Watts vs watt-hours, and why runtime isn't just capacity ÷ load

Watts are a rate. Watt-hours are an amount. A power station’s watt-hour rating says how much energy is in the battery, and its watt rating says how fast it can hand that energy out. An appliance’s watts say how fast it uses it. Runtime is the amount divided by the rate, minus some losses along the way, and those losses are why the simple division always comes out a bit optimistic.

The two numbers on every power station

Take the EcoFlow DELTA 2. EcoFlow rates its battery at 1,024 Wh, and its AC outlets at 1,800 W continuous.

They answer different questions. The watt rating decides whether something will run at all: plug in more than the inverter can supply and the station will usually trip its overload protection. The watt-hour rating decides how long things run once they do.

A ResMed AirSense 11 uses a tiny share of what the DELTA 2’s inverter can deliver:

So the AC rating is no problem here. For a CPAP, watt-hours are the number to shop on.

If you see amp-hours instead, that’s charge rather than energy. Multiply amp-hours by the battery voltage and you get watt-hours. A kilowatt-hour, the unit on your electric bill, is a thousand watt-hours.

The simple math

Divide watt-hours by watts and you get hours. A 100 Wh battery running a 10 W load would last 10 hours, on paper. That’s the right starting point, and it’s the core of every runtime number on this site.

But divide the DELTA 2’s capacity by the AirSense 11’s watts in the Why column above and you’ll get a bigger number than our table below shows:

The difference is the efficiency factor in the note under the table, and real life usually takes a bit more off still.

Why runtime comes in under capacity ÷ load

Conversion losses. The battery stores DC. To run anything from the AC outlets, the station’s inverter converts that to AC, and some energy turns into heat on the way. The 12V port converts too, from the battery’s own voltage to 12V, and usually loses less. This is the one loss our tables do account for, with one flat factor for AC and one for DC. We explain the AC and DC difference in inverter efficiency and AC vs DC outputs.

The inverter’s own draw. An inverter uses some power just being on, whatever is plugged in. With a big load that’s a rounding error. With a small one that runs all night, like a CPAP, it’s a bigger share. Our flat factor doesn’t model this separately, so for very small AC loads the real hours can come in lower than ours.

Not all of the rated capacity reaches the outlets. The battery management system keeps a margin at the top and bottom of the charge to protect the cells. How makers count this in their ratings varies, and manuals often don’t say.

The load isn’t steady. Appliances rarely draw the same watts all the time. A CPAP draws more at higher pressure or with the humidifier working hard. A 12V fridge cycles on and off, so what matters is its energy per day, not its running watts. For the Dometic CFX3 45, we take Dometic’s daily figure and spread it over the day, which you can see in the note under the table:

That’s fair for planning, but a hot afternoon with the lid opening every ten minutes isn’t an average day.

Cold and age. A cold battery gives less than its rating, and every battery holds less as it gets older. Our tables assume a new battery at its rated capacity at a comfortable temperature. LiFePO4 vs NMC covers how the two chemistries age.

What to do with this

Treat the tables and the runtime calculator as an estimate. If a station only just covers what you need, it probably won’t once the battery is a few years old or the night is cold. We’d size so the table shows a clear margin over your longest stretch without charging, and our van and RV sizing guide walks through adding up a day of loads.

The full formula and the factors we use are on our methodology page. When we measure a combination ourselves, the table will say “measured” and use that result instead.