Accurately estimating the solar radiation on tilted surfaces is essential for reliably predicting the energy yield of solar PV systems. In this study, irradiance models that estimate Global Plane Irradiance (GPI) from Global Horizontal Irradiance (GHI) measurements for South African climatic conditions are evaluated. Seven irradiance decomposition models namely Orgill and Hollands, Erbs, Louche, Reindl₁, Reindl₂, Disc and Dirint are evaluated as well as combinations of the Isotropic, Sandia, Klucher, Hay, Reindl and Perez transposition models with the best performing decomposition models. Thirty-minute interval measured GHI and coincident measured GPI data, from an operational 75 MW_p solar Photovoltaic (PV) plant in South Africa, were used for validation purposes. It was observed that all the transposition models underestimated the winter GPI for the reference site, with the Perez plus Dirint and Perez plus Louche combinations showing monthly mean bias errors within 2% under South African climatic conditions.