Here’s a story that ends in mild betrayal: you buy a 10,000mAh power bank. Your phone battery is 5,000mAh. Simple math says two full charges. You get one, maybe 1.2 if you’re lucky — and you quietly wonder if you got scammed.
You didn’t. You just did the math the manufacturer’s marketing hopes you’ll do. Here’s the real math.
mAh is a fuel tank measurement — nothing more.
mAh stands for milliampere-hour: how much current a battery can deliver over an hour. A 10,000mAh power bank genuinely holds that much energy — but at its internal cell voltage of 3.7 volts.
Your phone’s charging port runs at 5 volts (or higher). Converting 3.7V to 5V, plus heat lost during conversion, plus losses in the cable, means the power bank’s real-world efficiency is around 60–70%.
The worked example:
10,000mAh × 0.65 (typical efficiency) = 6,500mAh actually delivered.
Your phone then loses another 10–15% converting incoming power into battery charge. Net result: about 5,500mAh reaches your battery. For a 5,000mAh phone — that’s the 1.1 charges you experienced. The math checks out; the marketing just skipped the asterisk.
The rule of thumb you’ll never forget:
Divide the advertised number by 1.5 to estimate realistic output. Or flip it: for two full phone charges, buy a bank roughly three times your phone’s battery capacity.
What size fits whom:
- 5,000mAh — a pocket top-up, about half a charge. For emergencies, not weekends.
- 10,000mAh — the sweet spot. Roughly 1.5 phone charges, fits in a jacket pocket, no charging anxiety on a normal day.
- 20,000mAh — flights, festivals, camping: 3–4 charges. Nearly all come with two ports so you can share.
- Above 27,000mAh — read this carefully: airlines cap carry-on batteries at 100 watt-hours, which at 3.7V works out to roughly 27,000mAh. Anything bigger is not allowed on the plane, period.
Two specs people confuse: capacity vs. speed.
mAh is the tank size — how many charges. Watts is the pump speed — how fast each charge fills. A huge 30,000mAh bank with a 10W output will charge your phone at a crawl; a slim 10,000mAh bank with a 22W USB-C Power Delivery port can hit 50% in 30 minutes on modern phones. When comparing banks, look for “PD” or output wattage of 20W or more — that’s the number that decides your wait time.
How to spot an honest brand:
Well-labeled power banks print two numbers: cell capacity (the marketing number) and rated capacity (the realistic output, often stated “at 5V”). Brands that publish the rated capacity are telling you the truth up front. Brands that only print the big number? Now you know what it’s worth — divide by 1.5 and smile.