Power Bank Capacity Explained: How Many Charges Will You Really Get?

A 10,000 mAh power bank never delivers 10,000 mAh to your phone. Learn why, and how to calculate the real number before you buy.

mAh vs. Wh: why voltage matters

Milliamp-hours (mAh) tell you how much charge a battery holds, but not how much energy, because energy also depends on voltage. Watt-hours (Wh) combine both, which makes them the fair way to compare batteries.

Wh = mAh × V / 1000

Typical lithium cell: 3.7 V

A 10,000 mAh power bank:

Wh = 10,000 × 3.7 / 1000 = 37 Wh

A phone with a 5,000 mAh battery:

Wh = 5,000 × 3.7 / 1000 = 18.5 Wh

Why you never get the full capacity

Three things eat into the printed number:

Voltage conversion. The bank steps its 3.7 V cell up to 5 V or higher, and every conversion wastes some energy as heat.
Phone-side charging losses. Charging circuits and heat lose a bit more.
Cable and connector resistance. Poor cables lose power as heat, following P = I² × R.

A realistic overall efficiency is about 60–80%. Good fast-charging banks with quality cables sit at the top of that range, and cheap ones at the bottom.

How many phone charges will you get?

Charges = (Bank Wh × efficiency) / Phone Wh

10,000 mAh bank, 5,000 mAh phone, 75% efficiency:

Charges = (37 × 0.75) / 18.5 = 1.5

You get about one and a half charges, not two. That gap is the "missing" capacity.

Bank size	Energy	Charges for a 5,000 mAh phone (75%)
10,000 mAh	37 Wh	1.5
20,000 mAh	74 Wh	3.0
30,000 mAh	111 Wh	4.5

Fast charging: reading the watts

USB-C power delivery negotiates the voltage, and power is simply volts times amps.

Voltage	Current	Power
5 V	3 A	15 W
9 V	3 A	27 W
20 V	3 A	60 W
20 V	3.25 A	65 W

The charger, the cable and the phone must all support the higher wattage. The slowest of the three decides the speed. Learn the basics in our electrical power guide.

How long does the power bank take to recharge?

Time = Bank Wh / (input W × efficiency)

A 37 Wh bank charged with an 18 W input at 85% efficiency:

Time = 37 / (18 × 0.85) ≈ 2.4 hours

Charging slows near the end of the battery, so expect a little longer in practice.

Taking a power bank on a flight

Lithium batteries must travel in your carry-on bag, never in checked luggage. Most airlines allow banks up to 100 Wh without special approval, and many allow larger ones with approval. A 10,000 mAh bank (37 Wh) is well within that, and a 30,000 mAh bank (about 111 Wh) is not. Always check your airline's current rules before you fly.

Buying tips

Compare banks in Wh, not mAh.
Check the output watts match your phone's fast-charging needs.
Use a short, good-quality cable rated for the wattage.
Be cautious of unusually large capacities at very low prices, since real capacity is often much lower than printed.

Common mistakes

Expecting full printed capacity. Always allow for 20–40% loss.
Comparing mAh across different voltages. Convert to Wh first.
Using the cable that came with a cheap accessory. It can limit charging speed and waste power.
Charging in a hot place. Heat reduces efficiency and shortens battery life.

Practice problems

Try each one before you open the answer.

1. A 20,000 mAh power bank is rated at 3.7 V. How many Wh is that?

20,000 × 3.7 / 1000 = 74 Wh.

2. At 70% efficiency, how many full charges does a 20,000 mAh bank give a 4,500 mAh phone?

(20,000 × 0.7) / 4,500 ≈ 3.1 charges.

3. A charger outputs 20 V at 3.25 A. What is its power?

P = 20 × 3.25 = 65 W.

Frequently asked questions

Why does my 10,000 mAh power bank not charge my phone fully twice?

The rating is measured at the battery's internal 3.7 V, but power is lost in voltage conversion, heat and cable resistance. Real usable capacity is usually around 60–80% of the printed number.

Is mAh or Wh a better way to compare power banks?

Wh is better because it accounts for voltage. Wh = mAh × 3.7 / 1000 for typical lithium cells, so two banks with the same mAh but different voltages are directly comparable.

How many watts does fast charging use?

Watts = volts × amps. A 20 V, 3 A USB-C output gives 60 W, and 20 V at 3.25 A gives 65 W.

Can I take a power bank on a plane?

Power banks go in carry-on baggage, not checked luggage. Most airlines allow up to 100 Wh without approval, so a 10,000 mAh bank (about 37 Wh) is fine. Always check your own airline's rules.

Want to estimate runtime for your own battery and load? Use our Battery Life calculator, or convert watts with the Power Converter.

Open the Battery Life Calculator

Power bank capacity, explained: how many charges will you really get?