In this work, we present a simplified model that describes the energy performance of solar photovoltaic (PV) modules under real operating conditions. The model, validated against three full years of data collected from our outdoor test field for amorphous (a-Si) and crystalline (c-Si) silicon PV modules, agrees well in describing the energy performance of both technologies, including their peculiar counter-cyclical seasonal oscillations. The model focuses on clear-sky conditions and on four main loss/gain mechanism: (1) temperature, (2) spectral-effects, (3) reflection, and (4) irradiance, with the addition for a-Si of the Staebler-Wronsky effect. From the device side, the model requires a limited characterisation of the device under test: (a) power rating, (b) temperature coefficients, (c) spectral, (d) angle-of-incidence response and (e) irradiance dependence. Compared to approaches that are more conventional, our model, rather than focusing on the instantaneous power, concentrates on the large picture and directly attempts at providing a description of the daily performance ratio of the device allowing us to introduce a number of simplifications and, most notably, work with much smaller data sets. Input for our simulations are daily aggregate meteorological, and solar data weighted on the irradiance profile. For the a-Si device, slight discrepancies on the long term are attributed to an intrinsic degradation of the module's energy performance, which is not observed for c-Si. Moreover, for the devices investigated in this work we show that by neglecting the irradiance dependence parameter the model can further be simplified without loss of accuracy. (C) 2015 Elsevier Ltd. All rights reserved.