A power source circuit includes a power source detection unit, a control unit and a switch unit. The power source detection unit detects whether a plurality of power sources including a green power source is supplied to the power source circuit, and detects and outputting a power value of the green power source. In addition, the control unit is electrically coupled to the power source detection unit and a load, and the control unit further detects a power consumption value of the load, to generate a switch signal to the switch unit according to the power consumption value of the load and a result detected by the power source detection unit. Therefore, the switch unit selects the green power source and at least one of other power sources thereof to supply electric power to the load when necessary.