Technically speaking, solar energy is the only energy we have. Every other “energy” is a derivative of the sun's energy hitting the earth. Fossil fuels, wind power, biofuels, and even wood are all products of the sun. It is the most abundant and basic form of energy. Unfortunately, it is difficult to capture without some technology, though the simpler that technology is, the longer it will last.
Solar energy is most efficient if used directly as thermal energy for heating water, air, or a thermal appliance. Many systems are available for using the sun's heat, and by designing an appropriate system for your needs, a solar thermal system can assume a central role in your energy supply.