We’ve all been there: you are navigating an unfamiliar city, your phone hits 5%, and panic sets in. A portable power bank is an absolute lifesaver, but buying one can be incredibly confusing. You are instantly hit with numbers like 10,000mAh, 20,000mAh, and Watts.
If you buy the wrong one, it might be too weak to charge your phone, or so heavy that you leave it at home. Let’s cut through the math and figure out exactly what size you actually need.
The “mAh” Lie You Need to Know
Battery capacity is measured in Milliampere-hours (mAh). If your smartphone has a 5,000mAh battery, and you buy a 10,000mAh power bank, you should get exactly two full charges, right?
Wrong. Moving electricity from a power bank, through a cable, and into your phone generates heat. That heat is wasted energy. Because of this physical energy loss, a power bank is only about 70% to 80% efficient.
The Real Math: Take the capacity of the power bank and multiply it by 0.75. So, a 10,000mAh power bank will actually deliver about 7,500mAh of real juice to your devices. Keep this in mind when shopping!
Which Size Should You Buy?
The Pocket Saver (5,000mAh – 10,000mAh)
Best for: A night out, commuting, or emergency backup.
This is the sweet spot for most people. A 10,000mAh bank is roughly the size of a deck of cards and weighs almost nothing. It will give any modern smartphone one to one-and-a-half full charges. It easily fits in a purse or a jacket pocket without weighing you down.
The Weekend Warrior (20,000mAh)
Best for: Camping trips, long flights, or charging a tablet.
These are noticeably heavier (about the weight of a thick paperback book). They will charge your phone 3 to 4 times. If you are going away for a weekend without wall outlets, this is your best friend.
The Laptop Lifesaver (25,000mAh+)
Best for: Digital nomads, remote workers, and heavy travelers.
These are massive, heavy bricks. But capacity isn’t the only thing that matters here. If you want to charge a laptop, the power bank must support USB-C Power Delivery (PD) and output at least 45W to 65W. If it only outputs 15W, it will not charge your laptop—it will just slowly drain while plugged in.
Air Travel Warning: The FAA and most international airlines strictly prohibit power banks over 27,000mAh (or 100Wh) in your carry-on luggage. Never buy a monster 30,000mAh power bank if you plan on taking it on a flight, as airport security will likely confiscate it.