In northern Switzerland, a construction team is hard at work excavating a hole in the ground that will end up being over 88 ft (27 m) deep, and spanning the length of two soccer pitches. This pit will be home to Switzerland's first redox flow battery for storing clean energy – and it'll be the most…
But with 800 MW it takes an hour and a half to get to 1.2 GWh, and an hour is 3600 seconds or 3600000 ms. So “a few” is doing a lot of work in that quote.
The quote is obviously incorrect, perhaps the engineer meant to say “absorb 1.2 GWh and start providing power to the grid in a matter of milliseconds”, and it was lost in translation.