Comparison of pyranometers vs. PV reference cells for evaluation of PV array performance

As the photovoltaics (PV) industry has grown, the need for accurately monitoring the solar resource of PV power plants has increased. Historically, the PV industry has relied on thermopile pyranometers for irradiance measurements, and a large body of historical irradiance data taken with pyranometers exists. However, interest in PV reference devices is increasing. In this paper, we discuss why PV reference devices are better suited for PV applications, and estimate the typical uncertainties in irradiance measurements made with both pyranometers and PV reference devices. We assert that the quantity of interest in monitoring a PV power plant is the equivalent irradiance under the IEC 60904-3 reference solar spectrum that would produce the same electrical response in the PV array as the incident solar radiation. For PV-plant monitoring applications, we find the uncertainties in irradiance measurements of this type to be on the order of ±5% for thermopile pyranometers and ±2.4% for PV reference devices.