Comment by pfdietz

Hydrogen would be burned in turbines, not fuel cells. A simple cycle turbine powerplant might cost $0.60/W. Remember, the goal here is minimizing capex, not maximizing efficiency. Backing up the entire grid with such turbines is a small cost compared to powering that grid with, say, nuclear ($10/W).

A once-through steam cycle with steam generated by hydrogen-oxygen combustion heating additional water (save the oxygen from electrolysis also) might have even lower capex. The combustion would occur with water injection in a kind of rocket-like combustor, no heat exchangers needed. This is potentially cheaper because it avoids spending 50+% of the output of the turbine on the compressor.

Hydrogen, like natural gas, is extremely storable, very cheaply, in underground storage formations. Nations typically store months worth of natural gas, and the volume could be greatly extended. Europe for example has the geology to store millions of GWh of hydrogen, and that's not even touching the salt formations under the Mediterranean.