The Effect of Low Insolation Conditions and Inverter Oversizing on the Long-Term Performance of a Grid-Connected Photovoltaic System

The performance of a roof mounted grid-connected photovoltaic (PV) system inNorthern Ireland was monitored over 3 years on annual, seasonal and monthly bases.The overall system performance was adversely affected by low insolation conditions;19% of total incident insolation was absorbed at irradiance level below 200 W/m2 and67% below 600 W/m2, only 6.2% above 900 W/m2. In summer and winter, the PVandsystem efficiencies were 9.0 and 8.5%, and 7.8 and 7.5%, respectively and inverterefficiencies were 86.8 and 85.8%, respectively. The inverter for this particular systemwas oversized; 77% of the total DC energy produced when inverter’s operating loadwas 50% of its rated capacity. The annual average monthly system performance ratio(PR) was 0.61 with seasonal variation 0.59 to 0.63. The average monthly PV, system andinverter efficiencies over the whole monitored period were 8.8, 7.6 and 86.8%, respectively. The main losses of the system were inverter DC/AC conversion loss, inverterthreshold loss and low insolation loss.