Watts vs watt-hours vs surge watts, explained

Watts measure power, watt-hours measure energy

A watt is a rate. NIST defines it as the SI unit of power, equal to one joule per second, so it tells you how fast energy is moving at a given moment. A watt-hour is an amount: the energy used when one watt is drawn for one hour. One watt-hour is 3,600 joules, 1,000 Wh make a kilowatt-hour (kWh), and 1,000 W make a kilowatt. The hour isn't an SI unit, but NIST lists it as one accepted for use with the SI, which is why kWh appears on every power bill.

That's the whole difference between 1,000 W and 1,000 Wh. A 1,000 W rating is how much power a station can push out at once. A 1,000 Wh rating is how much energy its battery holds. In theory 1,000 Wh runs a 100 W load for 10 hours or a 500 W load for 2, before losses, and in neither case can the load be larger than the watt rating allows.

One more detail matters for AC appliances. NIST's definitions give AC power as volts times amps times the power factor. A label that lists only volts and amps can overstate the watts a motor or electronic device uses, so a measured or published wattage is a better starting point.

Continuous output decides what can run

The continuous (or rated) AC output is the power a station can deliver for as long as the battery lasts. Take the EcoFlow DELTA 3 Plus: EcoFlow's US manual lists a 1,024 Wh battery, 1,800 W of continuous AC output and a 3,600 W surge. Those three numbers answer three different questions, and a station can pass one and fail another.

An appliance whose running draw sits above 1,800 W won't run on this station, however full the battery is. A small load well under 1,800 W will run, and then the 1,024 Wh decides when it stops. When you add up a list of devices for an outage, it's their combined running watts that have to fit under the continuous figure. Our power station database lists capacity, continuous output and surge for every model side by side.

Surge watts matter when a motor starts

Motors and compressors draw far more for a moment as they start than once they're turning. DOE's motor tip sheet explains that after the first instant, a starting motor draws about its locked-rotor current until it nears running speed. A Danfoss data sheet for a small refrigeration compressor (the B60C5BL, a freezer-class unit, much smaller than a full-size fridge) shows the scale: 7.24 A locked-rotor current against 1.30 A at its rating point.

BLUETTI's manual warns that motors and compressors may start at 2 to 4 times their rated power. That's the surge rating's job: a higher output for the start, after which the load has to drop back under the continuous figure. No maker manual or page we checked, from EcoFlow, Anker, BLUETTI, Jackery or DJI, says how long a surge rating can be held.

Refrigerators show why the different numbers matter. ENERGY STAR data for full-size 16 to 30 cubic foot models puts a typical fridge at about 60 W averaged over a day and about 123 to 240 W while the compressor runs. DOE says a refrigerator runs at its maximum wattage about a third of the time it's plugged in, which is how a 123 to 240 W running draw averages out near 60 W. No refrigerator maker publishes a start-up figure for consumers, so BLUETTI's 2 to 4 times warning is the closest guidance there is. Our refrigerator guide covers sizing a station for one.

Why a boost mode is not a surge rating

EcoFlow's X-Boost, Anker's SurgePad and BLUETTI's Power Lifting all let a station keep running an appliance rated above its continuous output. None of them adds power. EcoFlow says X-Boost works by lowering the voltage, suits heating devices and doesn't support devices with voltage protection; its DELTA 2 manual says the mode still delivers only the rated output. Anker says SurgePad delivers rated power, works better with heating devices and doesn't support precision instruments. BLUETTI limits Power Lifting to purely resistive (heating) loads, keeps actual output at the rated power, and its AC500 manual says the mode stops if voltage falls below 70% of rated.

A heater or kettle simply runs cooler at a lower voltage. A motor is a different case, and the surge rating is the figure that applies to starting one. Boost figures also aren't always consistent. For the DELTA 3 Plus, EcoFlow's US manual gives 2,600 W with X-Boost at 120 V, and the US spec table on the product page gives 2,200 W. The two sources disagree, and we list both until EcoFlow settles it.

Work out runtime from watts and watt-hours

DOE's formula for energy use is watts times hours used, divided by 1,000, which gives kilowatt-hours. Turned around for a power station, runtime is roughly the usable watt-hours divided by the load in watts. The catch is "usable": some energy is lost turning the battery's DC power into AC. Our site assumes 80 to 90 percent of a station's rated capacity reaches AC appliances. That's our working assumption, not a maker figure.

With a fridge averaging 60 W on a 1,024 Wh station like the DELTA 3 Plus, the sums are 1,024 × 0.8 ÷ 60 ≈ 13.7 hours and 1,024 × 0.9 ÷ 60 ≈ 15.4 hours. That runtime uses the daily average, and the station still has to handle the 123 to 240 W running draw and the start. BLUETTI's manual gives its own version of the formula: battery capacity times depth of discharge times inverter efficiency, divided by the load plus the station's self-consumption. Our runtime calculator does this for any station and appliance list.

Reading a spec sheet without mixing up the numbers

On a product page these figures often sit side by side with similar labels. Sorted out, they read like this:

Check the continuous figure against what you want to run at the same time, the surge figure against anything with a motor or compressor, and the capacity against how many hours you need. Our power station finder takes your appliance list and does those three checks for you.