Abstract: A method and system for monitoring energy consumption is provided. The method for monitoring energy consumption estimates a number of people in a room based on an indoor CO2 concentration, and determines whether an amount of energy consumed indoors is proper or not based on the estimated number of people in the room. Accordingly, a waste of energy can be prevented by monitoring when a proper amount of energy is consumed in comparison with the number of people in a room. In addition, the number of people in the room can be easily estimated at low cost prior to determining whether energy consumption is proper or not.