Battery Capacity: mAh to Watt-Hours, Runtime, and How Many Charges a Power Bank Gives
October 29, 2026
Battery capacity is quoted in milliamp-hours (mAh) for small devices, but mAh alone doesn't tell you the energy because the voltage matters. Energy in watt-hours is Wh = mAh ÷ 1,000 × volts. Runtime is then Wh ÷ watts of load.
A phone with a 5,000 mAh battery at a nominal 3.85 V stores 5 × 3.85 = 19.25 Wh. If the phone averages 2.5 W of power, the battery lasts 19.25 ÷ 2.5 = 7.7 hours of heavy use.
Skip the math and get your answer instantly:
Open the Unit Converter →Power Banks
A 20,000 mAh power bank is rated at the cell voltage of 3.7 V, so it stores 20 × 3.7 = 74 Wh. When charging a device, losses in voltage conversion and heat mean only about 65% to 70% of that energy reaches the device. At 68%, the usable energy is about 50 Wh. A phone battery of 15 Wh (about 4,000 mAh at 3.85 V) can be charged 50 ÷ 15 = 3.3 times. A laptop battery of 60 Wh can be charged 0.84 times, so a 20,000 mAh bank can't fully recharge a typical laptop. Airlines limit power banks to 100 Wh in carry-on baggage, and up to 160 Wh with approval.
| Battery | Voltage | Energy (Wh) | Runtime at load |
|---|---|---|---|
| 5,000 mAh phone | 3.85 V | 19.25 | 7.7 h at 2.5 W |
| 20,000 mAh power bank | 3.7 V | 74.0 | 50 Wh usable (68%) |
| 10,000 mAh power bank | 3.7 V | 37.0 | 25 Wh usable (68%) |
| 12 V 100 Ah lead-acid | 12 V | 1,200 | 600 Wh usable (50%): 10 h at 60 W |
Larger Batteries: Amp-Hours and Depth of Discharge
For 12 V batteries, Wh = Ah × 12: a 100 Ah battery has 1,200 Wh. Lead-acid batteries should be discharged only about 50% to extend life, so the usable energy is 600 Wh, running a 60 W load for 10 hours (ignoring inverter losses of 10% to 15%). Lithium iron phosphate batteries can use 80% to 90%, so the same capacity gives about 1,000 Wh usable. To size a battery for a load, multiply watts by hours, divide by usable fraction and efficiency: a 100 W load for 8 hours needs 800 Wh ÷ 0.5 ÷ 0.9 = 1,778 Wh of lead-acid, about 148 Ah at 12 V.
Practical Tips
Compare batteries in Wh instead of mAh. Capacity declines with age and in cold temperatures. Fast charging is less efficient. Check the label for the rated capacity and the real capacity (some cheap power banks overstate). Keep batteries within allowed limits for travel. Solar systems and RV setups use the same calculations.
Charging Time
Charging time is energy divided by charging power, adjusted for efficiency: filling a 19.25 Wh phone battery at 18 W takes about 19.25 ÷ 18 = 1.07 hours, plus losses and the slower final phase, so closer to 1.5 hours.
Frequently Asked Questions
How do I convert mAh to Wh?
Wh = mAh ÷ 1,000 × volts.
How long will a battery run a device?
Divide the battery's Wh by the device's power in watts.
Why can't a power bank deliver its full rated capacity?
Conversion losses reduce the usable energy to about 65% to 70%.
How much of a lead-acid battery can I use?
About 50% to preserve its life.
What is the airline limit for power banks?
Generally 100 Wh in carry-on baggage.