Saved that pic from an Onan class I was at. They wouldn't hand out the pics of them painted green for whatever reason, but saw a few on the instructors computer hooked to a generator. They're massive. Can't remember the specs (something like 90 liters & the gen is rated in mega watts) but he said they bring them online once a month & power part of the grid to help out energy usage from the plant.
That's how our local municipality maintenance building is for the city of Columbus. During the summer on days where there are high usage they will kick the generator on to help the power company out.
We are not talking about your normal building though either. This place is HUGE! As is the generator. 24/7 365. Never shuts down