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It does say that there's a (hydrogen) fuel cell in it to convert the hydrogen back into energy. https://en.wikipedia.org/wiki/Fuel_cell?wprov=sfla1

They also say that it has pretty low efficiency, around maybe 50%.

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Yes, 50% seems to be a fair assesment of current PEM fuel cell efficiency. Apparently, the PEM fuel cell itself can get up to 60% efficiency, but there are auxiliary services (cooling, supply and exhaust pumps, etc) that most people don't think about that drive the system down to about 47% reliable output efficiency. That's not bad considering zero GHG emissions, and other benefits such as scalability and power density of the cells, but I'm still hopeful for significant improvements in the next decade (I suspect this may be significant offset in the form of greater H2 storage mass/volume energy density).

We can correct the price or capacity by the efficiency for comparison, but on it's own 50% is not good/bad really. I'd take cheaper per-adjusted-kWh-over-usable-years solution even if it has 10% roundtrip efficiency since I normally get a lot of spare energy from solar.

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