Reference
Watt-Hours and Amp-Hours
Two units show up on backup power listings and they are not interchangeable. Watt-hours describe energy. Amp-hours describe charge, and only become energy once you know the voltage. Getting this straight is the difference between comparing two products and comparing two marketing numbers.
A watt-hour is one watt for one hour
A watt is a rate. A watt-hour is that rate multiplied by time. A 10 W lamp burning for four hours consumes 40 watt-hours. A 1024 Wh pack, on paper, holds enough for that lamp to run about a hundred hours, and we will get to why the real answer is smaller.
Watt-hours are the honest unit for comparing storage, because they already include voltage. When a power station states its capacity in watt-hours, you can divide by the wattage of what you want to run and get hours directly.
| Stated capacity | What that is | Rough sense of scale |
|---|---|---|
| 89.6 Wh | A small carry-anywhere pack | Phones and a lamp |
| 288 Wh | A desk-sized pack | Router, laptop, lights |
| 292 Wh | A desk-sized pack | Router, laptop, lights |
| 1024 Wh | A two-handed pack | Adds a fridge for part of a day |
| 1070 Wh | A two-handed pack | Adds a fridge for part of a day |
An amp-hour means nothing until you know the volts
Jump starters and phone banks are usually rated in milliamp-hours, because that is the convention for the cells inside them. A rating of 26800 mAh is 26.8 Ah. On its own that number is not energy. Multiply amp-hours by the pack voltage and you get watt-hours.
The trap is that the voltage used for the mAh figure is often the raw cell voltage, not the 12 V the device delivers at the clamps, and not the 5 V it delivers at a USB port. Two products with the same mAh number can hold different amounts of energy. This is why the site compares power stations in watt-hours and does not try to rank jump starters by capacity at all.
- Watt-hours = amp-hours x volts. That conversion needs the voltage the rating was taken at.
- A milliamp-hour is a thousandth of an amp-hour, so 26800 mAh is 26.8 Ah.
- Where a listing states watt-hours directly, use that figure and ignore the mAh number.
- Where a listing states only mAh and no voltage, the honest answer is that the energy content is not stated, and this site records it as not stated rather than guessing.
Rated capacity is not delivered capacity
The rating describes the cells. The outlets are downstream of an inverter, which converts stored direct current into the alternating current a household plug expects, and that conversion is not free. Packs are also not run flat to protect cell life, and cold reduces what a pack will give up.
This site handles that with a single derate of 0.85 applied everywhere, so that hour figures on a stage page, a device page, and the Runtime Estimator all come from the same arithmetic. Written out:
- hours = (rated watt-hours x 0.85) / continuous watts
- The 0.85 covers inverter conversion loss and the portion of rated capacity a pack will not hand back.
- It is one planning figure applied to every unit. It is not a measurement of any particular model, and a specific unit may do better or worse.
What this arithmetic leaves out
Division is the easy part. The reasons a real outage does not match the division are worth knowing before you plan around a number.
- Startup surge. Anything with a motor pulls far more current for the first fraction of a second than it does while running.
- Duty cycle. A fridge compressor is not on continuously, so its nameplate wattage overstates its average draw by a wide margin.
- Temperature. A pack in a cold garage gives back less than the same pack indoors.
- Age. Cells lose capacity with cycles, and a pack several years into its life is not the pack on the label.
- Model-specific efficiency. Two units of the same rated capacity do not have the same inverter.
- Your actual load. Nameplate wattage is a ceiling, not a reading. The label on the appliance is the number to check.
Convert a milliamp-hour rating into watt-hours
Turn a battery capacity given in milliamp-hours into watt-hours so it can be compared against a power station rating.
- Find the voltage the rating was taken atLook on the listing or in the manual for the voltage that goes with the milliamp-hour figure. If it is not stated, stop here: the energy content is unknown and any conversion would be a guess.
- Convert milliamp-hours to amp-hoursDivide the milliamp-hour figure by 1000. A rating of 26800 mAh becomes 26.8 Ah.
- Multiply by the voltageAmp-hours multiplied by volts gives watt-hours. This is the figure that can be compared against a power station capacity.
- Apply the derate before planning around itMultiply the result by 0.85 to account for conversion loss and unusable capacity, then divide by the wattage of your load to get an estimated number of hours.