In order to properly size HVAC equipment for a building or industrial setting, engineers need to have an accurate estimate of the rate at which heat is added to a structure's environment. RP-1104 (White and Pahwa 2003) and RP-1395 (White and Piesciorovsky 2010) were projects aimed at improving the means of estimating the rate of environmental heat gain from indoor power distribution equipment. The main goal of this paper is to build a power loss model that can predict the rate of heat gain from power panelboards. The rate of heat dissipated from a panelboard depends upon the power loss of the breakers, fusible switches, motor starters, and bus bars. Information collected from measurements and published literature is used to build the power loss models for the panelboard components. Using these component models, the power panelboard loss model is created based on the sum of the component power losses. A portion of this paper is devoted to illustrating panelboard construction details. Finally, the power panelboard loss model is used in a heat gain example, demonstrating the model's use. The information in this paper is based on findings resulting from RP-1395.