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Semiconductor nanowires have different physical properties than their bulk counterparts due to their small physical dimensions and high surface-to-volume ratio. The nanowire geometry entails enhanced optical absorption, widened possibilities to grow material heterostructures and ability to withstand high levels of strain. The strain may alter the physical properties further and can be used to tune them. Because of their unique properties, solar cells based on III-V compound nanowires hold promise of becoming both more efficient and less expensive than conventional solar cells. However, nanowire solar cell efficiencies are still far below their theoretical maximum and further optimization is needed. This demands versatile characterization techniques where the microstructure of single nanowires can be related to their properties, and strain-induced effects may be investigated. In this thesis, a nanoprobing in situ electron microscopy technique has been developed to study the electrical and photovoltaic properties of single GaAs nanowires. Furthermore, the quantitative effects of uniaxial strain on these properties were investigated. The results show that the nanowires function as solar cells, with a highest measured single nanowire efficiency of 10.8% during white light emitting diode illumination. Optimization of the electrical contact was found to be crucial for the photovoltaic performance of the wires. Furthermore, tensile strain was shown to increase the photocurrent in the near-infrared spectrum due to a reduction in bandgap energy. These findings provide insights for further optimization of nanowire solar cells.