A predictive controller for a building energy system obtains a function that defines an amount of energy consumed by both waterside equipment and airside equipment during a time period as a summation of multiple equipment-specific energy components including a waterside energy component indicating an amount of energy consumed by the waterside equipment during the time period and an airside energy component indicating an amount of energy consumed by the airside equipment during the time period. The predictive controller performs a predictive control process using the function to determine values of the equipment-specific energy components during the time period. The predictive control process includes predicting the amount of energy consumed by both the waterside equipment and the airside equipment during the time period based on the values of the equipment-specific energy components. The predictive controller operates the waterside equipment and the airside equipment using the values of the equipment-specific energy components.