Mining bases may not be placed in adjacent hex-sides of a planet, and only one may be placed per hex-side. This limits standard mining planets to a maximum of three mining stations, and 120EP per turn (as noted on the planetary stats list). Larger planets may be found, but are rare. Most planets are on the order of 80EP maximum per turn.
Agro stations on a colony world produce 2EP per turn. There may be a maximum of three Agro stations on any colony planet.
Once transported to a Major Planet or permanent base within your territory, they are added to your treasury and may be used to build ships, stations, and colonies. If the EP are to be used as payment to another empire, they may simply be shipped to a Major Planet or base of theirs, at which point they will be added to that empire's treasury.