A photovoltaic system, a method for locating devices in a photovoltaic string, a MLPE apparatus, and a method for ranking MLPE apparatuses. Each MLPE apparatus sends, actively according to a predetermine rule, an initial message to the other MLPE apparatuses. The initial message includes a serial number corresponding to said MLPE apparatus and an accumulated operation duration of said MLPE apparatus. The MLPE apparatuses determine the rank of the accumulated operation duration of each MLPE apparatus according to the initial messages sent from all MLPE apparatuses. After each MLPE apparatus communicates with a communication host, the MLPE apparatuses reports the rank of the accumulated operation duration of each MLPE apparatus to the communication host. Thereby, the communication host may determine a physical location of each device in the photovoltaic string according to the ranks of the accumulated operation durations and a sequence of installing positions of the devices in the photovoltaic string.