In this paper, a method to estimate a decline in string STC power output has been proposed. Open circuit voltage (Voc), short circuit current (Isc), fill factor(FF)of a degraded string are calculated using the I-V characteristics of two strings, a reference string and a degraded string. Then STC power output is estimated by using these parameters. This method is based on an assumption that Voc, Isc, FF of the two strings have linearity. It is verified that this method would be effective when applied to PV strings which have different azimuth angle.