A measurement instrument capable of electrically isolating the connected photovoltaic (“PV”) module in an array of PV modules to perform a health diagnosis including of current versus voltage measurements on the attached device by using a resistive load to acquire the current-voltage (“IV”) curve in the positive power quadrant of the module. The instrument is capable of switching a charge storage element into the array during the period when the solar module is under test to provide uninterrupted electrical power to the PV array. The measurement instrument contains a battery and charger allowing the device to run from the connected PV module's energy. The instrument contains a microprocessor to allow a high degree of configuration through software, including altering the speed of an IV sweep, the interval between sweeps, and integrating temperature and tilt measurements. The instrument is equipped with low power radio devices to communicate wirelessly, further negating the need for a common ground.