[jgveill]

Again, just for fun, I found a white paper studying thermopile pyranometer uncertainties compared to PV uncertainties.  Conclusion may not go the way most people would think : here's an extract from conclusion : 

" The PVRD achieves an expanded uncertainty that is a factor of ~2 lower than the thermopile pyranometer. During the majority of the day uncertainties are on the order of +/-5% for a pyranometer, and +/-2.4% for a PVRD, both stated with 95% confidence intervals. We therefore conclude that 6 PVRDs provide superior irradiance measurements for PV power plant monitoring applications. 

The white paper is not for those allergic to maths, but you could learn few things having an eye on it. 

 http://imtsolar.com/wp-content/uploads/2...-Paper.pdf 

J Guy