They're chonky devices which were not really off-the-shelf until the last decade, as far as I know. There's very few images of them, but a smaller one looks like this: https://www.pv-magazine.com/2018/03/14/pilot-project-for-fly...
One of these units blew at one point. We had 4 and only needed two running, so no big deal. The company who managed the whole thing (Swiss) came to replace it. Amazing job, they had to put it on small rollers, like industrial roller skates, then embed hooks in the walls at each corridor junction, and slowly winch the thing along, it was like watching the minute hand of a clock.
Then the whole process in reverse to bring in the new one. Was fascinating to watch. The guy in charge was a giant, built like a brick outhouse. They knew their stuff.
The computer, storage, etc. ran off the generator, which first eliminated any risk of power spikes and surges (as the flywheel is a very effective low-pass filter), and the circuits controlling motor speed also ensured the AC frequency was better than the power company supply. This was located in a rural area, so the long power lines with few sinks (customers pulling power) made lightening spike risk spread further, and the rural voltage and frequency fluctuated a lot. Seemed like a really cool system that worked flawlessly in the years I was there.