[shatnersbassoon]

It's presented in Gigawatts here, but I expect the actual metric is probably something like "Gigawatts available for 99th centile of duration of demand peaks", as this would allow you to talk about capacity in the same units as grid supply and demand. The whole point of grid batteries is for near-instant availability, so total energy available is moot if you can't access it right away - therefore a metric of energy would be less useful. Anyway, that's my speculation.