Power Banks Explained: mAh, Watts and What You Actually Need

Ad: this guide contains affiliate links. If you buy through them we may earn commission, at no extra cost to you.

Quick answer

mAh tells you how much energy a power bank stores; watts tell you how fast it can charge. For most people, a 10,000mAh bank covers a day out with a phone, while a 20,000mAh bank gives several phone charges or a top-up for a tablet or small laptop. If you want fast charging, check the bank’s output in watts matches what your device supports, usually over USB-C. For flights, power banks must go in hand luggage, and most airlines allow those under 100Wh without approval.

mAh: how much is in the tank

Milliamp-hours (mAh) describe a battery’s capacity. A bigger number means more stored energy. But a 20,000mAh power bank will not give a phone with a 5,000mAh battery four full charges, for two reasons:

1. Voltage conversion. Power bank cells usually store energy at around 3.6 to 3.7 volts, but USB outputs at 5V or higher. Converting the voltage uses some energy.
2. Heat and losses. Some energy is lost as heat during charging.

As a rough rule of thumb, expect to get somewhere around two-thirds of the rated capacity into your devices. The exact figure varies by model and how you use it.

Rough size guide

Capacity	Best for	What to look for	Things to know
5,000mAh	Emergency phone top-up, small bags	Slim, light design	Usually around one phone charge or less
10,000mAh	A day out, commuting	Pocket-friendly size, USB-C	A good all-rounder for phones
20,000mAh	Weekends away, tablets, sharing	Multiple ports, higher wattage	Heavier, but more flexible
25,000mAh+	Laptops, camping, festivals	High-watt USB-C output	Check flight limits before travelling

Watts: how fast it charges

Watts (W) measure power: how quickly energy flows. A higher wattage output can charge compatible devices faster.

• Phones often support fast charging somewhere between around 15W and 45W, depending on the model.
• Tablets can take a little more.
• Laptops that charge via USB-C usually need much more, often 45W to 100W.

A power bank cannot make a device charge faster than the device allows, and a device will only charge as fast as the bank and cable can deliver. For the best results, match all three: bank, cable and device.

Fast-charging standards

• USB Power Delivery (USB-C PD) is the most widely supported standard for phones, tablets and many laptops.
• PPS (Programmable Power Supply), part of USB PD, is used by some Android phones for their fastest charging modes.
• Quick Charge is a Qualcomm standard used by some Android devices.

Check your device’s charging specs in its manual or on the manufacturer’s website, then look for a power bank that supports the same standard.

Wh: the number airlines care about

Watt-hours (Wh) express capacity in a way that does not depend on voltage. Airlines use Wh to set limits. You can estimate it like this:

Wh = (mAh × volts) ÷ 1,000

Using the typical cell voltage of 3.7V, a 20,000mAh bank works out at about 74Wh. Many power banks print the Wh figure on the casing; always use that if it is there.

Flying with a power bank from the UK

• Power banks must go in hand luggage, not checked bags.
• Banks under 100Wh are usually allowed without airline approval.
• Banks between 100Wh and 160Wh generally need the airline’s approval, and there is often a limit on how many you can carry.
• Banks over 160Wh are not normally allowed on passenger aircraft.

Airline policies vary and some have added extra rules about using or charging power banks on board. Check your airline’s website and the UK Civil Aviation Authority’s guidance before you travel.

Ports and extras

• USB-C in and out is the most useful feature on a modern bank. It charges the bank and your devices from the same cable.
• Multiple ports let you charge a phone and earbuds together, though total output is often shared.
• Pass-through charging lets you charge the bank and a device at the same time. Not every bank supports it.
• A display showing remaining charge is more precise than a row of LEDs.

Deals on our radar

Product	Best for	What to look for	Deal
INIU 20000mAh Power Bank, 45W Fast Charging	Phones and tablets over a long weekend, plus faster USB-C charging	Check which ports support 45W and which charging standards it lists	INIU 20000mAh Power Bank, 45W
AstroAI Car Jump Starter Power Bank	Drivers who want a jump starter that can also charge a phone	Check the USB output and how to keep it charged between uses	AstroAI Car Jump Starter Power Bank

If you are upgrading your phone too, phones such as the Google Pixel 11 Pro charge over USB-C, so a USB-C PD power bank and a good-quality cable are a sensible pairing. Check Google’s specifications for the exact charging speeds supported.

Safety tips

Power banks contain lithium-ion cells, which are safe when well made and used properly. To reduce risks:

• Buy from reputable brands and retailers, and look for a UKCA or CE mark.
• Don’t leave a bank charging unattended for long periods, especially overnight on a bed or sofa.
• Stop using a bank that swells, gets very hot, smells or is damaged. Recycle it at a battery recycling point; never put it in household rubbish.
• Keep banks out of direct sunlight and hot cars in summer.
• Use good-quality cables, especially at higher wattages.

What do you actually need?

• Commuter with one phone: 10,000mAh with USB-C PD.
• Family days out or weekends away: 20,000mAh with at least two ports.
• Laptop on the go: a bank with enough USB-C wattage to match your laptop’s charger, and check the Wh rating for flights.
• Car emergency kit: a jump starter with USB output, kept charged.