“Fleet” seems the standard collective term for all of the power plants of a certain type belonging to a single country or company in middle-brow journalistic writing. E.g., Japan’s fleet of nuclear reactors or PG&E’s fleet of gas-fired power plants. My memory is that this usage is relatively recent, probably since the 1990s, but I’m not at all certain about that. It definitely seems to currently be the standard term used by staff writers for The Economist and contributors to Foreign Affairs.
I don’t think I’ve seen it used in other contexts for referring to anything other than vehicles.