Here's my working. There are about 31.56 million seconds in a year, which I rounded to 30 million, and so

30 GWh/year
= 30x10^9 Wh / year
~ 30x10^9 Wh / 30x10^6 s
= 10^3 Wh/s
= 1 kWh / s
= 3600 kWs / s
= 3.6 kw

I used the duckduckgo autocalculator just now, and 30/31.56*3.6 is about 3.4, so it's much closer to 3.4kW.

(It's not power output, it's manufactured storage output. I think of it as like a factory that produces 3.4 litre capacity jugs per second, but they're not jugs, they're actually batteries. Big ones.)

They could make a 120kWh battery (which would give a family car a range in the region of 450 miles) every two minutes.