge and the furnace, then you run the microwave for four minutes with the furnace off.

Load management is the cheapest capacity you can buy. Deciding in advance which two things run together routinely halves the machine you need, and halves what it costs to fuel. Every sizing page here works two totals for that reason: everything at once, and a sequenced list.

Where our numbers come from

Appliance wattages are the manufacturers' and the trade's published figures, not ours — the reference table cites Champion Power Equipment's wattage chart and Honda's estimation guide, both linked on the chart page. Outage statistics come from the EIA's Electric Power Annual. Fuel prices are the EIA's weekly series, quoted with the week they were taken.

Every figure we compute on top of those is shown with its inputs, so you can substitute your own and get a different, better answer. That is the point. A sizing calculator that hides its arithmetic is asking you to trust it; one that shows the arithmetic is letting you check it.

Two things the arithmetic cannot tell you

Inverter efficiency varies. We use 85% as a working figure for battery systems, which is conservative for a good inverter at moderate load and optimistic for a cheap one running near its limit. Your real number is somewhere in that range and we cannot measure it without the unit in front of us.

Appliance draw is a range, not a value. A 2003 side-by -side refrigerator and a 2024 Energy Star model differ by a factor of two or more. Where a published range exists we give the range and size against the upper end, because a system that only works with the efficient appliance is not a backup system.