A driver is configured to provide an output voltage and an output current to a load according to an input voltage. The driver includes a power converter, first and second detecting devices and a controller. The power converter is configured to receive and convert the input voltage to the output voltage and the output current. The first detecting device is configured to detect the input voltage to generate a first signal. The second detecting device is configured to detect the output voltage to generate a second signal, and detect the output current to generate a third signal. The controller is configured to perform a calculation to the second signal and the third signal according to one of lookup tables corresponding to the first signal to generate a power value. An operation method of a driver and a light source system are also disclosed herein.