I thought I'd seen an article on them that gave an efficiency rating but can't find it now. I seem to recall it being some 30% as good as solar voltaic, which is itself pretty poor efficiency, but I don't know if that was the old version of these or the new one that improves the old version by 10x according to this article.
Off hand, if what you're looking for is a game use I think I'd treat them as batteries but they won't go dead as quickly just sitting like regualr power cells would or need recharging as often.
In T20 terms something like 1 year useful life for Crude batteries (wet and dry cell, rechargeable or single use), 3 years useful life for Basic batteries (modern dry cells, rechargeable or single use), 9 years useful life for Modern batteries (these radio decay cells or a rechargeable version), and 81 years useful life for the Advanced batteries (whose tech I can't imagine). Useful life being the maximum life, on the shelf or through recharge cycles.