Portable power banks are arguably the most critical mobile accessory for anyone spending extended periods away from an outlet. However, the market is saturated with misleading marketing, confusing metrics, and outright inflated capacity claims. Understanding the true capabilities of a portable charger requires looking past the big numbers printed on the box to understand battery chemistry, conversion efficiency, and regulatory limits.
Demystifying Capacity: mAh vs. Wh
The most prominent number on any power bank is its capacity, typically listed in milliamp-hours (mAh). A common assumption is that a 10,000 mAh power bank can charge a smartphone with a 5,000 mAh battery exactly twice. This is mathematically incorrect due to voltage conversion differences.
The lithium-ion cells inside a power bank operate at an internal voltage of 3.7V. Therefore, 10,000 mAh at 3.7V gives you a total energy pool. However, USB standards dictate that power must be transmitted at a minimum of 5.0V. To step the 3.7V up to 5.0V, internal circuitry must perform a conversion. This step-up conversion reduces the effective mAh available for charging. A standard rule of thumb is to multiply the advertised mAh by 0.65 to find the true transferable capacity. Thus, a 10,000 mAh power bank will reliably deliver about 6,500 mAh to your device.
A more accurate measurement of total energy is Watt-hours (Wh), which factors in both capacity and voltage. High-end power banks are increasingly listing their capacity in Wh to provide a more transparent representation of their power reserves.
Input and Output Wattage
Capacity tells you how much energy the bank holds; output wattage tells you how fast it can deliver it. Older or cheaper power banks max out at 10W or 15W, which is sufficient for slowly charging a phone but useless for a laptop. Modern high-performance power banks utilize USB-C Power Delivery (PD) to output 65W, 100W, or even 140W, allowing them to rapidly charge high-draw devices like MacBooks and high-end Windows ultrabooks.
Equally important is the input wattage—how fast the power bank itself can be recharged. Recharging a massive 26,800 mAh battery using a basic 10W charger can take over 12 hours. Premium banks support high-speed PD input, allowing them to refill entirely in under two hours when connected to a capable wall brick.
Pass-Through Charging and UPS Functions
Pass-through charging allows a power bank to be plugged into the wall while simultaneously outputting power to a device. This is a highly desirable feature for travel, as you can plug in one wall adapter before bed and wake up to both a fully charged phone and a fully charged power bank.
However, true pass-through circuitry is difficult to engineer. Many budget banks simply cycle power rapidly between the battery cells, generating extreme heat and severely degrading the lithium-ion chemistry. High-end models manage this safely by bypassing the battery entirely to power the device first, and routing excess wattage to charge internal cells. Some models can even act as a rudimentary Uninterruptible Power Supply (UPS) for low-power devices like Raspberry Pis or routers.
| Category | Typical Capacity | Best For | Airline Allowed? |
|---|---|---|---|
| Pocket/Lipstick | 3,000 - 5,000 mAh | Emergency smartphone top-up | Yes |
| Everyday Carry | 10,000 mAh | Full smartphone recharge (1-2x) | Yes |
| Laptop/Travel | 20,000 - 26,800 mAh | Tablets and Ultrabooks | Yes (under 100Wh usually) |
| Portable Power Station | 50,000+ mAh | Camping, multiple devices, CPAP | No (Strictly ground transport) |
Airline Regulations (TSA/FAA Limits)
Lithium-ion batteries present a fire hazard if punctured or short-circuited. Consequently, aviation authorities strictly regulate their transport on passenger aircraft. The universally accepted limit set by the FAA and international bodies is 100 Watt-hours (Wh) for carry-on luggage. Power banks must never be placed in checked baggage under the plane, as an cargo hold fire cannot be extinguished easily.
Most manufacturers design their largest portable chargers to specifically skirt just under this limit. A typical maximum travel power bank is rated at 26,800 mAh at 3.7V, which equates to roughly 99.1 Wh. If you carry a larger capacity unit, you must receive prior airline approval, and anything significantly over is strictly prohibited from flight.
Making the Right Choice
When purchasing a power bank, resist the urge to buy the largest capacity available unless you genuinely need to charge a laptop off-grid. For everyday smartphone use, a slim 10,000 mAh bank with 20W PD output offers the best balance of weight, pocketability, and daily utility. If you are buying a laptop-class bank, verify it supports at least 60W output, and invest in a high-quality GaN wall charger to refill it rapidly.