How battery life is worked out
A battery's real energy content is measured in watt-hours (Wh). Milliamp-hours (mAh) only tell half the story because they depend on voltage: a 3.7 V and a 7.4 V battery with the same mAh hold very different amounts of energy. Convert to watt-hours first, take off conversion losses, then divide by how fast the device uses power.
The formula
usable Wh = Wh × efficiency
run time (h) = usable Wh ÷ device watts
charges = usable Wh ÷ device battery Wh
Example: 10,000 mAh power bank running a 5 W device
| Step | Result |
|---|---|
| Energy: 10,000 × 3.7 ÷ 1000 | 37 Wh |
| Usable at 65% | 24.1 Wh |
| Run time: 24.05 ÷ 5 | 4 h 49 min |
Typical power draw
| Device | Approximate watts |
|---|---|
| Phone, light use | 1–3 |
| Phone, gaming or GPS navigation | 4–8 |
| Tablet | 5–10 |
| Handheld gaming PC (Steam Deck class) | 10–25 |
| Thin laptop, web and office | 8–15 |
| Gaming laptop on battery | 40–100+ |
Frequently asked questions
How do I convert mAh to Wh?
Multiply mAh by the battery voltage and divide by 1,000. A 10,000 mAh power bank uses 3.7 V lithium cells, so it holds 10,000 × 3.7 ÷ 1,000 = 37 Wh. Watt-hours let you compare batteries with different voltages fairly.
How many phone charges will my power bank give?
Divide the power bank's watt-hours by your phone battery's watt-hours, then multiply by about 0.6 to 0.7 for conversion losses. A 37 Wh (10,000 mAh) bank charges a 15 Wh phone battery about 1.5 to 1.7 times.
Why does my power bank give fewer charges than its mAh suggests?
Power bank mAh is rated at the internal 3.7 V cells, but USB charging happens at 5 V or higher, and both the bank and phone lose energy as heat. Typical real-world efficiency is 60 to 75%.
How long will a battery last at a given wattage?
Divide usable watt-hours by the device's power draw in watts. A 50 Wh laptop battery powering a 10 W load lasts about 5 hours, minus a little for conversion losses and the reserve the battery keeps to protect itself.
Can I take a power bank on a plane?
Most airlines follow the FAA and IATA rules: power banks must go in carry-on bags, those up to 100 Wh are allowed, 100 to 160 Wh need airline approval and over 160 Wh are not allowed. Check your airline before flying.
Rates and figures last checked October 2026.
This calculator gives estimates for planning. Check current rates and product labels before you buy, list or file.