Power Bank Buying Guide and What the mAh Number Hides

A 20,000 mAh power bank sounds like it should charge a phone with a 5,000 mAh battery four times. In practice it manages about two and a half. Nothing is broken and nobody lied outright. The number on the box is measured in a way that makes it look bigger than what reaches your phone.

This power bank buying guide explains what the mAh figure really means, which other specs decide whether a battery pack is useful, and what I check before recommending one. I have tested a lot of these over the years, from pocket sized 5,000 mAh sticks to brick sized laptop packs, and the same handful of details separate the good ones from the drawer fillers.

What the mAh Number on a Power Bank Hides

Power bank capacity is quoted at the voltage of the lithium cells inside, which is usually 3.6 or 3.7 volts. Your phone, though, receives power over USB at 5 volts or higher. Converting from one to the other costs energy, and the conversion circuit and cable lose a little more as heat.

The result is that a typical power bank delivers around 60 to 70 percent of its rated mAh to the device. A rough rule:

Rated capacity times 0.65 gives the usable capacity in phone terms.
Divide that by your phone battery size to estimate full charges.

So a 10,000 mAh bank gives about 6,500 mAh of real charge, or roughly one and a third charges for a 5,000 mAh phone. If a box promises far more than this, be suspicious of the product rather than impressed by it.

Watt Hours Are the Honest Number

A better figure to look for is watt hours (Wh), which accounts for voltage. It is usually printed in small text on the back of the power bank itself, often because airlines require it. You can calculate it too: mAh times 3.7, divided by 1,000.

Rated capacity	Approx. watt hours	Typical phone charges	Typical weight
5,000 mAh	18.5 Wh	About 0.7	100 to 130 g
10,000 mAh	37 Wh	About 1.3	180 to 230 g
20,000 mAh	74 Wh	About 2.5	330 to 450 g
26,800 mAh	99 Wh	About 3.4	500 to 600 g

That last row is no accident. Most airlines allow power banks under 100 Wh in cabin luggage without approval, so manufacturers build right up to the limit. Power banks are never allowed in checked bags, and a growing number of airlines now restrict using or charging them during the flight, so check your carrier before you fly.

Output Wattage Matters More Than Capacity

Capacity tells you how much energy is stored. Output wattage tells you how fast it gets out, and it decides what you can actually charge.

For phones

Look for USB Power Delivery (USB PD) on a USB C port, at 20 W or more. That covers fast charging on most current iPhones and many Android phones. Some Samsung phones need a feature called PPS to reach their fastest speeds; without it they fall back to a slower rate.

For tablets and laptops

A thin laptop typically needs 45 to 65 W to charge while in use, and larger machines want 100 W. A power bank rated at 30 W will keep a laptop alive slowly but may not charge it under load. The thinking is similar to what I described in what determines how long a laptop battery lasts: the drain while working sets the bar, not the idle figure.

Total versus per port output

A box that says 65 W total may only give 45 W from one port when two devices are plugged in. Check the small print for the split.

The Features Reviews Rarely Mention

After capacity and output, these are the details that decide whether I keep a power bank or hand it on:

Recharge speed. A 20,000 mAh pack that recharges at 10 W takes over eight hours to refill. Look for USB PD input of at least 20 W, ideally 30 W.
Pass through charging. This lets you charge the bank and a phone from one wall plug overnight. Handy for travel, though it warms the battery.
A real charge display. Four dots tell you very little. A percentage readout is far more useful.
Low current mode. Earbuds and smartwatches draw so little that many banks switch off after a minute. A low current mode keeps them going.
Standby drain. Cheap packs can lose several percent a month sitting in a drawer. Good ones hold their charge for months.

Why Bigger Is Usually the Wrong Choice

The common advice is to buy the largest power bank you can afford. I think that is wrong for most people. A 20,000 mAh pack weighs about as much as a full water bottle, and most people stop carrying it within a month. A power bank at home in a drawer charges nothing.

For daily carry, a 10,000 mAh pack with 20 to 30 W USB C output is the sweet spot. It fits a jacket pocket, covers a full day of heavy phone use and refills in two or three hours. Save the 20,000 mAh class for camping, long trips or laptop use.

The second piece of advice I push back on is that magnetic wireless packs are the best upgrade. They are convenient, but wireless charging wastes more energy as heat, so a 5,000 mAh magnetic pack often delivers less than two thirds of a phone charge. Use them for top ups, not as your only backup.

Safety Signs Worth Paying For

A power bank is a dense lithium battery you carry against your body, so cheap shortcuts matter. Buy from brands with a clear warranty and a support page, and look for certification marks for your region. Very light packs claiming huge capacity are a red flag, because real lithium cells have a fairly fixed weight per watt hour.

Retire a pack that swells, gets hot when idle, smells odd or has a cracked case. Do not throw it in household rubbish; take it to a battery recycling point. Treat accessories the same way you would judge any small gadget, as I did in the guide to budget wireless earbuds and the specs that matter: a few key numbers, read honestly, beat a long feature list.

Frequently Asked Questions

Can I leave my phone charging from a power bank overnight?

Yes. Phones stop drawing current once full, and decent power banks shut off output when the load drops. Keep both on a hard surface rather than under a pillow.

How long does a power bank last before it wears out?

Most are rated for around 300 to 500 full cycles before capacity drops noticeably. For a pack used a few times a week, that is roughly three to four years.

Is it bad to store a power bank fully charged?

For long storage, around half charge is kinder to the cells. Top it up every three months so it never drains completely.

What to Buy for Most People

Check the watt hours, not just the mAh. Choose a 10,000 mAh pack with USB PD output of 20 to 30 W, fast USB C recharging and a percentage display, and expect about one and a third phone charges from it. Only go bigger if you know you will carry the weight. For more hands on buying advice, browse the rest of our reviews.