With power banks and other types of batteries, the first thing you need to worry about in terms of wattage is the watt-hour. Watt-hours indicate how much energy you can get out of a battery and the rate at which that energy will be transmitted from the power bank to your device.
The second thing to worry about when it comes to wattage in your power bank is the overall energy capacity. Oftentimes, the wattage of a power bank refers to its maximum charging output. This may be higher or lower depending on how the power bank is being used. When used as a means of gaining energy from an external source, the maximum wattage output may be higher than when it is used as a power bank on its own. So, if you used a dual 45W power bank, you might expect the energy output to reach up to 45W when used as a charging station from an external source and up to 30W when used as a standalone power bank.