How to Choose the Right Power Bank for Your Laptop
Understanding watt-hours and output power for effective laptop charging.
1Watt-hours vs. mAh
Power banks list capacity in milliamp-hours (mAh), but watt-hours (Wh) are the true measure of energy storage. To convert mAh to Wh, multiply by the voltage (typically 3.7V for lithium-ion cells) and divide by 1,000. For example, a 10,000 mAh bank stores about 37Wh. This conversion is crucial because laptops, being high-drain devices, require more energy than phones.
2Laptop battery ratings
To estimate how much a power bank can charge your laptop, first find your laptop's Wh rating (usually on the manufacturer's site). Divide this by the battery's runtime to estimate average power usage. A 70Wh battery lasting 5 hours averages 14W. Note that actual usage varies—rendering video consumes far more power than browsing documents.
3Charging inefficiency
Power banks lose energy during charging due to heat and voltage conversion. While 20% loss is typical for standard charging, fast charging (like USB Power Delivery) can see 30% or more. For a 20,000 mAh (74Wh) bank, this means only ~52Wh effectively reaches your laptop. Always account for this when calculating extended runtime.
4Output power matters
A power bank's capacity doesn't indicate its output capability. A 50,000 mAh bank rated at 15W will barely slow a laptop's drain or may be rejected entirely. Look for USB-C PD (Power Delivery) labels, which ensure negotiation of higher charging levels. For most laptops, 45W+ output is necessary for effective charging.
5Per-port output details
Total output ratings can be misleading. A bank labeled '100W total' might split this across ports or reduce output when charging multiple devices. Always check per-port specifications—your laptop needs consistent high wattage (e.g., 65W+) from a single port to charge properly during use.
6Cable requirements
Not all USB-C cables support high-wattage charging. Standard cables handle up to 3A (60W at 20V), while 100W+ requires 5A-rated cables. For USB PD 3.1's 240W capability, all components (bank, laptop, cable) must support it. Avoid old or damaged cables that can't meet modern power demands.
7Ideal capacity choices
A 20,000 mAh (74Wh) bank suits emergency top-ups, while 27,000 mAh (100Wh) offers near-full charges for most laptops. Note that 100Wh is the maximum airline-approved capacity without special permission. Larger banks exist but may be impractical for travel due to size and weight.
8Compatibility checks
Ensure your laptop supports USB-C charging—some ports are data-only. Verify the bank's PD output matches your laptop's needs (e.g., 65W+ for MacBook Pros). If the laptop draws more power than the bank provides, it will supplement with internal battery, slowing drain rather than charging effectively.