When choosing a power bank to charge a laptop, the milliamp-hour (mAh) rating alone is not enough to determine its effectiveness. A more accurate measure is watt-hours (Wh), which considers both the voltage and capacity of the power bank. Most power banks use lithium-ion cells that operate at around 3.7 volts (V). To convert mAh to Wh, multiply the mAh value by the voltage. For example, a 10,000 mAh power bank has a capacity of about 37Wh (3.7V x 10Ah) before accounting for energy loss during charging. A 20,000 mAh power bank would then store about 74Wh. To find the right power bank, it's important to know the Wh rating of the laptop's battery, which can usually be found on the manufacturer's website. Estimating the laptop's average power usage involves dividing the battery's Wh capacity by its runtime. For example, a 70Wh battery that lasts 5 hours uses an average of 14 watts. However, this number can fluctuate depending on the tasks being performed. Modern MacBooks have battery capacities ranging from about 37Wh (MacBook Neo) to 100Wh (16-inch MacBook Pro). This means that a 10,000 mAh (37Wh) power bank may not provide a full charge for most laptops. Additionally, the capacity of the power bank doesn't indicate how quickly it can charge the laptop. The charging process is not 100% efficient, with an estimated 20 percent efficiency loss for normal charging and up to 30 percent or more for fast charging. To estimate how long a power bank will extend a laptop's runtime, the effective capacity of the power bank must be calculated after accounting for efficiency loss. For example, a 20,000 mAh 45W USB-C power bank with an effective capacity of approximately 52Wh (after a 30 percent loss) would extend the runtime of a laptop averaging 14W by about 3.7 hours. If the laptop requires more power than the bank can provide, it will draw from its internal battery alongside the power bank. The label "USB-C PD" indicates that a power bank supports USB Power Delivery, allowing for higher charging levels than an ordinary USB port. It's important to check the per-port output of the power bank, as the total output number can be misleading. For instance, a power bank with a total output of 100W may not deliver this full output on any given port or when charging multiple devices simultaneously. For most users, a 20,000 mAh/74Wh power bank is a reasonable emergency or partial-charge option, while a 27,000 mAh/100Wh power bank is closer to the optimal capacity. In the US, 100Wh is also the limit for power banks allowed on a plane without approval. If a power bank is unable to charge a laptop, several factors may be at play. The laptop may not support charging through USB-C, with some USB-C ports only handling data, video, or other accessories. The power bank may not support USB-C Power Delivery at a high enough per-port wattage, as a basic 15W USB-C output may not be sufficient for a laptop drawing 65W or more. Additionally, the cable used may not be rated to carry high-wattage power, with standard USB-C cables supporting up to 3 amps, while higher-power connections require a 5-amp-rated cable. It is also important to avoid damaged or outdated cables, as they may not handle the power requirements of modern devices.