Why a 20,000mAh Power Bank Never Gives You 20,000mAh

You buy a 20,000mAh power bank. Your phone has a 5,000mAh battery. You expect four full charges. You get somewhere between two and a half and three.

Nothing is wrong with the power bank. The arithmetic that produced "four" was the wrong arithmetic, and the reason is worth understanding because it also tells you which power bank to buy.

Milliamp-hours are not a unit of energy

This is the whole trick. A capacity in mAh only means something alongside a voltage. Energy is the two multiplied together — that is what watt-hours measure.

• The lithium cells inside a power bank run at about 3.7V
• The USB output must deliver 5V (or more, for fast charging)
• Your phone's own battery is, again, about 3.7V

So the pack's real energy content is 20,000mAh × 3.7V ≈ 74Wh. To push that out at 5V, the electronics must step the voltage up, and the charge measured in mAh shrinks in proportion: 74Wh ÷ 5V ≈ 14,800mAh at the output.

You have lost roughly a quarter of the number before any inefficiency at all. It was never there to begin with.

Then come the real losses

On top of the voltage conversion, three things take a further cut:

Notice that both ends get warm while charging. That warmth is the loss, made visible.

A rule of thumb that works

For a 5,000mAh phone: a 20,000mAh pack gives about two and a half full charges. That is the honest figure, and it is the one to plan a trip around.

What this means when choosing one

Compare watt-hours, not milliamp-hours. Most packs print the Wh figure somewhere on the casing or in the listing's specifications, because airlines require it. It is the only number that compares two packs fairly, especially if they use different cell chemistries.

Check output wattage, not just capacity. A large pack that only outputs 10W will charge a modern phone slowly no matter how much energy it holds. For phones, 18W–30W is the useful range; for a laptop over USB-C, look for 45W, 65W or higher, and check that the listing says Power Delivery rather than just "fast charging".

Watch the input. A 20,000mAh pack that only recharges at 10W takes most of a day to refill itself. If the pack supports fast input, the listing will state it separately from the output — the two are often different.

Weight scales with energy, not with cleverness. Roughly 350–400g for 20,000mAh is normal. A pack claiming very high capacity at a suspiciously low weight is claiming something that physics does not currently allow.

The airline limit

Power banks must travel in cabin baggage, never checked, and there is a size cap expressed in watt-hours. The common international limit is 100Wh without airline approval, which works out at roughly 27,000mAh at cell voltage. A 20,000mAh pack (≈74Wh) is comfortably inside it; some very large packs are not. If you fly often, check the printed Wh figure before buying, and check your airline's current rules — they vary and they change.

Choosing a size

Browse 20,000mAh power banks on Amazon

For most people a 10,000mAh pack is the better daily choice — it delivers a full phone charge, weighs half as much, and is far more likely to actually be in the bag when it is needed.

Browse 10,000mAh power banks on Amazon