The Mumbles Railway Electricity Sub-station or Blackpill Station was designed by the architect, Ernest Jones, and built in 1927. The building originally contained two 500kW rotary converters that converted 6600V alternating current to 650V direct current to operate trams by overhead wires.

The building has a tightly ordered, formal design and is constructed in brown brick with concrete dressings. The round arched windows have Georgian-wired glazing incorporating a 'pebbled' obscure finish, which is a typical neo-classical product of the 1920s.
The original internal rotary converters and associated plant have been removed, although the internal tiled floor and 'I' beams remained until 1997. Above the door there is a grille and the date of 1927.

RCAHMW, 19 October 2011.