The graph shows the energy of light hitting a square metre of ground in London for an average year. It varies between 5kWhr per day in the summer down to about 0.5 kWhr in the winter time.

A surface that is not horizontal, but tilted might collect 2 times this energy.

A family of four use about 9kWhr per day heating hot water for washing and bathing. So 1 square metre of your roof recieves enough light energy to heat your hot water in summer, and 10 square metres would in December.

Solar collectors absorb this light, and heat up - like a car dashboard on a sunny day. The heat is transferred to water pumped through the system, and then stored in the hot water tank ready for use. Not all of the energy from the light ends up in the water. Some light is reflected, and heat is lost from the collector to the environment by conduction, convection and radiation.

Solar collectors have clever designs to minimise the heat lost to the environment and thereby maximise what ends up in the water - many types work well on a cloudy day. The back and sides will have thick layers of insulation. The top is glazed to allow light in, but trap the heat like a greenhouse. The collecting surface may have smart coatings that absorb daylight well, but don't re-emit infra-red radiation anything like so strongly.

Some designs use the insulating power of a vacuum (like a vacuum flask). Air is pumped out of the system, which is then sealed. Convection from the collector to the outside world is eliminated.

So why don't we see more of these systems taking advantage of the free energy from daylight? It boils down to cost. Current solar collector systems are too expensive compared to cheap fossil fuels. Viridian is working to change this.