Solar Industry, July 2014, by James Mokri & Joseph Cunningham.
http://www.solarindustrymag.com/issues/SI1407/FEAT_04_Corrections-Needed-For-PV-Performance-Blind-Spots-During-Commissioning.html

"If a photovoltaic system is commissioned according to industry standards, then it must be performing as expected, right? Not necessarily. PV industry standards for commissioning do not include performance testing.

NEC code and IEC 62446 mention nothing about performance testing. ASTM E2848-11 outlines a method of performance measurement during the commissioning stage, but it is not a complete, consistent, standardized method of measurement and metrics."