I think what threw me off was when they say "capacity".
I don't work in power systems, I work with low-voltage DC stuff, so when I think "the capacity of the battery system" I think it would mean the maximum stored energy.
But "The maximum amount that can be produced" is also a valid definition of capacity, and I'm guessing this definition might be used in power generation more often ("the generator is operating at half capacity").
If you're interested, the context is that capacity was historically used in the utilities industry to refer to generation capacity, which is in MW or GW. E.g., the capacity of a power plant could be 500 MW, which for the decades of power production preceding renewables, could be sustained indefinitely as long as you're feeding it fuel.
By extension, when you talk about battery capacity in the context of the electrical grid, you're talking about the MW or GW of generation that you can replace during peak loads. The ability to distribute batteries across a grid to meet peak demand (and defer infrastructure/peaker plant construction) is the best way (today) to justify investments in batteries.
I don't work in power systems, I work with low-voltage DC stuff, so when I think "the capacity of the battery system" I think it would mean the maximum stored energy.
But "The maximum amount that can be produced" is also a valid definition of capacity, and I'm guessing this definition might be used in power generation more often ("the generator is operating at half capacity").