A 20,000mAh power bank stores about 74Wh when its cells are rated at 3.7V. Runtime depends on the connected load. Under an illustrative 80% usable-energy model, about 59Wh reaches the output, which is roughly 11.8 hours at 5W, 5.9 hours at 10W or 3 hours at 20W. These are planning estimates, not measured results for every model.
Calculate Runtime From Watt-Hours and Load
Convert the cell capacity to energy first:
20,000mAh × 3.7V ÷ 1,000 = 74Wh nominal energy
Then choose a transparent usable-energy assumption and divide by the average device load:
Estimated runtime (hours) = usable output energy (Wh) ÷ average load (W)
For illustration, an 80% model gives 74Wh × 0.80 = 59.2Wh. The 80% value is a sensitivity input for planning, not a tested category average or a claim for a particular Keku model. Use measured output Wh when an exact product result is available. The mAh-to-Wh calculator can convert other capacities and cell voltages.
Illustrative Runtime by Average Load
| Average load | Estimated runtime at 59.2Wh | Example planning context |
|---|---|---|
| 5W | About 11.8 hours | Low-power USB accessory |
| 10W | About 5.9 hours | Moderate continuous USB load |
| 20W | About 3.0 hours | Higher phone or tablet load |
| 45W | About 1.3 hours | Compatible laptop-class load |
These examples assume the power bank can sustain the requested output profile. A product limited below the device load may charge slowly, cycle on and off, or not operate the device. Actual loads also vary over time, so a meter-based average is more useful than a peak wattage label.
For the next power-bank procurement step, compare How Long Does It Take to Charge a Power Bank? Quick Guide!.
What Makes Actual Runtime Shorter or Longer?
- Conversion path: raising the cell voltage to 5V, 9V or a higher output voltage consumes energy.
- Load variation: screen brightness, radios, processors and background activity change demand.
- Temperature: very hot or cold conditions can reduce available energy and charging performance.
- Port allocation: connecting another device changes how energy and output power are shared.
- Cutoff behavior: some low-power devices fall below the bank’s minimum-load threshold.
- Battery condition: age, storage and cycle history affect remaining capacity.
Runtime and Full Phone Charges Are Different Questions
A runtime estimate uses the device’s average watts. A phone-charge estimate compares usable power-bank energy with the phone battery’s energy and charging conditions. If the question is “how many times can it charge my phone?”, use the separate 20,000mAh charge-count guide. Do not divide 20,000mAh directly by the phone’s mAh unless both values use the same voltage and losses are accounted for.
Use the Wh Label for Travel Planning
Power banks are spare lithium batteries and belong in carry-on baggage under FAA guidance. A typical 20,000mAh pack rated at 3.7V is about 74Wh, but the product’s own Wh label is the value to check. Protect the terminals and confirm the airline’s current requirements before departure.