Sustainable energy is energy that can potentially be kept up well into the future without causing harmful repercussions for future generations. A number of types of energy can be thought of as sustainable, and many governments promote their use and the development of new technologies that fit within this model. Increasing rates of energy consumption around the world have led to a corresponding rise in concerns about where the energy comes from and if it will become more scarce.
Several factors go into making energy sustainable. The first is whether or not the current use of the energy is something that could potentially persist into the future, which leads many forms of renewable energy to qualify as sustainable. People can generate energy from windmills, ocean waves, and the sun without running out of energy and resources, making these methods sustainable for use by future generations. By contrast, fossil fuels are not treated as sustainable because the Earth's supplies of crude oil will eventually run out.
Another consideration is energy efficiency. Some forms of renewable energy, for example, take quite a lot of effort to actually generate, meaning that almost as much energy goes into their production as the sources themselves generate. Energy efficiency can also be used to describe the technologies that use energy, such as homes, cars, and businesses. Increased efficiency in the way energy is used makes sustainable energy stretch further.