Guide · Updated 2026-10-08

How Big a Power Bank Do You Need? Examples

How to work out the power bank capacity you need in mAh and Wh for a phone, tablet or laptop, why you never get the full rated capacity, and the rules for flying.

A power bank advertised as 20,000 mAh will not charge your 5,000 mAh phone four times. Two things get in the way: capacity is quoted at the battery cell's voltage, and energy is lost as heat while it is converted. Working in watt-hours (Wh) gives a fair comparison and is the figure airlines care about.

Step 1: find your device battery in watt-hours

If the battery is listed in mAh, multiply by its nominal voltage and divide by 1,000. A phone with a 4,500 mAh battery at 3.85 V holds 4,500 ÷ 1,000 × 3.85 = 17.3 Wh. Laptops and tablets usually list Wh directly, or you can find it on the underside or in the specifications.

Step 2: allow for losses

Charging through a power bank is typically 65 to 75 percent efficient. To charge that phone twice with 70 percent efficiency you need 17.3 × 2 ÷ 0.7 = about 49.5 Wh of power bank. Power banks are rated in mAh at 3.7 V, so 49.5 Wh is about 13,400 mAh. Round up to a 15,000 mAh or 20,000 mAh model, depending on what is available.

Step 3: check the output power

Capacity is not speed. A power bank limited to 20 W will charge a phone that can accept 25 W more slowly than the phone allows, and a laptop that needs 65 W will charge slowly, or not at all while in use, from a 20 W output. Check both the power bank's maximum output and the device's maximum charging power, and use a cable rated for the wattage.

Flying with a power bank

Airline rules are based on watt-hours, and the same figure is on the power bank's label. As a general rule:

• Up to 100 Wh: normally allowed in cabin baggage.
• 100 to 160 Wh: usually needs airline approval, and there is often a limit of two.
• Over 160 Wh: not allowed on most flights.
• Power banks must go in cabin baggage, never in the hold.

Some airlines have recently restricted using or charging a power bank on board, so check your airline's current rules before you travel. A 20,000 mAh power bank at 3.7 V is about 74 Wh, which is within the usual limit.

Try your own numbers

The Power Bank Capacity and Flight Rules Calculator works through these steps for your device. For bigger needs such as camping or working off-grid, the Portable Power Station Runtime Calculator adds up several devices. If you are buying a replacement laptop charger, use the Laptop Charger Compatibility Checker.

Frequently asked questions

Why does my power bank never give its full capacity? Energy is lost as heat in the conversion, and the rating is at the cell voltage. Expect about 65 to 75 percent of the rated energy to reach your device.

Can I charge a laptop from a power bank? Only if the power bank has a USB-C Power Delivery output of enough watts, and the laptop supports USB-C charging. Check both.

Figures are estimates to show the method. Always check your device and power bank specifications and your airline's current rules.