The paper presents proposal of scope of control surface for marine power plant of merchant ship, being an element of ship’s energy system and consists of devices and energy plants exchanging fuel energy into other kind of energy, auxiliary devices and others irrelevant to the energy conversion processes, but indispensable because of international safety at sea and environmental protection provisions. Accepted scope of control surface allows to derive useful energy, or total generated minus energy consumed for autonomous shipping, this is why the suggested term “autonomous ship’s power plant”. This way makes possible (among others) valuation of results of waste heat recovery system used on boar