This thesis presents the outcome of investigations undertaken in the field of terrestrial spectral solar irradiance characterisation and its impact on photovoltaics. Spectral irradiance has not previously been widely researched in the context of photovoltaic applications. Long-term, natural environment spectral irradiance observations are practically non-existent with availability very limited in terms of covered period, temporal resolution and site location. The work presented concentrates on four major aspects of spectral irradiance: spectroradiometer calibration spectral irradiance calibration transfer standards natural spectral irradiance variability and its impact on photovoltaic device efficiency impact of reference sensor spectral mismatch on accuracy of reference irradiance measurements.