[adrianN]

It's of course not particularly great. You can make hydrogen at like 80% efficiency and use it at like 60% efficiency in a combined cycle plant. So 50% is probably in the right ballpark. Batteries are much more efficient, but efficiency is only weakly correlated with cost. I don't know the actual numbers, but from what I've heard hydrogen (or even methane) are the most economically promising storage options when you need weeks or months worth of backup.