I wonder if that's because of the difference between being a router versus being just a standard client.

I would say monitor your power usage for a while and take the highest one you see and make a battery set that will last something like 72 hours with the highest draw you see so that you get a good buffer zone.

so 170ma*72hrs = 12,240mah (12.2A) or 5v*12.24a = 61.2wh