The efficiency of energy supply and demand in the residential sector is of great importance in the UK. Using renewable sources of energy such as water, to supply the demand of some buildings, has proved to be a proper energy strategy. In terms of this dissertation, an old watermill has been upgraded. A “Kaplan” turbine has superseded the old wooden wheel of the mill and the building is been renovated to accommodate apartments of different occupancy. Then, it is assumed that it is placed into two rivers of different average flow rate. The study aims to evaluate how much energy could be supplied to residential buildings by a typical UK river. Results showed that installing a turbine in a building should be thought thoroughly, as there must be sufficient water in the location of the installation. Buildings of 300 m2 occupied area can be served by rivers of mean flow rate of 5.40 m3/s, while rivers of low mean flow rate, such as 1.0 m3/s provide only a small amount of the energy demand. Issues like cost of installation and maintenance of the turbine are also quoted and discussed.