Power Station Wattage and Runtime: The Math in Five Examples

How to Calculate Power Station Wattage: One Formula

Sizing a power station is division, twice. You do not need a watt hour calculator, and you do not need to trust the runtime table on a listing, which is written by the seller's marketing department. This post works the battery runtime formula through five real loads end to end, with every number on the page so you can swap in your own appliances and get your own answers.

The formula:

Runtime hours = (rated Wh × 0.85) ÷ device watts

The 0.85 is the inverter derate, and it deserves a paragraph of its own before the examples.

The Runtime Formula and Its Two Correction Factors

The tank on the label is not the tank at the outlet. Converting DC battery power to AC loses energy to heat, and CNET's 2026 lab testing measured the damage across popular units: the Anker SOLIX C1000 Gen 2 delivered 85% of its rated 1,024Wh as usable energy, the EcoFlow Delta 3 Classic delivered 92.5%, and round-trip efficiency across the tested range ran from 69% to 83%. That is why the formula derates by 15% as a planning default. Plan on 85% of the label and you will rarely be surprised; plan on 100% and the second night of an outage will teach you the difference.

The second correction factor is the engine, not the tank. Watts decide what you can plug in at all. A 600W unit cannot run a 1,000W appliance no matter how big the battery behind it, and motors surge at startup, two to three times their running draw, so the compressor that idles at 150W may ask for 450W for a second. Both numbers live on the spec sheet, and the five examples below run into both limits.

Five Loads, Worked Start to Finish

Each example uses the same reference unit, a 1,152Wh BLUETTI AC180-class station, which computes to 979Wh of usable energy at the 85% derate. Swap in your own rated capacity and follow the same three steps: derate the tank, divide by the load, sanity-check the surge.

How long will a power station run a refrigerator?

The fridge is a cycle, not a device. A full-size refrigerator compressor averages around 150W while running and runs roughly 40% of the time in normal use. The day-math:

150W × 24 hours × 0.40 duty = 1,440Wh per day, call it 1.5kWh

Divide the usable tank by the daily need: 979 ÷ 1,440 × 24 = about 16 hours of refrigeration. The surge check: compressor startup near 450W is well inside the AC180's inverter. So a 1kWh-class unit carries a full-size fridge through one overnight and most of a day. Two of those days need either a bigger battery or a recharge source.

A 60W television

Simple division, no surge to worry about: 979 ÷ 60 = 16.3 hours. A TV and a router together, at 85W combined, run about 11.5 hours. This is the load class where small units shine, and it is why a 288Wh station (about 245Wh usable) is a complete entertainment-and-communications kit for an evening: nearly three hours of TV plus a full night of wifi and phone charging.

A 700W kettle

Heaters are short, brutal loads, and the math makes them look harmless. Boiling a liter takes about four minutes: 700W × 0.067 hours = 47Wh per boil. The 979Wh tank holds about 20 boils. The lesson cuts the other way from the fridge: a device that runs for hours punishes you in watt-hours, while a device that runs for minutes barely dents the tank. Check the duty length before you check the wattage.

A CPAP at 40W

The machines most owners run draw in the neighborhood of 40W without the heated humidifier, which is the single biggest lever in this whole example. Per night: 40W × 8 hours = 320Wh. The tank: 979 ÷ 320 = just over three nights. Turn on the humidifier and the draw can double or triple per manufacturer manuals, collapsing three nights into one. The device choice, humidifier on or off, swings the battery size more than any brand comparison.

A 1,000W espresso machine

Here the battery is irrelevant and the inverter is the whole story. A 1,000W machine needs an inverter that delivers 1,000W continuously. The 1,152Wh AC180-class unit can, and would give just under an hour of total element time before the tank ran dry. But a 600W unit like the BLUETTI Elite 30 V2 cannot run it at all; the inverter shuts the port down before the element warms, and no amount of battery capacity fixes it. When a listing brags about watt-hours, this example is the reason you also read the continuous output line.

Reading a Spec Sheet Critically

Three habits keep the marketing from warping your math.

Watch the surge and continuous ratings separately. Surge is a one-second tolerance for motor startups; continuous is what the inverter delivers all afternoon. Sellers lead with whichever number is bigger.

Convert mAh claims yourself. Some listings quote the battery in mAh at cell voltage (3.7V), which inflates the number against everything else in the category. The conversion is Wh = mAh × 3.7 ÷ 1,000, so "30,000mAh" is about 111Wh, not three hundred. Work only in watt-hours.

Ignore self-awarded scores. Labels like eBCI-style efficiency grades are the seller grading its own homework. The replicable number is usable capacity, which independent testers measure, and CNET's spread of 69% to 92.5% in the quote above is the realistic band to expect.

When the Math Points to a Generator

Push any load past roughly 1,500W continuous for hours at a time and the watt-hour requirements stop being battery-shaped. An electric heater at 1,500W for two evenings, 8 hours a night, is 24,000Wh. At the best price-per-watt-hour in the current market, about $0.32/Wh at the 2kWh scale, that is nearly $7,700 of battery, bought to burn resistive heat. A fuel-burning inverter generator does the same job for a fraction of the outlay and refuels in minutes, which is why the five-year cost comparison ends in a split by runtime hours rather than a winner.

Doing This for Your Own Loads

List the devices you would run in an outage, write the watts next to each, multiply by the hours you need each, and add 15% for the derate. The total is the capacity you should shop for, and the biggest single continuous load is the inverter rating you should shop for. The sizing guide turns that arithmetic into a decision tree if you would rather branch than calculate, and the myths piece covers where labeled capacity goes missing in the real world.

Work your own load through the formula first. Then let Versa AI line up the two stations that fit the number, so the only thing left to compare is price.

Frequently Asked Questions

How many watt-hours does a fridge use per day?

A full-size refrigerator averages about 1.5kWh per day based on compressor cycle math: roughly 150W while running, running about 40% of the time. Compact and chest models run less, often under 1kWh. Every fridge has the estimate on its energy guide label, and that number beats any general rule.

Does inverter efficiency really cost 15 percent?

It costs at least that, and sometimes more. CNET's 2026 measurements found usable capacity from 69% to 92.5% of rated across popular units, with 85% being a typical mid-point. Losses come from DC-to-AC conversion and stand-by draw. The 0.85 factor in the runtime formula is a planning default, not a guarantee, so treat anything above it as margin.

What does 600W surge mean on a power station?

Surge is a short tolerance, usually a few seconds, for the startup spike that motors and compressors draw, often two to three times their running draw. A 600W surge rating means the unit can start a small compressor briefly while delivering less continuously. The continuous rating is the one that decides what runs all afternoon; the surge rating decides what starts at all.

How accurate are manufacturer runtime claims?

Treat them as best cases. They assume new cells, moderate loads, ideal temperatures and no conversion losses. Independent lab tests consistently find usable capacity below rated, and real loads with surges do worse still. Run the formula yourself with the 0.85 derate and a measured or labeled device wattage, and your estimate will beat the listing's table.