We have two Sony branded power banks. One Aussie purchased at JB and the other in Japan.
Perhaps the key issue in sizing is what the rating of the bank is referring to, and in consideration of ‘conversion efficiency’?
Both our banks are 5,000mAh nominal capacity. IE they have a 3.75v battery that can store an amount of power (18.75W).
At 5v output they are rated at 3,000mAh or 15W capacity. Sony says so on the pack ratings. This is a 20% loss.
There is obviously a voltage conversion loss in providing the 5v as well as recovery losses depending on loading conditions.
There is also going to be a conversion loss in the device being charged (voltage charge regulation). A few percentage points.
And there are losses within the chemistry of the battery as it is recharged. (10% or more depending on the charge rates).
I have not seen a manufacturer’s figure for input power required for a full charge as a key spec for any mobile device? The data must exist. We just don’t worry about it given how little the amount consumed from the mains?
Add this up and a 30% loss compared to nameplate in Choice’s real world testing would seem normal!
The other point noted by @gordon is still relevant. The marketing if based on the internal battery rating is a slight of hand, although technically correct in describing one aspect of the product. Probably in the same consumer bucket as motor vehicle fuel efficiency.
The Choice reviews do not detail the internal battery storage capacity or voltage. However the real world testing does reflect the true performance and capacity of each product. Perhaps a weighted steady discharge test into a constant current load might add some clarity to how efficient each pack can be.
P.s. the Sony packaging in Japan made it relatively clear that the packs had two different ratings. One at the nominal battery voltage and the other lesser rating at 5v output. It would be interesting to check how these items are labelled locally?