Methods used to test silicon solar cells are described, and an account is given of the effects on the battery-charging characteristics of having different numbers of high-output cells in series under different ambient light intensities. Outdoor evaluation tests indicate that an economical number of cells per storage cell is between four and five. Considerations are given to the design of cells for ground-level operation, where a long unattended life is important, and to the method of mounting; an angle of elevation between site latitude and site latitude + 15° is acceptable for many applications. A graph showing the average total insolation power available throughout the world is presented. In designing a solar-cell/battery power supply for a given application and site, use is made of these data in conjunction with monthly input-output figures for the chosen cell configuration. The battery capacity is chosen so that the excess charge produced in the summer at least equals the deficiency to be made up in the winter. The most recently developed cell will, under favourable conditions, produce electricity at about 1d./Wh, estimated for a five-year life. This is shown to compare favourably with conventional power sources operating at remote locations. The list of applications for solar-cell-generated power is constantly growing, and the conversion of various equipment from valve to transistor design is expected to give impetus to this growth.