Power banks are vital tools for many people, allowing us to get a full working day out of our smartphones. While most people look at the size, weight, or price, the savvier buyer will pay attention to the battery capacity, which is measured in milliamp hours, referred to as mAh for short.

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Some eagle-eyed power bank owners may have noticed that their 10,000 mAh power bank won't give them two recharges of their smartphone, which only has a 4,000 mAh battery.

What gives?

Predictably, bigger numbers are being used to sell. After all, a 10,000 mAh power bank sounds more impressive than a 5,000 mAh, and a 20,000 mAh pack sounds even better.

Still, none of those power banks will give your smartphone anywhere close to the power listed on the packaging or the spec sheet, and I'm going to show you why. It's going to get a little mathy, but don't worry, if you can drive a calculator, you'll be able to follow and do the calculations yourself.

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Most power banks output at a standard voltage of 5V. This is what smartphones, tablets, and earbuds need. It's the power that we were used to getting from an old USB-A wall wart charger. Predictably, USB-C has brought higher voltages into the game to cater to things like laptops. Some power banks can go higher than 5V, but for the purposes of this exploration, we'll limit ourselves to looking at 5V outputs. 

However, the internal lithium battery typically operates at a lower voltage, around 3.7V. This difference is crucial for calculating the actual usable capacity. It's also part of the reason why (yes, just part -- there are a few things at play here) there's a discrepancy between the capacity listed on the box or power bank and its real-world output.

How to calculate your power bank's capacity 

The first thing I do is look for the capacity of the power bank rated in Watt-hours, or Wh, as this offers a truer, more accurate measure of the energy available for use within a battery. Sometimes this is listed with the specs, but sometimes it isn't. If it isn't, you can work it out as follows:

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Wh = (Advertised Capacity measured in mAh)