Power banks seem to be the consensus on a lot of different forums. I'll have to look into that. However a drawback is that most powerbanks are fairly heavy.
The weight will depend upon the powerbank's capacity (ie. how many mAh or "battery size") - that also determines how long the pi0w can remain powerd on for of course. Whilst I don't have any info. on their weights, FWIW, here is some info. on powerbanks I've used/tested:
http://www.cpmspectrepi.uk/raspberry_pi ... 2FChargers
Edit: Link corrected to be the one "at the top of the list"
Still running Raspbian Jessie on some older Pi's (an A, B1, B2, B+, P2B, 3xP0, P0W) but Stretch on my 2xP3A+, P3B+, P3B, B+, A+ and a B2. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm