I first looked into zinc/air fuel cells by Metallic Power back in 2000, to see if they would be practical for use in place of a generator for our remote cabin or for power outages here at home. Unless coupled with solar for regeneration, they'd have been of no use at the cabin (no grid). For emergency use it would have been cost prohibitive compared to a gasoline or natural gas generator, and we'd have needed to keep a significant volume of pellets on hand for the 3 - 5 day power outages. Not as convenient as a couple of 5 gallon gasoline cans.

Every conversion has inefficiencies. It's the amount and cost of the inefficiency that will make or break it. If fuel was free, would we care about fuel economy?