A secure supply of energy resources is generally agreed to be a necessary but not sufficient requirement for development within a society. Also, sustainable development demands a sustainable supply of energy resources. The implications of these statements are numerous, and depend on how sustainability is defined. As stated earlier, one important implication of these statements is that sustainable development within a society requires a supply of energy resources that, in the long term, is readily and sustainably available at reasonable cost and can be utilized for all required tasks without causing negative societal impacts. Supplies of such energy resources as fossil fuels (coal, oil, and natural gas) and uranium
are generally acknowledged to be finite; other energy sources such as solar, wind and hydro are generally considered renewable and therefore sustainable over the relatively long term. Wastes (convertible to useful energy forms through, for example, waste to energy incineration facilities) and biomass fuels are also usually viewed as sustainable energy sources. A second implication of the initial statements above is that sustainable development requires that energy resources be used as efficiently as possible. In this way, industry maximizes the benefits it derives from utilizing its energy resources, while minimizing the negative impacts (such as environmental damage) associated with their use. This implication
acknowledges that all energy resources are to some degree finite, so that greater efficiency in utilization allows such resources to contribute to development over a longer period of time, i.e., to make development more sustainable. Even for energy sources that may eventually become inexpensive and widely available, increases in energy efficiency will likely remain sought to reduce the resource requirements (energy, material, etc.) to create and maintain systems and devices to harvest the energy, and to reduce the associated environmental impacts.