Power Banks Explained: mAh vs Wh, Real Capacity and Signs of a Fake
With unreliable power supply in many areas, a power bank is one of the most useful accessories you can own. It is also one of the easiest to buy badly, because the number on the box, such as “20,000mAh”, does not tell you how much charge you will really get. This guide explains the maths in plain terms and shows how to choose a safe, honest product.
What mAh means, and why it is misleading
mAh (milliamp-hours) measures how much electric charge a battery holds, but the total energy also depends on voltage. Power bank makers quote capacity at the voltage of the internal lithium cells, which is usually about 3.6 to 3.7 volts. Your phone, however, charges at around 5 volts (or more with fast charging), and the power bank has to convert between them. That conversion wastes some energy as heat.
Convert to watt-hours (Wh) to compare fairly
Watt-hours give you real energy, and they let you compare any two batteries. The formula is simple:
Wh = (mAh ÷ 1000) × voltage
For example, a 20,000mAh power bank with 3.7V cells holds (20,000 ÷ 1000) × 3.7 = 74Wh. A well-made power bank might lose 20% to 40% of that in conversion, so you can expect roughly 44Wh to 59Wh of usable energy.
How many phone charges is that?
A typical modern phone with a 4,500mAh battery holds about 17Wh. Dividing the usable energy by that gives a realistic number:
- Usable energy of about 44Wh to 59Wh ÷ 17Wh per charge is roughly 2.5 to 3.5 full charges from a 20,000mAh power bank.
- A 10,000mAh power bank (37Wh rated) would give roughly one and a half charges.
If a seller promises that a 10,000mAh power bank will charge your phone five times, the maths does not work.
Choosing the right output
- Phones: a power bank with 10W to 20W output is enough for basic charging; look for fast-charging support if your phone uses it (such as USB Power Delivery).
- Tablets and laptops: these need much more power. Check that the power bank supports USB Power Delivery at the wattage your device needs, often 45W to 65W or more for a laptop.
- Ports: more ports let you charge several devices, but the total output is shared between them.
- Charging the power bank itself: a large power bank with slow input can take many hours to refill, so check its input rating.
Weight is a useful honesty test
Lithium cells are heavy. As a rough rule of thumb, a genuine 20,000mAh power bank typically weighs several hundred grams. If a power bank claims a very large capacity but is remarkably small and light, it almost certainly contains far less battery than advertised. Compare the weight and size with similar models from well-known brands.
Warning signs of a fake or low-quality power bank
- A capacity that seems impossible for the size and weight, or a price far below comparable models.
- No watt-hour rating, no clear input and output ratings, or vague printing on the case.
- No recognisable brand, no safety or regulatory marks, and no manufacturer contact details.
- Spelling mistakes, blurry printing or a flimsy case.
- It gets very hot when charging, charges much more slowly than it should, or dies after only a fraction of the expected charge.
Poorly made lithium batteries can overheat, swell or, in rare cases, catch fire, so safety is a good reason to buy from a seller you can return the product to.
Safety and travel
- Do not use a power bank that is swollen, damaged or very hot. Stop using it and dispose of it safely.
- Do not leave it in a hot car or in direct sun, and do not cover it while it charges.
- Use the cable it came with, or a good-quality cable, to avoid overheating.
- Airlines generally require power banks to travel in carry-on luggage, not checked bags. Most airlines allow power banks up to 100Wh without special approval, so a 20,000mAh power bank at about 74Wh is typically fine, but check your airline’s current rules before you fly.
Get more life from your power bank
Charge and store it at moderate temperatures, avoid running it completely flat all the time, and top it up every few weeks if you store it. Lithium batteries wear out gradually, so expect the real capacity to fall after a couple of years of regular use.
Summary
Convert mAh to Wh so you can compare honestly, expect to use about 60% to 80% of the rated energy, match the output to your devices, and be wary of impossibly cheap, light or high-capacity claims. Buy from a seller with clear return terms, and keep the receipt.