Watt Hours Explained: Wh vs W vs Ah (Power Stations)

What a watt-hour actually measures

A watt is a rate: it says how fast energy is being used at this instant. A watt-hour is a quantity: it says how much energy was used in total. The difference is the same as between a car's speedometer and its odometer.

Multiply the rate by the time and you have the quantity:

watt-hours = watts × hours

That single line explains most power-station marketing. A 100 W television running for 7.5 hours uses 750 Wh. A 10 W phone charger uses the same 750 Wh over 75 hours. Same energy, radically different power. A station's watt-hour rating pays for the total; its watt rating decides whether a load can start at all.

Wh vs W vs Ah: the three units side by side

Batteries are sold in amp-hours, devices in watts, and power stations in watt-hours. They are all the same family of measurement — one is a rate, one is a quantity, one is charge.

The conversion between Ah and Wh needs the voltage, which is the step people skip. Wh = Ah × V. A 100 Ah battery at 12 V is 1,200 Wh; the same 100 Ah at 24 V is 2,400 Wh. Amp-hours alone cannot size a power station, because they say nothing about voltage.

How many watt-hours is a 12 V 100 Ah battery?

On paper, 1,200 Wh. In practice, less — first because the pack should not be drained flat, and second because the inverter loses energy on the way out. Here are the common battery sizes converted to usable energy at a 90% depth of discharge and 88% inverter efficiency:

Usable = nameplate × 0.90 × 0.88 = nameplate × 0.792. A lead-acid pack uses a 50% depth of discharge instead, which cuts the same 1,200 Wh battery to 528 Wh.

The three losses between nameplate and outlet

"1,000 Wh" is measured at the cells with a slow, perfect discharge. Three things stand between that lab number and the energy your laptop receives:

• Depth of discharge (DoD). The battery management system stops the pack before it is truly empty to protect its lifespan. LiFePO4 allows about 90%, NMC lithium about 85%, lead-acid about 50%.
• Inverter efficiency. Turning DC battery power into 120 V AC wastes some as heat. Most stations publish 85–90%; the site's default is 88%.
• Standby draw. An inverter that is switched on burns a few watts even with nothing plugged in — typically 5–15 W on larger units. Over a 20-hour run, 5 W is 100 Wh gone.

Two "1,000 Wh" boxes with different chemistry are therefore different products. The lead-acid one delivers barely more than half the runtime of the LiFePO4 one, and it weighs considerably more.

What capacity buys in real runtime

Run the usable figures against two everyday loads and the capacity ladder becomes concrete. Each row assumes the device runs alone, with the 5 W standby draw included:

The full runtime chart covers eight devices across the same five capacities, and the runtime calculator lets you tick your own combination.

Watt-hours, kilowatt-hours and what the grid charges

Your electricity bill is also denominated in energy, just in a bigger unit: 1 kWh = 1,000 Wh. A 1,000 Wh power station therefore holds exactly one kilowatt-hour.

At the site's 18.16¢/kWh reference rate, that kilowatt-hour costs about 18¢ from the wall. Charging the station is not free of losses — if the charger is 90% efficient, drawing 1.11 kWh costs about 20¢. Compare that with a gasoline generator at 5 kW and 75% load: 0.60 gal/h at 3.50 a gallon is 2.10 an hour for 3.75 kWh, or about 56¢/kWh. The battery is roughly a third of the fuel cost per unit of energy, before maintenance and noise.

Why two 1,000 Wh stations run different lengths

Two units with the same label can differ by 30% in delivered runtime. The variables, in order of impact:

• Chemistry and depth of discharge — the 792 vs 440 Wh gap above.
• Inverter efficiency — an 85% unit delivers 765 Wh where a 90% unit delivers 810 Wh from the same 90% DoD pack.
• Standby draw — a 15 W inverter idles away 360 Wh in a day; a 5 W unit loses 120 Wh.
• Temperature — a pack stored below freezing delivers noticeably less than its rating, and charging it cold can damage it.
• Age — lithium packs are usually rated to 80% of original capacity after 3,000–5,000 full cycles, so a two-year-old station no longer matches its label.

The calculator lets you change chemistry, DoD, inverter efficiency and standby draw so the answer reflects the unit you actually own, not the brochure.