I tested a small powerbank that I have, and it’s very inefficient.
It takes 10.07 Wh to charge:
And it delivers 5.95 Wh when I discharge it (5V at 0.250 mA).
So I only got 59% back of the energy I put in.
This powerbank is quite old, it might have become so inefficient over the years. Google searches tell me that you should get at least 85% efficiency.
Although this powerbank still works fine, and his very handy to me because of its small form factor, I’ll see if I can get a more efficient one with a similar form factor.
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Didier Stevens 
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