So, how large would the ‘stack’ of fuel cells have to be to generate the electricity needs of New York City, for instance?
As I recall, a unit to run a single-family home is about the size of a refrigerator. But there would not be a "single stack to run NYC". The power generators would be installed on a distributed basis (which is the whole idea behind a "microgrid"). Distribution would more likely be determined by existing pipeline infrastructure distributing natural gas.