When we use sustainable energy, we use energy sources that will not run out as we use them. We use renewable energy sources that will be available today and in the future. Most sources of energy such as oil are finite and are likely to end at a certain point in the future hence the need to use sustainable energy. Sustainable energy is also used to protect the environment. Most sources of current sources of energy have high emission levels. These destroy the environment the longer they are produced and used.