Watts vs. watt-hours: what's the difference?
Short answer
Watts measure power at a moment; watt-hours measure stored energy. A power station's output in watts decides what you can plug in. Its capacity in watt-hours decides how long it lasts. A 1,000 Wh battery can run a 100 W load for about 8.5 hours after inverter losses, but it cannot run a 2,000 W heater at all if its output is only 1,500 W.
A simple way to picture it
Think of a water tank. Watt-hours are the size of the tank: how much is stored. Watts are how fast water flows out of the tap. A big tank with a narrow tap lasts a long time but cannot fill a bucket quickly. A small tank with a wide tap empties fast.
Power stations work the same way. Capacity (Wh) is the tank. Output (W) is the widest the tap can open.
The one formula you need
Watt-hours equal watts multiplied by hours. Turn it around and you get runtime:
Hours of runtime = usable watt-hours ÷ watts drawn
Usable watt-hours are roughly 85% of the rated capacity, because the inverter loses some energy converting battery power to household AC power.
A 1,000 Wh unit has about 850 Wh usable. Running a 60 W laptop: 850 ÷ 60 = about 14 hours. Running a 1,500 W space heater: 850 ÷ 1,500 = about 34 minutes.
Three numbers on every spec sheet
| Spec | Unit | What it tells you |
|---|---|---|
| Capacity | Wh (or kWh) | How much energy is stored. Decides how long things run. |
| Continuous output | W | The most it can supply nonstop. Everything running at once must fit under it. |
| Surge (peak) output | W | A short burst for starting motors in fridges, pumps and AC units. |
What about amp-hours?
Batteries are sometimes rated in amp-hours (Ah). To convert, multiply by the battery voltage: a 100 Ah battery at 12.8 V holds about 1,280 Wh. Amp-hours on their own are not comparable between batteries with different voltages, which is why watt-hours are the better number to shop by.
Kilowatt-hours on your electric bill
Your utility bills you in kilowatt-hours (kWh). One kWh is 1,000 Wh. A typical US home uses around 30 kWh a day, which is why a single portable power station can cover essentials but not a whole house.
To turn all of this into a real answer for your appliances, use the runtime calculator, or look up typical values in the appliance wattage chart.
Frequently asked questions
Is a higher watt or watt-hour rating better?
It depends on what you need. Higher watts let you run bigger appliances like microwaves and pumps. Higher watt-hours let you run things for longer. For outages, most people need at least 1,000 Wh and 1,500 W of output.
How many watt-hours do I need to run a fridge for a day?
A typical modern full-size fridge uses roughly 1 to 1.5 kWh a day. Allowing for inverter losses, plan on about 1,500 to 2,000 Wh of capacity for a full day.
How do I convert amps to watts?
Multiply amps by volts. A device drawing 5 A on a 120 V outlet uses about 600 W.
Sources and method
Runtime figures assume 85% of battery capacity reaches AC appliances and typical US appliance ratings. See exactly how we calculate. Found a mistake? Tell us and we will fix it.