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What common power bank sizes can do
With a 3.7 V battery, 85% conversion and a battery in full health. Phone charges assume a 4,500 mAh, 3.85 V phone.
| Device | 5,000 mAh | 10,000 mAh | 20,000 mAh | 26,800 mAh |
|---|---|---|---|---|
| Phone charges (4,500 mAh phone) | 0.8 | 1.6 | 3.3 | 4.4 |
| USB desk fan, 5 W | 3 h 9 min | 6 h 17 min | 12 h 35 min | 16 h 51 min |
| Bluetooth speaker, 8 W | 1 h 58 min | 3 h 56 min | 7 h 52 min | 10 h 32 min |
| Laptop, 45 W | 21 min | 42 min | 1 h 24 min | 1 h 52 min |
| Car tire inflator, 120 W (tires at 5 min each) | 1.6 | 3.1 | 6.3 | 8.4 |
| Portable blender, 150 W (blends at 1 min each) | 6 | 12 | 25 | 33 |
What the label means at 5 V
Power banks are rated in mAh at the 3.7 V of their cells, but your devices charge at 5 V or more. That makes the label look bigger than what you can draw.
| Label | Energy stored | Delivered (85%) | Same as, at 5 V | Cells weigh at least |
|---|---|---|---|---|
| 5,000 mAh | 18.5 Wh | 15.7 Wh | 3,145 mAh (63%) | 69 g |
| 10,000 mAh | 37 Wh | 31.4 Wh | 6,290 mAh (63%) | 137 g |
| 20,000 mAh | 74 Wh | 62.9 Wh | 12,580 mAh (63%) | 274 g |
| 26,800 mAh | 99.2 Wh | 84.3 Wh | 16,857 mAh (63%) | 367 g |
The last column is a sanity check. Lithium-ion cells hold at most about 270 Wh per kilogram, so a bank cannot hold more energy than its cells allow. A 20,000 mAh bank has at least 274 g of cells, and the case, circuit board and cable come on top of that.
How the numbers are calculated
- Energy stored (Wh) = mAh × battery voltage ÷ 1,000. If the label gives Wh, it is used directly.
- Delivered energy (Wh) = stored energy × condition × conversion efficiency. Conversion is 85% for typical USB or boosted outputs.
- Runtime (h) = delivered energy ÷ device watts.
- Uses per charge = delivered energy ÷ (device watts × minutes ÷ 60).
- Phone charges = delivered energy × 0.9 ÷ phone battery Wh. The 0.9 covers the phone’s own charging loss.
- Capacity at 5 V (mAh) = delivered energy ÷ 5 × 1,000.
- Lightest cells (g) = energy stored ÷ 270 × 1,000, which uses the best lithium-ion cells.
It does not model charging time, fast-charge protocols, temperature or the way some devices stop early at low charge.
A quick checklist before you buy a gadget battery
- Compare in Wh, not mAh. A 5,000 mAh pack at 7.4 V holds twice the energy of one at 3.7 V.
- Check the weight. Use the weight check above. A big capacity in a very light case is a warning sign.
- Read the power rating carefully. A listing’s “150 W” may be a peak value, not what it supplies continuously.
- Look for the safety markings and a Wh figure printed on the battery itself.
- Test it if you can. A cheap USB power meter shows the real Wh a bank delivers.
Using it anywhere in the world
- mAh, Ah, Wh and W are the same everywhere, so no conversion is needed.
- The weight check works in grams or ounces.
- Numbers follow your own region’s format.
- Airlines state battery limits in Wh, and their rules differ and change. Always check your airline before you fly.
Why real results fall short
- Comparing mAh across different voltages. Only Wh is comparable.
- Ignoring conversion loss. Boosting 3.7 V cells to 5 V costs about 15% or more.
- Heavy loads and cold. Both reduce what you get out.
- Old or cheap batteries. Capacity falls with age, and some unbranded batteries are rated optimistically.
- Using peak watts. Devices rarely run at their peak rating all the time.
Common questions
Why does my 10,000 mAh power bank not deliver 10,000 mAh?
The label counts mAh at the cells’ 3.7 V, but your phone or fan draws power at 5 V, and the circuit that raises the voltage wastes some energy. A 10,000 mAh bank stores 37 Wh and delivers about 31 Wh at 85% efficiency, which equals about 6,290 mAh at 5 V.
How do I convert mAh to Wh?
Multiply mAh by the battery voltage and divide by 1,000. At 3.7 V, 10,000 mAh is 37 Wh and 20,000 mAh is 74 Wh. Multi-cell packs use higher voltages, so a 5,000 mAh pack at 7.4 V is also 37 Wh. If the label already shows Wh, use that number.
How many times can a power bank charge my phone?
A 10,000 mAh bank charges a 4,500 mAh phone about 1.6 times, and a 20,000 mAh bank about 3.3 times, once conversion and charging losses are counted. That is fewer than the label suggests, because mAh at 3.7 V is not the same as mAh at 5 V.
How can I tell if a power bank’s capacity is realistic?
Check the weight. Lithium-ion cells hold at most about 270 Wh per kilogram, so a 20,000 mAh bank (74 Wh) needs at least 274 g of cells before the case is added. A listing that claims more capacity than its weight allows cannot be right. A USB power meter that shows the delivered Wh is the most reliable test.
How many tires can a cordless inflator fill per charge?
Multiply the inflator’s power by the time per tire to get the energy each one takes. A cordless inflator with a 2,500 mAh, 11.1 V pack stores about 28 Wh. If it draws 100 W and takes 5 minutes per tire, each tire uses about 8 Wh, so you get about three tires per charge. Enter your own pack and watts in the calculator.
Can I take a power bank on a plane?
Usually yes, in carry-on baggage only and not in checked bags. Airlines set the limit in Wh, commonly 100 Wh per battery with approval needed above that, and some limit the number of banks or how you may use them in flight. Rules change, so check your airline’s current policy before you fly.
Related calculators
For bigger batteries, use the battery runtime calculator or the power station runtime calculator. To size a full off-grid system, see the solar and battery calculator.