It annoys me to see "MW" described as the capacity of a battery. Even people who ought to know better do it.
MW is fair enough as the capacity of a gas-powered station, because you can run it more or less continuously. But for a battery that can only run for a short time until you have to recharge it, MW is instantaneous output. Capacity is MWh. They don't like telling you how many MWh they have. That's because many of these grid-connected batteries don't even run an hour at max output. And that's because the money lies in short-term reinforcement of the grid, not longer term balancing.
For comparison, Dinorwig pumped storage scheme, has a maximum output of 1728 MW, and a capacity of 9,100 MWh. That's a little over 5 hours worth. Though I think in practice it takes over 8 hours to run it to empty, as the output reduces when it's less full. So if that battery can run an hour, and thus is 100MWh, then it's only just over 1% of a Dinorwig in terms of capacity.
For further comparison, to run Britain on wind and solar alone, and balance the grid with storage devices, would require something approaching 500 Dinorwigs. Or perhaps 50,000 of those batteries, the largest in Europe. So I am not expecting batteries to play much role in in balancing the grid "as wind and solar power grow", as that article suggests. The Climate Change Commission and National Grid Future Energy Scenarios take a similar view.