''The phrase, "carrying coals to Newcastle," means spending an inordinate amount of energy on something useless, fruitless, or redundant. This idiom arose in the 15th century because Newcastle, England was known throughout the country as a major exporter of coal. Therefore, "carrying coals to Newcastle" would do you no good, because there was more coal there than anywhere else. Variations on the saying include "bringing," "taking," or "moving" the coal.
Newcastle-on-Tyne was founded at a convenient place to cross the Tyne River in 1080 as a harbor town. During the Middle Ages, the city exported wool by boat down the river. Then, in the 1400s, Newcastle became famous for its abundant coalmines. They exported the fuel all over England, especially to the growing metropolis of London. At the time, "coals" referred to the lumps of raw coal, whereas in today's vocabulary, "coal" is plural.
The surrounding counties of Northumberland and Durham supported a biannual fair in Newcastle where peddlers sold their goods. Perhaps, "carrying coals to Newcastle" arose as advice between peddlers not to try to sell coal at the bazaar. The first recorded instance of the contextualized saying appears in 1538 in England.'' [Wisegeek.com]