A portable instrument for displaying an efficiency metric for a photovoltaic power generation system based on an estimate of the source power available from the photovoltaic modules. The source power is a value scaled from the rated DC source power computed with data from irradiance and temperature sensors or from a photovoltaic sensor's deviation from a rated level of power generation. A ratio of output power divided by source power is one measure of efficiency and relates to the health of the physical photovoltaic power generation system.