Power Bank & Battery Runtime Calculator
How many times a power bank will charge your phone, and how long a portable power station or 12 V battery will keep your things running in a power cut.
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Phone charges from a power bank
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How long a battery runs your things
| Device | Watts | Hours a day | How many | Remove |
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How to work out battery runtime
- For a power bank, enter the capacity printed on it (mAh or Wh) and your phone’s battery size.
- For a power station or 12 V battery, choose the type and enter its capacity from the label.
- Add the things you’d run in a power cut, with their watts (on the label or charger) and how many hours a day you’d use them.
- Add a solar panel, if you have one, to see how much longer it keeps going.
- Read how many days it would last; cut the hours, or the devices, to make it last longer.
How the estimate works
Power banks are rated at their cells’ voltage (3.7 V), so a 10,000 mAh bank holds about 37 Wh. Raising that to USB’s 5 V and charging the phone loses about a third of it, so the calculator counts on 65% reaching the phone, which is typical: a 10,000 mAh bank charges a 4,500 mAh phone about 1.4 times, not the 2.2 times the numbers on the labels suggest.
Batteries: watt-hours = amp-hours × volts. A power station can use almost all of its rated capacity; a lead-acid or leisure battery shouldn’t go below half charge, or it’s damaged, so only half counts; a lithium (LiFePO4) battery can give about 90%. Running mains-type (AC) devices through an inverter loses about 15% more. A car battery is meant for starting the engine, so only a quarter is counted, to leave enough to start it.
Solar panels make their rated watts only in full sun, so the calculator multiplies the rating by your full-sun hours a day (often called peak sun hours: roughly 5 to 7 in a southern European summer, 1 to 3 in a northern winter; the EU’s free PVGIS gives them for any place) and counts on 75% of that reaching the battery, after heat, dust, angle and charging losses. If the panel makes more than you use, the battery keeps going on sunny days.
Device watts in the list are typical figures; the real ones are on the device, its plug or its charger (volts × amps = watts). Fridges and freezers switch on and off, so their figure is an average across the day. For a CPAP or other medical device, check its manual: a heated humidifier can use several times more, and the manufacturer can tell you its battery options.
Questions about battery runtime
How many times will a 10,000 mAh power bank charge my phone?
About 1.4 times for a phone with a 4,500 mAh battery, or about twice for a 3,000 mAh one. Power banks lose about a third of their energy converting to USB power and charging, so the real figure is lower than the numbers on the labels suggest.
How long will a 500 Wh power station run a Wi-Fi router?
A 10 W router uses about 240 Wh a day, so roughly 1.8 days through the power station’s mains sockets, or a little longer from its USB or 12 V sockets.
How do I work out watt-hours from amp-hours?
Multiply by the voltage: a 100 Ah, 12 V battery holds 1,200 Wh. For a lead-acid battery only use half of that, to avoid damaging it.
Can a solar panel keep a power station going?
Often, for small loads: a 100 W panel with 5 hours of full sun puts about 375 Wh a day into the battery, more than a router and phone charging use. A fridge as well needs a bigger panel or more sun.
How long will a fridge run on a power station?
A typical fridge averages about 50 W over a day, roughly 1.2 kWh, so a 1,000 Wh power station would run it for about 17 hours. Starting the compressor needs a burst of power, so check the power station’s surge rating too.