This is perhaps the most exciting area because it is capable of supplying large amounts of energy both on a local and more central scale. It is also interesting because the devices (the cells, modules or arrays) are constantly being improved by research and development, and production costs are being reduced. Modern cells are capable of operating at relatively low lighting intensities and therefore providing energy when it is most needed. Currently available cell technology seems to be based largely, but not exclusively, on silicon in various fabrications which can be flexible sheets or replacement roof tiles for example. It is claimed that PVs can be incorporated into industrial, commercial, domestic and even mobile situations. Undoubtedly the range of applications is almost endless.

Normally a large array of cells is interconnected and positioned sensibly to expose the surface area to the sun as much as possible. Practical examples are south-facing roofs and walls . The UK law requires that electricity companies must allow you to  install solar power and they must also buy your surplus electricity.

Once the electrical energy is generated it can be converted to ac (technically, inverted) and used in a variety of ways, it can even be fed into the national grid. These secondary, but important, enhancements can add significant cost and may reduce the reliability somewhat.

Preferably you will need 2-4m2 of southeast to southwest facing roof receiving direct sunlight for the main part of the day. You'll also need space to locate an additional water cylinder if required.

Theory
Radiation from the sun ('sunlight') hits a photovoltaic cell. These cells are made up of two layers , Depending on the intensity of the sunlight a voltage difference builds up between the upper and lower layer of the cell. This difference is in the order of magnitude of 0.5 V. If you short-circuit the upper and lower layer a current runs of about 3 Amps. If you arrange sufficient cells in series, the result is a PV module or PV panel. Let's say 36 cells in series produce 36 x 0.5V = 18V at 3 Amps = 54Watts

Sunlight, especially in the UK and in the Low Countries, arrives in packets in between between clouds. No problem: the voltage and the amps go up and down a bit, just like a lamp with someone playing with the dimmer.
 
Theory
 Sunlight can be considered as light waves (wave theory) or as packets of energy (photon theory). The explanation of the photovoltaic effect is easiest when you think of light as energy packets: photons.

A solar cell is made up of two layers of semi conducting material: P and N. The boundary between P and N acts as a diode: electrons can move from N to P but not the other way. Photons with sufficient energy hitting the cell cause electrons (-) to move from the P layer into the N layer. An excess of electrons build up in the N-layer while the P layer builds up a shortage. This is the voltage difference that can be used as a power source. As long as the panel is hit by sunshine, the voltage difference is maintained. 

The photovoltaic principle was discovered in 1839 by Edmund Becquerel in France. Solar panels were used for the first time in 1958 to generate electrical power (0.1 W) for the US Vanguard spacecraft. In 2005, a total of over 1.000.000.000 Wp (1 GWp) is reported to be installed world wide.

The amount of incident sunlight
 The efficiency of the photovoltaic at converting this light to electricity
Efficiencies of 15% are common amongst commercial photovoltaic cells. This means that of the energy in the light falling onto the cell only 15% is converted into useful electricity. For this reason, photovoltaic are most effective in areas with a large amount of daily sunlight. However, as you can see on the page solar maps, even in the UK there is sufficient daily sunlight to exploit this ‘free’ resource with success The UK receives on average more hours of sun than Holland!