What mAh Actually Tells You
The number printed largest on any power bank box is almost always its mAh (milliampere-hour) rating — and it's the most misunderstood spec in portable charging. mAh measures stored charge, not how much of that charge your device will actually receive.
Because of internal voltage conversion and heat generated during charging, a typical power bank delivers roughly 70–90% of its rated capacity to your device. A bank labeled 10,000 mAh realistically transfers about 7,000–9,000 mAh. To estimate how many full charges you'll get, divide that usable number by your phone's battery capacity. A 4,500 mAh phone battery, charged from a 10,000 mAh bank, will likely get one full charge and a meaningful partial second — not two complete charges.
For a deeper look at how charging habits affect long-term battery health, see our guide to common battery myths.
Output Wattage: The Speed Number That Matters Most
While mAh tells you how much energy a power bank holds, wattage (W) tells you how fast it can deliver that energy. Wattage is calculated from voltage multiplied by current (V × A = W). A port rated at 5V/2A delivers 10W. A USB-PD port rated at 9V/3A delivers 27W.
This matters because slow output extends charging time significantly. A basic 5W output might take five or more hours to top up a modern smartphone. A 20W USB-PD output on the same bank could do it in under two hours — assuming your phone also supports that charging standard.
Check both the power bank's output spec and your device's supported input. Neither device alone determines charging speed — the slower of the two always wins. A 65W power bank connected to a phone that only accepts 18W will charge at 18W.
Ports, Protocols, and Pass-Through Charging
Modern power banks often include multiple ports with different capabilities. A USB-A port is typically slower (5–12W) and suited for basic devices. A USB-C port with USB Power Delivery (USB-PD) can handle smartphones, tablets, and even some laptops at much higher speeds. Some banks include proprietary fast-charge protocols that work only with compatible devices.
Pass-through charging — recharging the bank while it simultaneously charges your device — sounds convenient but isn't universally supported. Where it does work, efficiency drops further, and some manufacturers note that frequent pass-through use can modestly reduce long-term battery lifespan. Use it as a convenience feature, not a permanent setup.
Also pay attention to the bank's input rating — how fast it can be recharged. A 10,000 mAh bank with only 5W input charging will take four or more hours to refill. Higher-input banks (18W+ via USB-PD) recharge in a fraction of that time.
Watt-Hours, Airline Rules, and Reading the Fine Print
The spec that actually matters at airport security is watt-hours (Wh), not mAh. Wh represents total energy and is what airlines and regulators use to classify battery size. If your power bank doesn't display Wh, calculate it: multiply the mAh rating by the battery's voltage (usually 3.6V or 3.7V), then divide by 1,000.
For example: 20,000 mAh × 3.7V ÷ 1,000 = 74 Wh — well within the FAA's standard 100 Wh carry-on limit. A 27,000 mAh bank at 3.7V works out to roughly 100 Wh — right at the limit.
Beyond travel rules, Wh helps you compare banks with different stated voltages more accurately than mAh alone. Two banks with the same mAh but different cell voltages will not store the same amount of energy.
