Answer:
Energy is used to power the "night mail" is not specified in the works of J.R.R. Tolkien, who was the author of "The Lord of the Rings" and "The Hobbit." The "night mail" is a reference to a postal service that delivers letters and packages at night, as described in the poem "Night Mail" by W. H. Auden. However, there is no information available about what energy source was used to power this service.