There are two main ways of collecting energy from the sun to use in an existing building. Either by turning the energy into heat (solar thermal) or into electricity (solar photovoltaic).

Even in the UK the ground receives up to 1kW of energy per meter square. This means that enough energy from the sun will fall on the roof of the average house to supply that property with all of its energy requirements, if we could capture all of the energy. 

Solar thermal

The principal is to take the energy from the sun and use it to heat water. The main elements of the system are the solar collector which absorbs the energy from the sun, a transfer system which takes the energy from the collector into the building and an energy store which in its simplest form would be your domestic hot water tank.

The solar collector comes in two main types, the flat panel collector and the solar evacuated tube collector. The flat panel collector looks like a large roof window and can be fitting into an existing roof or mounted on the ground. The evacuated tube system is slightly more efficient and looks like a series of fluorescent tubes in a frame. Again this type can be roof or ground mounted.

The orientation of the panel is extremely important and as far as possible should face south in order to maximise the efficiency of the system. Most systems are designed to provide domestic hot water only, although some systems are designed to contribute to the heating requirements of the property. A well specified system on a typical house will provide up to 75% of the annual domestic hot water requirements and up to 100% during the summer months.

Solar Photovoltaic (PV)

The principal in this case is to directly convert the suns energy into electricity using silicon semi-conductor technology. Panels can by quite small (1meter square) but are modular, such that a very large system of several tens of square metres could be developed. The average system on a house in the UK would be about 4 square meters. The PV panel would normally be connected to the local electricity network and the energy sold to an electricity company to maximise the use of the energy from the panel although you could have a system which allowed you to use all of the energy in your home. An average costing panel will produce about 75w per square metre on a bright day. PV panels like solar thermal panels should be oriented to the south and should not be overshadowed by buildings or trees as this will have a detrimental impact on the output of the system.