The Power Plant is an original building from the game, The Conquerors. It is mainly used for providing income during the match.
Much like other production buildings, it is used solely for the purpose of providing income. It can only be built near an Energy Crystal and an existing building or unit that you own. It also cannot be built if an existing production building you own has already claimed the crystal. Once placed and has finished building, it provides 10 cash per minute. It isn't as effective as the Nuclear Plant, and best used only early in a match.
Early in a match, it usually isn't a good idea to attack enemy plants. However, middle to late war, Destroying enemy plants will prove advantageous as unit production and building will be slower.