A power-source element is defined and its properties are studied. The power source describes the output characteristics of the loss-free resistor, as well as high quality rectifiers, some types of switched-mode convertors, and other loss-less two-ports. It is shown that controlled power sources are inherent in the large class of buffered power-conservative two-port networks. This approach has inspired some new realisations and applications of the loss-free resistor. A family of high frequency switched-mode convertors is shown to exhibit naturally power-source output characteristics