efficiency of power bank checked and found efficiency of 65% to 68%for new one.

 https://www.amazon.in/dp/B0C2YR4MV6

which is 5kmah 3.7 volts = 18.5 watts rated capacity

it could charge up to 17 watts with 9 volts

but at discharge it could give only 11.7 watts

which is 68.9% efficiency in 1.4 hours time

when charged with 5 volts it gave 65% efficiency