The phenomenon of low efficiency and waste is prevalent in public buildings. One of the reasons is the bad habit of people. There is no way to judge the level of waste ever since a long time ago. In order to analyze the level of building energy utilization to implement low energy building, this paper presents a concept of energy efficiency. It divides the energy of building into two parts. One part if effective energy, the other one is ineffective energy. The answer of effective energy divided by total energy is energy validity. Because the effective energy of building terminal is very difficult to measure directly, we can only use existing examination technology to calculate the parameters such as temperature, illumination and sensation to get change rate of time. Then unifying the behavioral characteristics of people, we gain the time of effective energy using. The ratio of both times can represent energy validity indirectly. In order to confirm this method, we have calculated the energy validity by using data of different buildings. The result indicates that the energy validity can reflect the level of artificial energy waste. It would be a quantitative indicator to obtain reasonable energy consumption of building.