Power Bank Charge Calculator
Enter the capacity of the power bank and of your device battery. You get the number of full charges you can expect after losses.
This is an estimate. Results are lower in cold weather and with fast charging.
Worked examples
A phone and a power bank
10,000 mAh at 3.7 V against 4,000 mAh at 3.85 V, 70% efficiency, 18 W charger
1.7 charges, 2.4 hours to recharge
37 Wh at 70% is 25.9 Wh against 15.4 Wh for the phone.
About Power Bank Charge Calculator
A power bank charge calculator tells you how many times a power bank will charge your device. The capacity printed on a power bank is not the amount that reaches your phone. It is measured at the voltage of the cells inside, usually 3.7 volts, and some energy is lost as heat when the power bank raises its voltage to 5 volts or more to charge a device. In practice you get about 60% to 80% of the printed figure, and the calculator uses 70% by default.
It turns both capacities into watt hours so that batteries with different voltages can be compared fairly. The usable energy divided by the energy in your device is the number of full charges. If you enter the power of the wall charger you use for the power bank, it also estimates how long the power bank takes to recharge, allowing about 15% for losses.
How to use Power Bank Charge Calculator
- Enter the capacity of the power bank and its voltage, usually 3.7 V.
- Enter your device battery and its voltage.
- Adjust the efficiency and the wall charger power.
- Read the number of charges and the recharge time.
Common uses
- Choosing a power bank for travel
- Checking a phone will last a festival
- Comparing power banks
- Planning a camping trip
Frequently asked questions
How many times will a 10,000 mAh power bank charge a phone?
A 4,000 mAh phone battery at 3.85 V holds 15.4 Wh, and the power bank holds 37 Wh. At 70% efficiency that is 25.9 Wh, or about 1.7 full charges.
Why do I get less than the number on the box?
The printed capacity is at the cell voltage, and the step up to the charging voltage wastes energy as heat. Cables, a hot day and fast charging all lose more.
How long does the power bank take to charge?
The energy divided by the charger power, plus losses. A 37 Wh power bank on an 18 W charger takes about 2.4 hours.
Can I take a power bank on a plane?
Rules limit the size, usually to 100 Wh without permission, and they must go in hand luggage. Check with your airline.
What does mAh mean?
Milliamp hours. It is a measure of how much charge a battery holds, and you need the voltage to compare different batteries.