How to know how much energy storage has powerbank?
If you already have the power bank a good way to evaluate is to first fully charge the unit. Then put on a constant current discharge load. Monitor the discharge voltage duration with a low cost USB voltage data logger to determine how long of time it takes till the unit cuts out.

From this you can directly determine the power bank energy density at the chosen load rate.

This procedure is valuable to learn the real capabilities on the power bank. You cannot trust the numbers given by manufacturers because they are often fake inflated data in marketing claims.