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One Power Bank Is Never Enough: How to Actually Keep a Phone Alive on a 16-Hour Sightseeing Day

I've watched my phone die at 4pm in a city I couldn't navigate, with my boarding pass and map both trapped inside it. Here's the real mAh math on the best power bank capacity for travel — and why the number on the box lies to you.

The best power bank capacity for travel isn’t the biggest number you can find — it’s the smallest one that actually gets you through your day, and figuring that out means understanding why the number printed on the brick is basically marketing. I learned this the hard way in Porto, phone at 2%, boarding pass inside the phone, power bank “fully charged” but somehow already empty. Standing on a corner. Brilliant.

Let me do the math nobody does before they buy.

That 10,000mAh on the box? That’s the capacity of the cells inside, measured at 3.7 volts. Your phone charges at 5 volts (or higher with fast charging). Pushing power from one voltage to another wastes energy as heat — conversion loss — and then there’s the bank’s own circuitry sipping a bit too. The real-world number you get out is roughly 60-70% of the label.

So a 10,000mAh bank gives you about 6,000-7,000mAh of actual charge. A modern phone battery is around 4,000-5,000mAh. Do the division. That’s about 1.5 full charges. Not the “charge your phone three times!” the listing implied. One and a half. Maybe two if your phone’s small and old.

This is why your single bank dies before you do. You did the napkin math wrong, because the box wanted you to.

Be honest about your drain. A heavy sightseeing day — maps running constantly, photos, a couple of videos, hunting for signal which absolutely murders battery, plus the heat if you’re somewhere warm — can chew through 1.5 to 2 full phone charges on top of your morning’s start.

So you want real usable capacity of roughly two phone batteries. That’s a 15,000mAh bank in printed terms (which gives ~9,000-10,000mAh usable), or two 10,000mAh banks if you’d rather spread the weight and have a backup. I travel with a 20,000mAh now — an Anker 737, runs about £100 but it’s a workhorse — for the days I know I’ll be out from dawn till a late dinner. For a light day a single 10,000mAh slab is plenty and barely weighs anything.

Here’s a trick that costs nothing. The order you charge things in matters.

Overnight, charge the power bank off the wall, not your phone. Phones charge fast; banks charge slow. So plug the bank into the wall before bed and charge the phone from the bank if you must, or top the phone briefly in the morning. You wake up with a full bank, which is the thing that takes hours to fill, and a phone that fills in 40 minutes while you shower.

During the day, charge the phone before it gets low, not after. Lithium batteries charge fastest between about 20% and 80%, and slow to a crawl near full and near empty. Topping up from 40% to 80% over coffee is far more efficient than draining to 5% and trying to claw it back. Little sips, not one big gulp.

And use the wall whenever you can. Every cafe, every museum lobby, every train seat with a socket — plug into mains, not your bank. Save the bank’s juice for when there’s no wall in sight. Treat the power bank as your emergency reserve, not your daily driver.

Before you buy a bigger brick, strangle your phone’s drain. These add up fast:

• Low Power / Battery Saver mode all day. It throttles background nonsense and easily adds a couple of hours. Just leave it on while traveling.
• Download offline maps for the city before you go. Live GPS with constant data is one of the biggest drains there is; offline maps with GPS-only cut it hugely.
• Drop to 4G/LTE instead of 5G. 5G hunts harder for towers and drains faster, and you don’t need it to read a map.
• Turn the screen brightness down and the auto-lock to 30 seconds. The screen is the single hungriest thing on the phone, by a long way.
• Kill background app refresh for everything that isn’t maps and messages.

Do all of that and a day’s drain can drop by a third. That third might be the difference between needing a 20,000mAh monster and getting by on a pocketable 10,000.

So the answer to “what capacity” is: figure your real day’s drain, double a phone battery, buy a bank with about 1.5x that in printed mAh to cover the conversion loss, stay under 100Wh, and charge in the right order. The number on the box was never the real number. Now you know what is.

Carry one 10,000mAh for light days, one 20,000mAh for the dawn-to-midnight ones, screenshot the things that matter — and never stand on a Porto corner at 2% again.

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