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.

More appliances, worked through

The deep-dives that replace a guess with a number: a space heater is always 1,500W and that is set by the outlet rather than the design; a microwave draws about 50% more than the number on its door; and a computer uses a fraction of its power supply rating. A washer and dryer are two completely different problems sitting side by side.

Two more that answer a question rather than an appliance: how many watts you need for camping and 30 amp versus 50 amp RV service, where the names hide a factor of more than three.

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.

Appliance by appliance

The pages that take one load and work it through properly: a sump pump and a well pump are both tiny energy loads with brutal starting surges, which is the pattern that defeats correctly sized batteries. And if you arrived here asking about floor area, what size generator a 2,000 square foot house needs explains why square footage is the wrong input.

The cheapest fix for a compressor that will not start

When an air conditioner trips the generator, a soft starter is usually the cheaper answer than a bigger machine — but only if the failure is at the moment of starting. That page separates a surge problem from a running-watts problem before you spend anything.

Four more loads, and one device

The appliance series keeps growing as people ask: a freezer is the cheapest thing in the house to back up, a television costs less than people expect, an electric stove is the load that ends most backup plans, and power tools are sized on their starting surge rather than their running draw. Sizing a UPS is its own exercise, because VA is not watts.

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.

Seasonal loads, published rather than guessed

Winter adds appliances nobody sizes for. A pellet stove is published at about 336 watts running and 504 starting, and a hydronic boiler can be as little as a 44-watt circulator, which makes those two the cheapest heating systems in the house to back up. At the other end of the season, holiday lighting carries a published 210-watt cap per end-to-end run that decides how the whole display gets wired.