This document specifies two procedures to check the performance of solar thermal collector fields. This document is applicable to glazed flat plate collectors, evacuated tube collectors and/or tracking, concentrating collectors used as collectors in fields. The check can be done on the thermal power output of the collector field and also be on the daily yield of the collector field. The document specifies for the two procedures how to compare a measured output with a calculated one. The document applies for all sizes of collector fields.