Yesterday I unearthed a Poweradd X7 20,000 mAh unit purchased in 2016, and decided to do a capacity test. I have used it maybe 20 times since purchase. From empty, it took 18,600 mAh to fill, and then delivered 12,800 mAh to a small USB fan before pooping out about 15 hours later.
I know about 70% “efficiency” is rather normal for these things, but I think that just under 65% of “nameplate” for a pack that old seems pretty good to me. Too bad it is just USB-A output.