III–V concentrator photovoltaic systems attain high efficiency through the use of series connected multi-junction solar cells. As these solar cells absorb over distinct bands over the solar spectrum, they have a more complex response to real illumination conditions than conventional silicon solar cells. Estimates for annual energy yield made assuming fixed reference spectra can vary by up to 15% depending on the assumptions made. Using a detailed computer simulation, the behaviour of a 20-cell InGaP/In0.01GaAs/Ge multi-junction concentrator system was simulated in 5-min intervals over an entire year, accounting for changes in direct normal irradiance, humidity, temperature and aerosol optical depth. The simulation was compared with concentrator system monitoring data taken over the same period and excellent agreement (within 2%) in the annual energy yield was obtained. Air mass, aerosol optical depth and precipitable water have been identified as atmospheric parameters with the largest impact on system efficiency. Copyright © 2012 John Wiley & Sons, Ltd.