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This paper reports new data and findings related to the decreased performance of a mono-crystalline silicon (c-Si) photovoltaic (PV) system in the northeastern United States when compared with an amorphous silicon (a-Si), thin film system. These findings are based on a kWh per installed kW basis during a warm summer period with relatively high ambient temperatures. Electric utilities will become increasingly dependent upon the performance of renewable energy systems during peak demand periods to meet their renewable portfolio standard obligations to public utility commissions as their investments in these systems expand. At present there is little data available to correlate the performance of lower efficiency thin film PV modules with higher efficiency mono-crystalline modules in a side-by-side test environment. The research findings demonstrate that while amorphous (a-Si) PV systems are generally regarded as inferior (due to their lower overall efficiency on a kW/m2 basis) their performance on a kWh/kW basis during periods of high ambient temperature is shown herein to be superior to higher efficiency single crystal silicon modules. These performance measurements, completed over the summer of 2008, provide a detailed analysis of energy and temperature measurements on an hourly basis during the higher demand periods for summer peaking electric utilities. The summary data shows a clear correlation where a-Si modules outperform mono-crystalline PV modules when ambient conditions lead to increased operating cell temperatures above ca. 30°C. Below this temperature threshold single crystal PV materials performance generally exceeds that of the thin film devices. At present, the cost differentials between the two technologies make a-Si more attractive for many utility scale applications and these findings indicate that PV power plants of this construction will outperform their more efficient competitors during the typical weather conditions of many su- mer peaking utility systems.