How to Choose a Power Bank: mAh, Watts and the Numbers That Actually Matter
Power banks are the most misunderstood accessory people buy. The number printed on the box — 10000 mAh, 20000 mAh — is real, but it is not the number that decides whether the pack is any good for you. Here is what to look at instead.
Rated capacity is not usable capacity
The cells inside a power bank run at roughly 3.7 volts. Your phone charges at about 5 volts. Converting between the two loses energy as heat, and the conversion is never perfect. In practice you get somewhere around 60–70% of the printed figure delivered into your phone.
So a 10000 mAh pack realistically hands over about 6000–7000 mAh. If your phone has a 5000 mAh battery, that is a little over one full charge — not two, whatever the marketing suggests. This is physics, not a defect, and it applies to every brand.
A quick rule of thumb:
- 10000 mAh — roughly one full phone charge, plus a little. Pocketable, good for a day out.
- 20000 mAh — roughly two to three charges. Heavier, better for travel or two-device households.
- 5000 mAh — a top-up, not a charge. Useful as an emergency reserve.
Watts decide the speed, not the capacity
Capacity tells you how much energy is stored. Wattage tells you how fast it comes out. These are completely independent, and a large pack with a slow output is a common and frustrating combination.
Wattage is volts multiplied by amps. A 5V/2A output is 10W — an older, slow standard. A 22.5W or 20W output is meaningfully quicker, often taking a phone from flat to half full in around half an hour rather than well over an hour.
Two things have to line up for fast charging to work:
- The power bank must support a fast-charging protocol — usually USB Power Delivery (PD) or Quick Charge (QC).
- Your phone must support the same one.
If either side is missing, the two devices fall back to the slowest speed both understand. That is why a fast power bank can still charge slowly on the wrong phone, and why the cable matters too.
Check the ports before you buy
Look at what is physically on the pack:
- USB-C in/out is what you want. A single USB-C port that both charges the pack and charges your phone is the modern standard.
- Micro-USB input only is a warning sign on any pack sold today. It means the pack itself will take several hours to refill.
- Built-in cables save you carrying one, but they are the part most likely to fail, and you cannot replace them.
Pass-through charging
Pass-through means the pack can charge your phone while it is itself plugged into the wall. It is genuinely convenient when you have one free socket, but it generates more heat and not every pack supports it. If it matters to you, confirm it before buying rather than assuming.
Safety is not optional
A power bank is a lithium cell in your pocket. Cheap, unbranded packs cut costs on exactly the components that stop them failing badly — the protection circuit that prevents overcharge, over-discharge, short circuits and thermal runaway.
Practical checks:
- The pack should feel dense. Suspiciously light usually means smaller cells than claimed, sometimes padded out with filler.
- It should get warm in use, never hot enough to be uncomfortable to hold.
- Any swelling, casing that no longer sits flush, or a burning smell means stop using it immediately.
- Airlines cap what you can carry in the cabin — generally up to 100 Wh without approval, which covers essentially all consumer packs up to about 27000 mAh. The pack should have its Wh rating printed on it.
Matching the pack to how you actually use it
Be honest about your day rather than buying the biggest number you can afford. Someone commuting with a phone that lasts until evening needs a slim 10000 mAh pack they will actually carry. Someone on the road for two days with a phone, earbuds and a watch needs 20000 mAh and multiple ports, and will accept the weight.
The best power bank is the one that is in your bag when the battery hits 5%. A 20000 mAh brick left at home because it is too heavy has a real-world capacity of zero.
