For completeness sake, let me say that there is a lot of research going on in other solar energy applications also, such as, solar desalination, solar refrigeration and cooling, solar photocatalytic environmental clean-up, daylighting and passive solar uses.
We concentrate on the use of grid-connected solar-powered generators to replace conventional sources of electricity.
For the more than one billion people in the developing world who lack access to a reliable electric grid, the cost of small-scale PV generation is often outweighed by the very high value of access to electricity for lighting and charging mobile telephone and radio batteries.
A tiny fraction of the solar energy that falls on the earth is sufficient to take care of all of the needs on the earth (for details please see Chapter 1 of Principles of Solar Engineering, 3rd Ed. So it is obvious that if we are to depend on solar energy for our needs, we must be able to store it efficiently and cost effectively.
By the way, wind, biomass, ocean energy, hydro etc.