The problem of expert system development for selecting efficient reserved power source for small ship electrical system is important for sustainability of on-board power supply system. It is important to create an expert system which can estimate and choose the necessary electric equipment conform to set criteria. Problem of possible equipment alternative ranking expert system development for selecting efficient electrical devices for small ship electrical system consists of three main blocks: development of the knowledge base; the database and the rule interpreter. The rule interpreter is often known as an inference engine and controls the knowledge base using the set of facts to produce even more facts. In this article ELECTRE III based rule interpretation are used. Nowadays often energy saving decision making problems for small ship use only one criteria, as energy consumption level, but usage multi criteria decision making methods, as ELECTRE family methods for development power supply system for small ships, as also choosing reserved power supply equipment could help to make decision by multi criteria, as short term and long term costs, repairing time, safety and others. This problem is actual also in reserved electric power equipment and other equipment choosing. The difficulty of expert system building, for this problem solving, that it is not possible to define a completed set of the criteria in general set of criteria, it could be defined just only for very special cases like choosing reserved power supply equipment for small ships.