You can’t really go wrong with a power bank. You choose one, buy it, plug in your phone (or whatever other device that’s low on battery), and it’s ready to charge. Perfect for when you’re traveling!
However, it can be problematic when, for example, your power bank has a stated capacity of 20,000mAh but seems to run out a lot faster than you’d expect. It should be able to charge your phone around 4 to 5 times, but you get nowhere close to that.
As it turns out, there’s a big difference between the stated capacity of a power bank and it’s true capacity. Here’s why.
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Why 20,000mAh isn’t always true
The capacity of a power bank is how much energy it can store at full charge, usually provided in terms of mAh (milliampere-hours). A power bank with a capacity of 20,000mAh can store and release more energy than a power bank with only 10,000mAh of capacity. But you can’t just rely on what’s written in the product description.
Almost every power bank uses lithium-ion cells for energy storage. These cells work internally with a voltage of 3.7 volts (V). However, phones and tablets are usually charged with at least 5 volts. If you connect your phone to the power bank via USB port, the power bank must first convert its voltage from 3.7 V to 5 V.
When a manufacturer states that its power bank has a capacity of 20,000mAh, they’re basing it on 3.7 volts. If the voltage is converted during charging, the actual capacity will also change!
You can calculate it like this:
Stated capacity x 3.7 / converted voltage = converted capacity
For a 20,000 mAh power bank, this would result in a converted capacity of 14,800mAh. But it doesn’t stop there. In practice, the true capacity of a power bank can be even lower than that.
Losses due to heat and physics
When voltage is converted, some of that energy is lost as heat thanks to the physics of thermodynamics. Modern power banks have an efficiency of between 85 and 90 percent, at least for the reputable models. Cheap power banks often only achieve 70 percent. When converting to 5 volts, an efficiency of 85 percent is a good average value.
With that much efficiency, a 20,000 mAh power bank would ultimately have a capacity around 12,600mAh. That’s 63 percent of the stated capacity. Keep this in mind the next time you buy a power bank.
Even lower capacity with Quick Charge, Power Delivery, or laptops
But what happens if you connect a device to the power bank that requires a higher voltage than 5 V? In this case, the capacity will be even worse than stated. While 5 volts is the standard voltage for USB charging, nowadays a power bank might actually deliver more:
- 9 volts for Quick Charge (for faster phone charging)
- 12 volts for charging smaller laptops
- 15 volts for USB Power Delivery
- 20 volts when charging a MacBook or most laptops
The higher the output voltage, the lower the usable capacity. Firstly, converting from 3.7 V to a much higher voltage is a big change. Secondly, the higher the conversion, the lower the efficiency.
It may be helpful to use the following values as a guide for power banks with lithium-ion cells:
| Output voltage | Approximate efficiency | Approximate real proportion of the usable capacity | Calculation (using 20,000 mAh as an example) |
| 5 V (standard USB) | 85% | 63% | 12,600mAh |
| 9 V (e.g. Quick Charge) | 80% | 55% | 11,000mAh |
| 12 V (e.g. smaller laptops) | 75% | 46% | 9,200mAh |
| 15 V (e.g. USB Power Delivery) | 70% | 39% | 7,800mAh |
| 20 V (e.g. bigger laptops) | 65% | 32% | 6,400mAh |
Why Wh is more meaningful than mAh
When shopping for a power bank, check whether the watt-hours (Wh) is also specified in addition to its mAh. This is because Wh describes the actual energy stored, regardless of the voltage.
If you charge a smartphone with 5 V or via USB Power Delivery with 15 V, the actual capacity changes, but the energy remains the same. The Wh specification helps you to better compare power banks with each other. For example, a 20,000 mAh power bank has approximately 74 Wh.
In addition, the watt-hour specification is also the benchmark for safe power banks on flights. Power banks up to 100 Wh can be safely taken in carry-on luggage without raising any issues.
Your power bank mileage may vary
The calculation of a power bank’s capacity using real percentages and watt-hour specifications are good points of reference when buying, but those values may vary in practice.
Possible factors that influence true capacity include:
- Protection mechanisms against deep discharge, so that the cells never become completely empty.
- Temperature, as cold and heat reduce efficiency.
- Quality of the wires and circuit boards used, with poor components increasing conversion losses.
- Quality of the battery cells used.
The latter in particular can push the true capacity extremely low. Our friends at PCWelt were able to see this over a long period of time in their large power bank comparison test, in which they tested all devices with a constant consumption at 5 volts. Some power banks performed extremely poorly, with the worst one only providing 29 percent of its stated capacity (27,000mAh) as true capacity (7,830mAh).
Why did these devices perform so poorly? Cheap power banks skimp on their electronics. The voltage converters are inferior, so the conversion efficiency is often below 70 percent. On top of that, they have poor-quality wires and inefficient protection mechanisms.
The best power bank models, tested
Every now and then, some power banks also surprise on the positive side. The RealPower PB-20k PD, for example, scored the best over the years with 79 percent (unfortunately, it isn’t widely available).
Of the power banks that are readily available in the US, the following seemed to provide above-average capacities as relating to their stated capacities: Iniu Power Bank 22.5W 20,000mAh (73%) and Anker 325 Power Bank 15W 20,000mAh (73%).
What makes these models better? Reputable brands like Anker and Iniu rely on high-quality voltage converters with great efficiencies.
How to buy the right power bank
- Pay attention to watt-hours (Wh): The most crucial bit of information to know, often printed on the back. A 20,000 mAh power bank should have around 74 Wh.
- Go for brand quality: Brands like Anker, Iniu, Ugreen, and Baseus regularly perform better in tests than no-name products.
- High-quality USB cables: If you really want to eke out every bit of efficiency, use short USB-C cables of high quality.