How Many Watts Do You Actually Need in a Blackout?

Every backup-power sizing mistake is one of three errors: using an appliance's nameplate watts instead of what it actually draws, forgetting that compressors cycle, or trusting a battery's rated watt-hours instead of what survives the inverter. This page is the correction for all three — and the formula fits in one line:

(sum of each appliance's watts × its duty cycle) × hours ÷ 0.85 = the usable watt-hours you need.

The 0.85 is inverter efficiency. The duty cycle is the fraction of each hour the thing actually draws power — a router never stops (1.0), a cold fridge's compressor runs about a third of the time (0.35). Both corrections are large, and spec sheets apply neither.

Do the math on your own house

Check off what you'd actually run. The defaults are the suburban-outage load this whole site sizes against — and the numbers behind the two biggest lines are sourced, not folklore: DOE purchasing data puts a typical 18 cubic-foot refrigerator at 41–46W continuous average (a ~120W compressor at ~35% duty), and ResMed's own spec sheet puts an AirSense 10 with its humidifier at 53W typical.

The units list applies the same 85% haircut to every manufacturer's rated capacity — "usable Wh," not the number on the box. Capacities and outputs come from each maker's own spec page; every one is in the source ledger below with the date we checked it.

Reading your result

Under 1,000Wh needed: the essentials-overnight tier. Any 1kWh-class unit covers a fridge, CPAP, and comms until morning — pick on price per watt-hour, which is what our home backup rankings are sorted by.

1,000–2,500Wh: the full-day tier. This is the honest requirement for the default household trio across a 24-hour outage (~2,200Wh usable), and it's why the 2kWh class exists. One 2kWh unit — or a 1kWh unit you can recharge from solar or a car — is the play.

Over 3,000Wh, or any 240V load: you've left the portable conversation. Well pumps, central HVAC, and multi-day whole-home coverage mean the 4kWh-plus tier with expansion batteries, or a different machine entirely — our generator-vs-power-station guide covers that fork.

The two mistakes the calculator can't make for you

Don't size output to the nameplate. A "700W" microwave draws roughly double its cooking wattage from the wall; a fridge's start-up surge is several times its running draw. If your peak-stack estimate is within 20% of a unit's continuous rating, size up or plan to not run those two things at once.

Don't budget 100% of the battery. Beyond the inverter's 15%, packs protect themselves at the bottom of their charge range, cold reduces delivered capacity, and you will not execute perfect appliance discipline at 3 a.m. The 0.85 factor is the floor of honesty, not the ceiling.