Photovoltaic cells are the materials responsible for converting the energy in photons of light from the Sun into energy that can be stored or used. Individual cells usually range in size from areas of roughly one square inch to one square foot, and thousands of cells can be used simultaneously to harvest large amounts of energy. When photons of light strike the photovoltaic material, they excite electrons from a piece of silicon that has been treated such that the excited electrons will gather on one side. This creates a potential difference within the photovoltaic cell such that there is now a positive and negative side (similar to a battery). At this point, the photovoltaic cell has now converted (some of) the energy from the photons into electrical potential energy. This potential difference can be discharged to transfer the energy for immediate use or to be stored while the photovoltaic cell continues to collect additional photons.