A solar energy includes a charging control unit to output a number of charge voltages, a switch control unit, and a comparing unit. The switch control unit receives a break control signal and a charging control signal from the charging control unit to switch the charging control unit to charge the rechargeable battery, and receives a voltage detecting signal from the charging control unit to output a detect voltage corresponding to a charge voltage of the rechargeable battery. The comparing unit receives the detect voltage and correspondingly outputs a selecting signal to the charging control unit. The charging control unit determines the charge voltage of the rechargeable battery according to the selecting signal, and outputs a corresponding charge voltage to charge the rechargeable battery.