Almost every disappointing battery purchase traces back to mixing up two numbers on the spec sheet. They look similar — watts and watt-hours — but they answer completely different questions, and a battery has to pass both tests to do the job you’re buying it for. Five minutes here saves real money later.
Watts: how fast power flows
Watts measure the rate of energy use — how fast a device drinks. Think of a car’s speedometer, or how far open a faucet is.
Every device has an appetite: a Wi-Fi router sips around 10W, a laptop drinks 30–60W depending on what it’s doing, a small TV around 60–100W, and a space heater gulps 1,500W. A device’s watts tell you nothing about how long anything lasts — only how fast the meter spins while it runs. You can look up typical figures for common devices in the Device Wattage Library.
Watt-hours: how big the tank is
Watt-hours (Wh) measure stored energy — the size of the fuel tank. A 500Wh battery holds enough energy to deliver 500 watts for one hour, or 50 watts for ten hours, or 10 watts for fifty hours — before real-world losses trim those numbers.
Runtime is just the tank divided by the appetite:
runtime ≈ watt-hours ÷ device watts (then reduced for losses and reserve)
That one line is most of battery shopping. Everything else is refinement.
Two gates: capacity is not power
Here’s the part that catches people: a battery must clear both bars, and they’re independent.
- The watts gate. The unit’s continuous inverter output must exceed what your device draws. A 1,000Wh station with a 300W inverter simply cannot run a 500W appliance — not for one minute. The tank is huge; the tap is too narrow.
- The watt-hours gate. Clearing the watts gate says nothing about duration. A station with a 2,000W inverter but a small 500Wh battery will happily start a 1,500W space heater — and be empty in under twenty minutes.
So evaluate every candidate with two questions, in order: Can it run my device at all? (inverter watts vs device watts), then For how long? (watt-hours ÷ device watts, minus losses).
This is also where money quietly disappears. If your largest device is a 60W laptop, paying extra for a 1,800W inverter buys nothing you’ll use — and if you actually need to run a 700W appliance, no amount of extra watt-hours on a 500W-output unit will help. Knowing which gate is your constraint tells you which spec deserves the budget.
Reading a spec sheet in sixty seconds
Three numbers matter on any power station listing:
- Capacity (Wh) — the tank. Sometimes buried under the marketing name.
- Continuous AC output (W) — the sustained rate the inverter can deliver.
- Surge or peak output (W) — a brief allowance for startup spikes from compressors and motors. “600W (1,200W surge)” means 600W is the real limit for continuous use.
If a listing leads with mAh instead of Wh, or hides the continuous output behind a peak number, treat that as a small warning sign — reputable spec sheets state all three plainly.
mAh vs Wh: voltage matters
Phone and power bank capacity is usually quoted in milliamp-hours (mAh), which only makes sense at a known voltage:
Wh = (mAh ÷ 1,000) × volts
Most power banks use lithium cells at a nominal 3.7V, so a 20,000mAh bank stores about 74Wh, and a 27,000mAh bank about 100Wh. This is why mAh numbers can’t be compared across battery types — 20,000mAh at 3.7V and 20,000mAh at 12V are wildly different amounts of energy. Power stations skip the ambiguity and state Wh directly; convert everything to Wh and comparisons become honest.
Worked examples
The table below uses this site’s standard assumptions — 85% conversion efficiency and a 10% reserve, leaving about 76.5% of rated capacity usable. Runtimes come straight from the Battery Runtime Calculator:
| Device (typical draw) | 300Wh battery | 500Wh battery | 1,000Wh battery |
|---|---|---|---|
| Wi-Fi router (10W) | ~23 hr | ~38 hr | ~76 hr |
| Small fan (30W) | ~7.7 hr | ~12.8 hr | ~25.5 hr |
| Laptop, working (60W) | ~3.8 hr | ~6.4 hr | ~12.8 hr |
| Small TV (80W) | ~2.9 hr | ~4.8 hr | ~9.6 hr |
Two things jump out. Low-draw devices run a surprisingly long time on modest batteries — a router barely dents a 300Wh unit. And doubling capacity roughly doubles runtime, so you can buy hours with money — but only after the watts gate is cleared.
What this means when you shop
Match the inverter watts to your single largest device first; that’s pass/fail. Then buy watt-hours against the hours you actually need, not the biggest number in the product line. Beware of mismatched units in both directions — a big inverter strapped to a small battery runs heavy loads for minutes, while a big battery behind a weak inverter strands capacity you paid for. Our guide to choosing a power station without overspending turns this into a full checklist, and why runtime estimates are often wrong explains where the losses in the table above come from.
Next steps
- Run your own device numbers in the Battery Runtime Calculator.
- Look up realistic wattages in the Device Wattage Library.
- Compare capacity classes side by side in the comparison hub.