Caterpillar provides some really neat small scale flywheel UPS - used in places like hospitals where it would be very bad to lose power. They last long enough for the diesel gennies to start up.

I saw these in the basement of a data center about 11-12 years ago. Most steampunk thing I've seen in real life.

Here's a bigger one at ASDEX (a fusion experiment):

https://www.ipp.mpg.de/4244138/generatoren

I've worked on mine sites that use this as well.