Selection of best methods to calculate degradation rates of PV modules

Three different PV systems were evaluated to compute degradation rates using four different methods and the methods are: I-V measurement metered raw kWh, performance ratio (PR) and performance index (PI). I-V method, being an ideal and the best method for degradation rate computation, was compared to the results obtained from other three methods. The median degradation rates computed from kWh method were within ±0.15% from I-V measured degradation rates (0.9-1.37 %/year for three systems). The degradation rates computed from the PI method were within ±0.05% from the I-V measured rates for two systems but the calculated degradation rate was remarkably different (±1%) from the I-V method for the third system. The degradation rate computed from the PR method was within ±0.16% from the I-V measured rate for only one system but they were remarkably different (±1%) from the I-V measured rates for the other two systems. Thus, it was concluded that metered raw kWh method is the best practical method, after the I-V method and PI method (if ground mounted POA insolation and other weather data are available) for degradation computation as this method was found to be fairly accurate, easy, inexpensive and fast.