My own opinion is that they will remain so expensive that if our civilization had to rely on them the standard of living would go down a lot. Some enthusiasts think they wouldn't mind that. This generates about 20 percent of the world's electricity. Its expansion is temporarily held up by the influence of environmentalist ideology. See the Nuclear FAQ for details.
Nuclear energy is humanity's best hope for the long term.
These terms are often confused, because in ordinary language they are sometimes synonyms. However, the distinction is important for science and engineering and is simple to state. Power is the rate at which energy is generated or used. The basic unit of energy is the joule, and the basic unit of power is the watt. A watt is one joule per second.