The electrical efficiency of a photovoltaic-thermal system for coolant inlet temperatures ranging from 25°C to 75°C and concentrations from 500 to 1500 suns was investigated experimentally and theoretically. In this system absorbed radiation and thermal losses from the electric circuit are collected in a thermal circuit. This allows one to directly drive a thermal desalination process thereby contributing to an improved system efficiency. A triple-junction solar cell was tested in two different configurations. At 1500 suns the electric efficiency of a silicon microchannel cooler package exceeded the efficiency of a reference package with a copper cooler by 2% and it remained fully functional up to concentrations of 4930 suns. We present a general model for concentrated photovoltaic-thermal systems in which the standard efficiency modeling approaches for triple-junction cells are extended by temperature and concentration dependencies. The currents were modeled both following the Shockley-Queisser and a "real" cell model with temperature dependent material parameters, radiative recombination, and high injection conditions. © 2011 American Institute of Physics.