A diagnostic device (40) having layers (25, 27, 29) corresponding to respective layers (24, 26, 28) of a given photovoltaic module (20). The diagnostic device is provided for testing and monitoring a condition of the given photovoltaic module. The layers of the diagnostic device may be made of respective materials with the same or substantially the same electrical resistance and aging characteristics as the respective layers of the given photovoltaic module under operational conditions. Electrodes (42, 44, 50A-C, 52A-C, 54A-C) of the diagnostic device are configured to independently measure electrical resistance along at least two different current resistance paths in the diagnostic device corresponding to respectively different current leakage paths (R1, R′1, R2-4, R5, R5) of the given photovoltaic module.