Solar Energy is a renewable and unlimited source of energy which directly or indirectly delivers most of the other sources of energy to Earth. Solar energy in the narrow sense means the amount energy that is transmitted by solar radiation and is expressed in J.  Solar energy in its original form is commonly used for conversion into heat energy for domestic hot systems water and heating (as additional energy source) and in solar power where for conversion of electricity is using photovoltaic systems.

Solar radiation is the short-wave radiation that Earth receives from the Sun. It is expressed in W/sqf, and depending on its raid on the surface on Earth can be:

* directly: radiation rays
* diffuse sky radiation: radiation scattered all over the sky due phenomena in the atmosphere
* diffuse radiation horizon: the part of diffuse radiation that emanates horizon
* round sun diffuse (circle solar) rays: diffuse radiation of the vicinity of solar disc as can be seen from Earth
* refused radiation: radiation that can be deducted from the surrounding area and falls to observed surface.

Do of the solar radiation is about 3,8 W - 1026W, of which Earth gets 1,7 - 1017W. Earth of sun per year gets about 4 - 1024 J   of energy, which is several thousand times more than the total amount annual energy consumption of all primary sources. The average intensity of solar radiation is about 1367 W/m2 (so called solar constant).

The spectrum of solar radiation includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and Y-rays. The largest part of energy when it is the IC radiation (wavelength> 760 nm), visible light (wavelength 400   - 760 nm) and UV radiation. In the spectrum of their share as follows: 51% makes the IC radiation, 40% of UV radiation, and 9% visible light.

Under the concept of using solar energy in a narrower sense, thinks only of its immediate use in original form, not as for example wind energy or fossil fuels. Solar energy can be exploiting actively or passively. Active use of solar energy is its direct conversion into   heat or electricity. Therefore the thermal energy from the sun gets through solar collectors or solar cookers, and electric using photovoltaic solar cells. Passive use of solar energy means direct utilization of solar radiant heat adequate performance buildings (location in space, using appropriate materials, appropriate layout of the rooms and glass surfaces, etc.).