I received this battery today and I have to say, I am unimpressed. I drained my iPhone 13 pro max to zero and charged it from the power bank to 80%. this drained the power bank down to 67%. My iPhone has a 4352 mah battery so technically, I should be able to charge it around four times before the battery in the power bank is drained.

Of course, there is the question of how much power it takes to charge the phone but I am very disappointed in the performance of this battery. Unless I have a defective unit, there is no way this is a 20000mah battery.

Caveat Emptor.

*Edit:

Ok, after doing some reading I've learned that the power efficiency dips the lower the wattage is on these batteries. Charging at 100 watts is more efficient than charging at 30 watts. Since my iPhone only has a 27 watt charging speed, the lower efficiency effectively brings down the mah substantially. As u/StopwatchGod pointed out, the efficiency for lower-wattage devices just isn't there.

So, I am assuming I would be much better off with a lower-wattage battery. My older 26000mah battery is USB A and does multiple full charges of our phones. I just want something like this but with USB C. I am returning this power bank and looking for something that has slower speeds but higher efficiency and is USB C. Pref by Anker

Thanks for all of your responses.

FINAL EDIT:

After doing some more research, it has become clear that virtually every power bank manufacturer is lying. whatever capacity you were buying, it is effectively half that in most cases. A quick Google search confirms this.

I got this new anker 20,000mah battery pack. It weighs 12oz. I think the Nitecore 20000 weighs just a tiny bit less. This battery pack allows pass thru charging. Has digital % reading, so no guessing if you have 20% or 1% left with 1 light on. I tested this to charge from 0 to 100% in 7 hours using 30w PD charger. Paid $34+tax on amazon.

https://www.amazon.com/gp/product/B0CC1CS6J4/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&th=1

I'll be using this on my next trip and retire my old 10,000mah anker, which I think weighs a little under 8oz.