I changed my PF1 to NPFI recently, but the old phone still remains in our family, my 14 y.o. son is quite happy to get it instead of his previous Samsung Galaxy Mini 2.
I like my new phone, it is definitely better than PF1 except one point: I noticed that it works a bit less time from battery in case of medium or high load on CPU (WiFi+Skype or gaming or WiFi+movies etc.). Its cell is definitely enough for me for a day, but PF1 usually had more juice before overnight charging.

I hope this may be fixed with new firmware, but so far I have more need in an external battery in case of vacations, flights etc. when I may use NPFI for a long time over day far from wall electrical outlet. Old PF1 is also in scope, so it could be useful to have external power supply with 2A 5V USB and Asus 12V 40pin Cable connector together.

Some time ago I bought EasyAcc® 15000mAh External Battery Pack from Amazon for PF1, but it works very well for NPFI too due to both 2A microUSB and 12V Asus connectors (12V->Asus connector cable was in the box):



I also checked - this external battery can charge simultaneously NPFI (5V 2A USB) and PF1 (12V), both tablets+phones, but I charged them together for several minutes only.

I tested this battery with NPFI on this week: just used it for overnight charge of phone+tablet together (in "intelligent mode"):
- First time EasyAcc completely loaded phone+tablet batteries from 35% (phone) and 9% (tablet) juice level.
- Second time EasyAcc was capable to achieve just partial device batteries saturation and was completely empty after this:
- - Phone battery was charged from 16% to 99%;
- - Tablet's one got 50% of charge from the initial 1%.

So assuming battery level indicator is well calibrated, EasyAcc 15000mAh external battery pack provided 65+83=148% of phone battery(2400mAh) and 91+49=140% of the tablet battery (5000mAh) capacity.
So it's fair to say that this battery pack can charge both NPFI's batteries 1.5 times or extend for about a day work of both NPFI and PF1 together.
It's not bad, IMHO .

And finally the "counted" efficient capacity of this external battery is about 10600mAh, so it's marketing 15000mAh sounds reasonable taking into account <100% efficiency of phone's battery charging and <100% efficiency of external battery discharging.