Solar thermal collectors including hybrid photovoltaic-thermal (PV-T) collectors can be certified in accordance with the ISO 9806 standard for an assessment of their reliability and thermal performance. The ISO 9806 standard relies on the Perers model for the latter though the model does not differentiate between hybrid (fluid circulation and electricity generation) and non-hybrid (no electricity generation) operation of PV-T collectors. The present communication proposes an extension of the Perers model explicitly accounting for the effective absorption factor reduction caused by electricity generation in PV-T collectors under maximum power point (MPP) operation, and discusses its suitability for parameter estimation via multi-linear regression (MLR). The solution devised assumes a linear MPP cell efficiency temperature dependence and employs the collector efficiency factor construct on which the original Perers model already implicitly relies, thus avoiding the need to measure additional quantities while remaining compatible with MLR. In doing so, more detailed thermal performance modelling of PV-T collectors is possible than with the standard Perers model, with minimal additional effort and while retaining the original's modularity. The extended model's added relevance is nonetheless predicted to be positively correlated with the thermal and electrical performance levels and the cell efficiency temperature dependence.