For PV system performance verification and system efficiency calculations, NEO Virtus Engineering has developed a portable data acquisition system capable of achieving a high accuracy, high resolution irradiance and temperature data set. For performance verification, NEO will deploy a Campbell Scientific data logger with environmental sensors to monitor the two key environmental parameters, plane-of-array (POA) irradiance and PV module cell temperature, which are required to calculate the PV system array output. The POA irradiance is measured with a Kipp & Zonen CMP11 thermopile pyranometer which is classified by the World Meteorological Organization (WMO) as a secondary standard instrument which has an achievable uncertainty of 2%, with a 95 percent confidence level. This is far more accurate than typical photodiode based pyranometer. The system ac production over the relevant time period is measured directly from the revenue grade meters on site.