DC distribution network in building has attracted more and more attention due to its advantage of on energy saving with onsite DC energy sources and avoiding power conversions. However, few studies have concerned on the realistic component efficiencies at varied load conditions in details, but this will greatly raise the uncertainty on system energy efficiency prediction in the practical application of DC network. In this study, an experiment system of DC power distribution has been built to investigate energy loss and efficiency for system components at varied load conditions. The components include PV, battery, gird, high voltage load, and low-voltage load. The loads are equipped with HVAC and other DC electrical appliances. Through the experiments, a reliable set of data were collected and mathematical analysis was carried out to investigate the relationship between load and efficiency. For each component and its electricity branch, the results can demonstrate how the energy loss and efficiency varies with the load condition. In addition, the regression models are built to estimate the component efficiency in a quick way for DC system design in the practical projects.