At the most recent Asus investors conference, CEO Jerry Shen was asked about Zenfone accessories. Here is what he said:
power bank is the best seller among all accessories and sells very well in many countries
5 to 10 power banks would contribute the same amount of revenue as 1 phone
our expectation is that in the future contribution from power bank could reach more than 5% – in Q3 / Q4 maybe the range will be 5%
Jerry Shen went on to say that their sales of the power bank is a few hundred thousand at this point. Certainly the power bank for Asus is the best selling accessory. This makes sense. A power bank is very handy and it means that you can push your phone without having to worry so much about running out of battery and having to find a plug-in power source.