If I had to guess she's talking in terms of each individual tank/unit set-up. So, one tank has a capacity or power rating of 500kW (capacity in the sense of the way a utility refers to size/ability of generating units). This link[1] possibly covers some of the confusing terminology here, and why it is that way.
EDIT: Scrolling to the bottom of their homepage[2] explains it, their system components are independent of each other.
[1] https://gigaom.com/2012/01/04/how-to-compare-energy-storage-...