Marketing departments love big numbers because they assume you failed high school physics. If I had a dime for every time someone complained that their “10,000mAh” power bank only charged their 4,000mAh phone twice before dying, I could buy a better UPS for my server rack. The number on the box is technically true but practically a lie. Here is how to actually calculate what you need so you don’t end up stranded with a dead laptop in an airport terminal.
Why the Number on the Box is Wrong
The problem is voltage conversion. The lithium-ion cells inside the power bank operate at a nominal 3.7 Volts. However, your USB cable pushes power to your phone at 5 Volts (or 9V/12V/20V if you are using fast charging standards like Power Delivery). To bridge that gap, the power bank has to boost the voltage. This process isn’t magic; it costs energy. You lose roughly 15% to heat and resistance during this conversion, and another chunk because the voltage boost consumes capacity to maintain the higher pressure. When you buy a “10,000mAh” bank, you are paying for the capacity of the internal cells at 3.7V, not what actually reaches your device.
The “Two-Thirds Rule” Calculation
Stop trying to do complex efficiency math in your head while shopping on Amazon. I use a simple “dirty” formula that has never failed me in the field. Assume you only get 60% to 70% of the advertised capacity.
- Find your device’s battery size: Google your phone model + “battery capacity mAh”. For example, a standard modern flagship is around 5,000mAh.
- Apply the efficiency tax: If you are looking at a 10,000mAh power bank, multiply it by 0.65. That gives you 6,500mAh of actual usable juice.
- Do the division: 6,500mAh (usable bank) / 5,000mAh (phone) = 1.3 charges.
If you need to charge that phone three times for a weekend camping trip, you don’t need 15,000mAh (3 x 5000). You actually need roughly 23,000mAh (15,000 / 0.65). I always round up to the next standard size, so I’d grab a 26,800mAh unit.
Common Pitfalls
The “Fast Charging” Efficiency Hit
I realized this the hard way while testing a 65W laptop charger. Fast charging protocols (PD, QC 3.0) push the voltage up to 9V, 15V, or 20V. The higher the voltage boost, the harder the conversion circuit has to work, and the more energy is lost to heat. If you fast-charge your phone, that 65% efficiency estimate might drop to 55%. If you are desperate for maximum capacity, use a standard “slow” 5V/2A port. It takes longer, but it wastes less energy.
The TSA Limit (100Wh)
Don’t buy the biggest brick you can find if you plan to fly. The FAA and EASA have a hard limit of 100 Watt-hours (Wh) for carry-on batteries. To convert mAh to Wh, use this formula: (mAh / 1000) x 3.7V. A 26,800mAh bank is exactly 99.16Wh. I once saw a guy get his massive 40,000mAh off-brand battery confiscated at security because he didn’t check the Wh rating. Stick to 26,800mAh or lower for travel.
Input Charging Speed
Nobody looks at how long it takes to charge the power bank itself. I bought a cheap 20,000mAh unit a few years ago that only accepted 5W input. It literally took 18 hours to recharge. It was useless. Ensure the power bank supports USB-C PD Input (at least 18W or 30W) so you can refill the brick in a few hours instead of overnight.
Calculate your needs based on the physics, not the marketing, and always buy 30% more capacity than you think you need.
