By the Feed-in Tariff in Japan enforced in July，2012，quantity of introduction to photovoltaic system （PV system） for residence and business suddenly increases. Though each panel maker expands guarantee about output lowering and trouble of the PV system, the method to judge the degree and cause of output lowering is not established. The authors study on output lowering diagnostic method using the % electric energy. At first in this paper, we examined an output lowering judgment using the % electric energy. It became clear that the judgment of the output lowering using the % electric energy was possible，even if it was the day when the sunlight strength was unstable. In addition，it became clear that the evaluation of % electric energy was possible by using different weather data（ambient temperature, wind speed and global solar irradiance)when we could not measure weather data(total solar irradiance and module temperature)to be necessary for the evaluation. We were able to confirm that there was strong correlation between a change of the % electric energy and change of the evaluation. We were able to confirm that there was strong correlation between a change of the % electric energy and change of the output. Finally the authors propose the value that attracted standard deviation from the mean of the % electric energy as a decision method of the criteria of the output lowering using the % electric energy.
