The headline number on a power bank's packaging—milliamp-hours—can mislead anyone trying to charge a laptop. To know how much energy a bank actually stores, multiply the mAh figure by the cell voltage (typically 3.7V) and divide by 1,000. That gives watt-hours: a 10,000mAh bank stores roughly 37Wh, while a 20,000mAh bank holds about 74Wh. Modern laptop batteries range from about 37Wh to 100Wh, so a small bank may not even deliver a full top-up.
Charging inefficiency makes the math worse. The conversion process loses about 20% of energy in normal charging and around 30% when fast charging. A 74Wh bank, after 30% overhead, has an effective capacity of about 52Wh—enough to extend a laptop averaging 14 watts by roughly 3.7 hours. If the laptop's power draw exceeds the bank's output, the bank only supplements the internal battery rather than recharging it.
For business buyers, the practical checklist is USB-C Power Delivery support, per-port wattage, and a cable rated for high current. A basic 15W USB-C port will not charge a typical 65W laptop and may be rejected entirely. The article suggests a 20,000mAh/74Wh bank as a reasonable emergency option, while 27,000mAh/100Wh is closer to ideal—and 100Wh is also the US limit for carrying a power bank on a plane without airline approval.