If it's a completely closed-loop process, efficiency doesn't matter nearly as much as scalability. Let's say the solar cells are 20% efficient, and electrolysi

If it's a completely closed-loop process, efficiency doesn't matter nearly as much as scalability. Let's say the solar cells are 20% efficient, and electrolysis is 50% efficient... So 10% of the solar energy makes it into the tank, and some subset of that makes it through the fuel cell... But we started with water and photons and end up with evaporated water and heat. That's where we were going to be, anyway... If it scales, we're okay. If that electricity is generated from some non-renewable resource, efficiency is of huge importance. If it's something like wind or solar, it can take a back seat.