The fundamental limits for the measurement of the efficiency of single nanowire solar cell devices are presented. We evaluate the effect of the substrate, light polarization, and existence of Mie resonances in the absorption of the solar spectrum for nanowires with diameters from 10 to 300 nm. We find that the efficiency measured under such configuration can be underestimated between a factor 1.6 and 7.0 for GaAs nanowires and between 6.7 and 15.9 for silicon nanowires. These results constitute a reference for understanding the limits in the measurement of single nanowire devices. (C) 2011 American Institute of Physics. [doi:10.1063/1.3672168]