Battery backup for a window AC: hours per kWh at 8,000 BTU, and the surge rule that decides the unit

Air conditioning is the load that turns a battery purchase into a math problem, because a window unit is the one appliance most people own that draws half a kilowatt and starts with a spike its maker never prints. An 8,000 BTU window AC draws about 533 W while its compressor runs, by its own ENERGY STAR figure, and cycles about half the time in a hot room; a 2 kWh unit is a night, a 1 kWh unit is three hours, and only an inverter that clears 1,800 W continuous and 3,000 W surge should be asked to start it. The table above is the arithmetic; the five picks are the units that clear the rule.

Watts from the yellow label, not the box

The EnergyGuide label gives a CEER, the combined energy efficiency ratio, and the BTU rating. Divide one by the other and the result is watts while the compressor runs: 8,000 BTU at a CEER of 15 is 533 W, which is the Whirlpool WHAW-081IN’s ENERGY STAR listing and the figure the table uses. A 6,000 BTU unit at the same efficiency is 400 W; a 12,000 BTU unit is 800 W. The compressor does not run all hour. Once the room reaches the setpoint it cycles, and the share of each hour it runs is the duty cycle that decides the watt-hours: 50% in a hot room with the door shut, less on a mild night, close to 100% in a sunny room with a gap under the door. The watt-hours guide has the conversions, and the sizing calculator lets you set the duty for your room.

The number that is not on the label

A compressor is a capacitor-start motor, and for a fraction of a second at start it draws several times its running current. Window AC makers publish a running amperage and nothing about the start, so there is no figure to size against. This site’s rule for an unpublished compressor, explained in running versus surge watts, is an inverter with at least 1,800 W continuous and 3,000 W surge. Three of the five picks state a surge above that; two meet the continuous half and print a boost-mode figure their makers do not call a surge rating, and their cards say so. A 288 Wh unit with a 600 W inverter, the kind that runs a router for a day, will not start an AC, and a unit that trips on the start is worse than no unit at all, because the room warms while you reset it.

Sizing the night

At 533 W and 50% duty the average is 266 W. Usable capacity, the sticker at 85% inverter efficiency per the usable capacity guide, divided by that average gives the hours: about 3.3 for a 1 kWh unit, 6.5 for 2 kWh, nearly 10 for the 3 kWh F3000. A summer night is eight or nine hours, so the honest class for sleeping through an outage is 2 kWh and up, and the two 1 kWh picks are for the hours after dark before the house cools or for a unit that also has to be carried up the stairs. The 2 kWh roundup compares that class as stations, and the expandable roundup covers the units that grow when one night is not enough.

What a fan does to this page

A box fan draws about a tenth of a window AC and moves the air the body actually feels. On a battery, a fan and a wet towel buys hours of comfort that an AC spends in minutes, and it is the first thing to plug in while the AC waits for the hottest part of the night. The same logic in reverse is on the space heater page: heat and cold are the two loads a battery is worst at, and the cheap alternative is usually the better plan.

What we left out

Every 1 kWh unit ranked elsewhere on this site, the DELTA 3, C1000 Gen 2, AC180, Jackery 1000 v2 and DELTA 3 Plus, would clear the continuous rule here; they are on three pages each already, which is this site’s limit for one product. The Bluetti AC200L publishes neither weight nor cycles on its page and stays an honorable mention elsewhere. No maker here states a UL 2743 listing for a portable power pack; the Anker C1000’s four standards are the exception in the lineup, and the certification guide explains what each covers.