As well as using PV cells, Dr Roaf's solar house uses 5 square metres of solar panels to collect energy to heat water.
Solar panels contain metal or plastic pipes filled with water. They are usually put on the roof of a building, where the pipes absorb the sun's rays and pass the heat to the water. In countries where it is warm all year round, the hot water is stored in a tank on the roof above the panels. In northern Europe, because heat can be lost easily and there are frosts in winter, the storage tank has to be kept inside the house and insulated. In this case, a pump is used to send the heated water down to the tank.
Even in the UK, houses can, on average, get 50% of their hot water from the sun - a cheaper and more energy-efficient way of using solar energy than PVs. About 40,000 UK homes are already taking advantage of this.