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Humanity has been '''harnessing the sun's energy''' for thousands of years. We have been perfecting the art of solar-powered heating since the 1700s when Europeans started to use sun heated walls to ripen the fruits, 1760 the Swiss naturalist Horace de Saussure run some experiments to find out how much sun heat can be trapped inside of an insulated box with glass cover. He built a rectangular box out of half-inch pine, insulated the inside, and had the top covered with glass, and had two smaller boxes placed inside. When exposed to the sun, the bottom box heated to 228 degrees F (109 degrees C) or 16 degrees F (9 degrees C) above the boiling point of water. The hot box experiment made possible the development of the future solar collectors. In the 1800s the rich heated the water by burning coal or gas. A cheap, economic and easier way was to placing into the sun a metal water tank painted black to absorb as much solar energy as possible. 1891in Baltimore US, Clarence Kemp combines the principale of the hot box with the solar water heated tank system giving us the world’s first solar water heater, called '''Climax'''. This was a big improvement in the daily life, but also a scientific progress. In 1909, William J. Bailey revolutionized the business with the '''Day and Night''' water heating system, solving the problem that water heated by the sun during the day, cooled during the night and never stayed hot enough to do the wash the next morning or to heat the bath. By 1941, more than half the population heated its water with the sun! Unfortunatelly after the WW II an agressive lobby was made for increasing the electric water heating. Their very competitive prices made the change from solar water systems to electric ones possible. [http://www.californiasolarcenter.org/history_solarthermal.html/] Today's solar thermal and photovoltaic cells systems are much more powerful and efficient, and scientists are improving them every year. 

The distance from the Sun to the Earth is approximately 149.6 million km, the perfect distance to ensure the Earth get's enough heat and light but isn't too close so as to frazzle.  The light takes 8 min and 19 sec to reach the Earth. [http://en.wikipedia.org/wiki/Sun] If you are in UK and the sun is shining you could receive between 94 sunshine power per m² up in Edinburgh and 109 in London. However, it is often rather cloudy, and last year 2008 Cambridge (in the driest part of the UK) had just 1400hours of sunshine throughout the whole year. [http://www.metoffice.gov.uk/climate/uk/stationdata/cambridgedata.txt]


But how can we use these precious hours of sunshine and convert it into '''electricity'''? '''Photovoltaic solar panels''', everybody has heard about them and probably has seen already one. They need to face South and can be installed up on the roof or be self standing. Their '''energy conversion efficiency''' is normally 10%, with some adaptations like a sun tracking system they can reach 20% or more efficiency. The power of the sunshine at midday on a cloudless day is 1kW per m², but because Cambridge is tilted about 52° compared to the Equator the sunshine intensity is reduced to 60%. They are some other reasons like clouds that reduce the sun intensity when reaches UK, so we can receive on average just roughly 110W/ m² per hour from the sun. [http://www.withouthotair.com/]


Let’s look at converting this with the PV.  The PVs can be between 10% and 20% efficient so if we look at the best  case and assume 20% efficiency and given 110W/ m² sun power we can hope to convert  22W per m²per hour. So if we have a 10 m² PV array that will create 220W per hour. As these are all averages over a 24 hour day this means 5.3kw (24hr x 220W) of energy are generated. Just to explain some units used 1 kWh/day  means over 24 hours it creates 1kW of energy.
The above calculation has been proven in practice with  the average power delivered by south-facing PV panel in a 25 m² array in Cambridgeshire delivering 12kWh/ day. 
The average UK household’s electrical energy consumption for a year is about 3800 kWh, or 10.4 kWh/ day (Scottish Power) ; so we will probably need PV solar panels with a surface of =10.4 kWh per day/ 5.3kwh per day x 10m2  = 20 m² per average house


On the basis of a 60m total population and an average of 2 people per house, this results in 30m households in the UK. We would need 20m2 x 30m households = 600 km² covered with solar PV panels. This will be a little bit more than '''0.25% of UK’s area''' of 244,820 km2. BUT this is just for household energy consumption, without the office buildings or industrial and retail units.
The Energy Saving Trust that administers government grants for domestic photovoltaic systems, the '''Low Carbon Building Programme''', estimates that an installation for an average-sized house would cost between £5,000- £8,000 per kWp installed, with most domestic systems usually between 1.5 and 3 kWp, and yield annual savings between £150 and £200.[http://en.wikipedia.org/wiki/Solar_power_in_the_United_Kingdom]



But how much do you need? As we are surrounded by water we can’t exclude the idea of having a floating platform covered with PV solar panels that would use also the water reflection? Maybe we should just install the panels in sunnier countries and export the energy? It has been proposed by scientists at the '''International Institute for Applied Systems Analysis''' [http://www.timesonline.co.uk/tol/news/environment/article5887597.ece] that if a section of the '''Sahara desert''', roughly the size of Wales, was covered with PV farms, enough electricity would be generated to power the whole of Europe. The project would cover ALL energy requirements, from homes to industries. Energy produced in PVs, can, unlike wind, be easily stored and transmission cables would send the electricity North. The main objection to such a fantastic project is, unfortunately, the realm of geo-politics, as the Sahara is within Libyan and Egyptian territories. Both countries are considered unstable. However, the project could be carried out in Algeria, which has been relatively stable since the 1990s, is on decent terms with Europe, and has a vast expanse of land in the Sahara.  
The DESERTEC project is a concept developed by TREC Trans-Mediterranean Renewable Energy Corporation and it is still 10-15 years to going online. Despite the financial crisis the big players on the market, companies like Siemens, Deutsche Bank and energy companies RWE and E.on as well as German Insurer Munich Re are interested to invest in the project. The solar PV farm would be 80 times larger then the PG&E and Bright Source project in the Mojave Desert, the present world largest solar installation. The investment until 2050 is estimated for 400milliarden Eur; peanuts when you think it will eliminate the dependecy on Fossil Fuel for the whole of Europe. [http://www.inhabitat.com/2009/06/22/worlds-largest-solar-project-sahara-desert/]


Solar panels can definitely increase the production of green energy, but like all the other alternative sources it has a price. The Sun is a free energy supplier, but we need to invest in capturing and storing it.