  The time averaged efficiency of photovoltaic modules in the field is
generally lower than the efficiency measured in the laboratory under standard
testing conditions due to the combined effects of temperature and spectral
variability, affecting the bankability of power plant projects. We report
correction factors to account for spectral effects ranging from -2% to 1.3% of
the produced energy for silicon modules depending on location and collector
geometry. In high irradiance locations, the energy yield advantage of trackers
is underestimated by 0.4% if spectral sensitivity effects are neglected. We
find a correlation between the locations most favourable for tracking, and
those most favourable for multijunctions. As the photovoltaic market grows to a
multi-terawatt size, these seemingly small effects are expected to have an
economic impact equivalent to tens of billions of dollars in the next few
decades, far out-weighting the cost of the required research effort.
