In a PV cooling duct, heat transfer from the heated side to the cooling airflow takes place Partly by convection at the walls and partly by radiation exchange between them. A method is developed for representing these effects in combination, avoiding the uncertainties and iterations involved in treating the two mechanisms as independent and parallel. Though the radiative element introduces two further parameters, the procedure has a straightforward closed form, convenient for routine engineering calculations.' An approximation, that treats the radiation exchange as determined by the local wall temperatures, is validated by comparison with published results in which the diffusion due to the axial temperature distribution is fully represented. The method is applicable to both laminar and turbulent flows, employing coefficients already available in the literature. The incorporation of duct heat transfer within thermal models of the PV installation is discussed briefly, highlighting further areas which are being refined by on-going work.