Photovoltaic systems use cells to change sunlight into electricity. The cells consist of one or two layers of a semi conducting material. When light shines on the cell it creates an electric field across the layers causing electricity to flow. The greater the intensity of the light, the greater the flow of electricity. PV cells are referred to in terms of the amount of energy they generate in full sunlight; know as kilowatt peak or kWp. The benefits PV systems produce no greenhouse gases and each kWp can save approximately 325kg of carbon dioxide emissions per year, adding up to about 8 tonnes over a system's lifetime. You can use PV systems for a building with a roof or wall that faces within 90 degrees of south, as long as no other buildings or large trees overshadow it. If the roof surface is in shadow for parts of the day, the output of the system decreases.