A solar pump uses power derived from sunlight that is converted into electrical power by Solar Photo Voltaic(SPV) modules, which give higher power output in the afternoons and lower power output in the mornings and evenings. As a result, a solar pump works on varying power input and gives varying water output at a given pump head. On the other hand, a grid powered/diesel operated pump set, while working at a particular pump head, works with constant power input from the grid/diesel engine and provides constant water output at the given pump head.
E.g. standard 5 HP pump working on grid power/diesel engine for say, 2.5hours a day, may give the same water output in a day as a solar pump of 3HP working for 8hours using sunlight. This is an important fact to keep in mind for appropriate solar pumps selection and application engineering.
The most important parameters to select a solar pump are: how much daily water needed, at what pump head, and at which location. The location is important because solar energy varies from region to region, and sizing of solar panels depends on the solar energy of a region.