This application discloses a power supply and a power system. A first power supply is configured to set an output voltage of the first power supply to a plurality of voltage values. A second power supply is configured to send a plurality of first parameters to the first power supply, where the first parameters are used to identify power efficiency of the second power supply. The first power supply is further configured to obtain a plurality of second parameters, which are used to identify power efficiency of the first power supply. The first power supply is further configured to determine, based on the plurality of first parameters and the plurality of second parameters, maximum power efficiency of the power system and a voltage value corresponding to the maximum power efficiency.