A box that says 20000 mAh and up to 112 hours is not lying, but it is telling you two numbers that almost never happen together. Capacity is a fuel tank. Runtime depends on how hard you drive. Once you can convert between the two, comparing fans stops being guesswork.
Capacity needs a voltage to mean anything
Milliamp hours only describe charge, not energy. Multiply by cell voltage, usually 3.7 volts, and divide by a thousand to get watt hours. A 20000 mAh pack is about 74 watt hours. That single number is what you should compare across models on pages like https://socoolfan.net/, because it survives every marketing trick.
Then divide by the draw
A small wearable motor on its lowest speed might pull under a watt. On maximum, with two motors and a light running, it can pull eight or ten. Seventy four watt hours divided by nine watts is about eight hours. Divided by two thirds of a watt, it is over a hundred. Both numbers are real, and only one of them is on the box.
Where the rest of the energy goes
Conversion from cell voltage to motor voltage is never free, so expect to lose a fair share to the electronics. A built in flashlight, an LED display, or a wireless remote receiver all draw power the whole time they are lit. Cold mornings cut usable capacity too.
How to translate a spec sheet
- Convert everything to watt hours before you compare two models.
- Assume the headline runtime is speed one with every extra turned off.
- Look for a stated runtime at the top speed, and be suspicious if it is missing.
- Check input wattage, since a big pack on a slow charger means overnight.
- Note whether it runs while plugged in, which matters more than capacity indoors.
Capacity fades with age
Lithium cells lose usable capacity over hundreds of cycles, faster if they live hot or sit at full charge in a truck all summer. A two year old pack that once ran a full shift may quit at lunch, and that is normal wear, not a defect.
Padded numbers are common
Cheap packs are sometimes rated at cell capacity rather than delivered capacity, or simply overstated. A useful sanity check is weight: lithium cells of a given capacity weigh a predictable amount, so a very light fan claiming a very big pack is worth doubting.
Match the number to your day
A roofer needs a full shift at medium speed, which is a real capacity requirement. A commuter needs twenty minutes twice a day and should buy something light. Camping is different again, because you want days between charges at low speed.
The one line version
Watt hours divided by watts equals hours. Everything else on the box is a description of the best case. Pick the capacity that covers your actual shift at the speed you actually use, then stop reading the marketing copy.