Technical guideline 5 of FGW e.V. describes the technical procedure for determining the reference yield.

Since the introduction of the EEG in 2000, the two-stage reference yield procedure with an initial and basic remuneration has been an integral part of the support system for onshore wind energy. With the EEG 2017, this support system was changed to a single-stage procedure. Annex 2 of the EEG 2014 and EEG 2017 states that the reference yield must be determined according to the generally accepted rules of technology. This is the case when the current version of the FGW Technical Guideline Part 5 (TR5) is applied. The TR5 “Determination and application of the reference yield”, defines all terms and technical regulations for the calculation of the reference yield. The current version, revision 7 as of 01.01.2017, refers to the EEG2017 and regulates the procedure in the single-stage and two-stage reference yield system. The system is similar for both procedures. A standardized model environment is defined, the so-called reference site, whose site conditions are specified in the EEG. This model environment is used to calculate the maximum yields that a wind turbine could generate at this reference site within five years. These yields are the so-called reference yields. The reference yield is compared with the site yields measured or determined by an expert opinion. The comparison is made on the basis of the so-called site quality, which determines the subsequent remuneration level. In the two-stage reference yield procedure, the adjustment of the remuneration level takes place via the Additional Period of Increased Initial Remuneration (ZZA). In the single-stage reference yield method, the so-called correction factors (KR) are used.