A unique system for the synchronous measurement of photovoltaic module electrical parameters and solar spectral irradiance is described. Data quality control is addressed in depth and the practices followed to ensure integrity for the ensuing analysis are explained. Irradiance spectra have been characterised with a single-number descriptor, the average photon energy. Unlike previous spectral characterisation work, the figure employed here is independent of artificially imposed references, such as the spectral response of a solar cell. This has allowed it to be used in a full analysis of spectral variation in the UK, with comparison made to a second site of significantly different climate. This simple characterisation has also allowed spectral irradiance measurements to be included for the first time in a thorough investigation of the effects of different environmental factors on photovoltaic device performance in real operating conditions. Discussion is entered on each of the main influences on key device parameters and concludes with a quantification of the principal effects on electrical energy generation. The development of an enhanced spectral irradiance model is described, which can simulate solar spectra under non-ideal weather conditions, as experienced by many locations. In validation against measured data, it is proved a convenient solution to the problem of poor availability of spectral irradiance data for use in photovoltaic performance modelling. The inclusion of spectral effects into photovoltaic device modelling is undertaken and shown to significantly improve existing modelling approaches. Finally, improvements are identified and suggestions made to further the work.