A method of supplying power from a solar cell module to a load includes measuring the output of the solar cell module, and controlling a power converter having a preset duty ratio to start operating in accordance with the measured output. Additionally, a power supply apparatus and a power supply system include a solar cell module having at least one solar cell to absorb sunlight, a power converter which converts power output from the solar cell module and supplies the converted power to a load, and a driving controller which measures the output of the solar cell module and controls the power converter having a preset duty ratio to start operating according to the measured output. Thus, electrical stress on the power converter is significantly decreased even though incident solar energy and/or the load is largely varied, so that the power converter may be stably driven to output the power.