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Power Bank Buying Guide: How Many mAh Do You Actually Need?

The mAh number on a power bank’s box is measured under lab conditions that have almost nothing to do with how it charges your iPhone. Voltage conversion losses alone typically eat 20-30% of that number before it ever reaches your battery. Here’s how to translate the label into something useful.

Tiny Tech Box Team · Aug 6, 2026 · 6 min read

The rough math

A modern iPhone battery holds somewhere between 3,000 and 4,500 mAh depending on the model. A power bank rated at 10,000mAh will realistically deliver about 6,500-7,500mAh of usable charge — roughly one and a half to two full iPhone charges, not “three charges” as some listings imply.

Quick reference

5,000mAh ≈ one partial top-up. 10,000mAh ≈ 1.5-2 full charges. 20,000mAh ≈ 3-4 full charges, but adds real pocket weight.

Match the size to how you'll use it

A pocket-size 5,000mAh bank is enough for most single-day top-ups and is the one people actually remember to carry. A 20,000mAh brick makes sense for multi-day travel, but it’s the one that ends up left in a drawer if it’s too bulky for daily use.

Don't ignore output wattage

Capacity gets all the marketing attention, but output wattage determines how fast that capacity actually reaches your phone. A high-mAh bank with a weak output port will still charge slowly — check the watts, not just the mAh, before you buy.

How many mAh power bank do I need?

Start from how you actually run out of battery, not from the biggest number on sale.

How you use it	Size to buy	Example
A top-up to get through the evening	5,000mAh	Belkin 5,000mAh Qi2 ($59.99)
A full extra day for one phone	10,000mAh	Anker 10,000mAh 30W ($25.99)
A weekend away, or two people	20,000mAh	CUKTECH 45W 20,000mAh ($29.99)
A phone plus a laptop	20,000mAh or more, with 45W+	Anker 737 (24,000mAh, 140W) ($109.99)

Where the missing mAh goes

Two things eat into the number on the box. First, the bank itself loses some energy converting its battery voltage to USB power. Independent testing measured the Belkin BoostCharge at about 89.5% efficient, which is good. Then your iPhone’s own charging circuit loses a little more. Together, that’s why a 10,000mAh bank gives about 1.5–2 iPhone charges, not three.

Magnetic or cable?

Magnetic banks snap onto the back of an iPhone with no cable. That’s convenient, but wireless charging is slower and wastes more energy as heat than a cable. The Belkin 5,000mAh Qi2 adds a kickstand, which is handy for watching while it charges. If you care most about speed and getting the most from the capacity, use a cable. The best power banks for iPhone guide covers both.

Check the airline limit before you fly

Airlines allow power banks up to 100Wh in carry-on bags without approval, and never in checked luggage. To convert, multiply mAh by 3.7 and divide by 1,000: a 20,000mAh bank is about 74Wh, and even the 24,000mAh Anker 737 is under the limit.

A built-in cable is worth having

Many newer banks, including the Anker 10,000mAh and CUKTECH, have a USB-C cable built in, so there’s nothing to forget. The trade is that a built-in cable is a wear point: if it frays, that port is done. A bank with a second USB-C port gives you a backup.

Bottom line

Buy the smallest capacity that covers your actual daily gap, not the biggest number you can find. The power bank you’ll actually carry beats the one that promises the most charges on paper.