According to Paragraph 4, the "ideal energy source":

          What are the best energy sources? "Best" depends
     on many factors - how the energy is being used, where
     it is being used, what energy sources are available,
     which sources are most convenient and reliable, which
5   are easiest to use, what each costs, and the effects on
     public safety, health, and the environment. Making smart
     energy choices means understanding resources and their
     relative costs and benefits.
          Some energy sources have advantages for specific
10  uses or locations. For example, fuels from petroleum
     are well suited for transportation because they pack a
     lot of energy in a small space and are easily transported
     and stored. Small hydroelectric installations are a good
     solution for supplying power or mechanical energy close
15  to where it is used. Coal is widely used for power
     generation in many fast-developing countries - including
     China, India, and many others - because domestic
     supplies are readily available.
          Efficiency is an important factor in energy costs.
20  How efficiently can the energy be produced, delivered,
     and used? How much energy value is lost in that process,
     and how much ends up being transformed into useful
     work? Industries that produce or use energy continually
     look for ways to improve efficiency, since this is a key to
25  making their products more competitive.
          The ideal energy source - cheap, plentiful, and
     pollution-free - may prove unattainable in our lifetime,
     but that is the ultimate goal. The energy industry is
     continuing to improve its technologies and practices, to
30  produce and use energy more efficiently and cleanly.
          Energy resources are often categorized as
     renewable or nonrenewable.
          Renewable energy resources are those that can be
     replenished quickly - examples are solar power,
35  biomass, geothermal, hydroelectric, wind power, and
     fast-reaction nuclear power. They supply about seven
     percent of energy needs in the United States; the other
     93 percent comes from nonrenewables. The two largest
     categories of renewable energy now in use in the U.S.
40  are biomass - primarily wood wastes that are used by
     the forest products industry to generate electricity and
     heat - and hydroelectricity.
          Nonrenewable energy resources include coal, oil,
     natural gas, and uranium-235, which is used to fuel
45  slow-reaction nuclear power. Projections of how long a
     nonrenewable energy resource will last depend on many
     changeable factors. These include the growth rate of
     consumption, and estimates of how much of the remaining
     resources can be economically recovered. New exploration
50  and production technologies often increase the ability of
     producers to locate and recover resources. World
     reserves of fossil energy are projected to last for many
     more decades - and, in the case of coal, for centuries.

In: http://www.classroom-energy.org/teachers/energy_tour/pg5.html