In order to size the HVAC equipment for facilities using uninterruptible power supplies (UPS), an accurate estimate of the heat load produced by these devices is required. A model for the power dissipation of UPS is presented. The basis of the power loss model is the UPS efficiency. Different UPS configurations are classified and a corresponding efficiency, which is a function of load, is presented. A total of five different configurations are illustrated and a total of four different efficiency models are provided. The data used in the models are obtained from a publically sponsored study. The models are compared with UPS efficiency information obtained from manufacturer Web sites and the agreement between the efficiency models and the manufacturer data is shown to be reasonable. The power loss models were part of the work scope for RP-1395.