Mixing 1 m3 of river water and 1 m3 of sea water can theoretically generate 0.5 kilowatt-hour (kWh). The energy density of fresh and salt water as fuel is significantly less than fossil fuels such as coal or petroleum; however, there are large quantities of water available that make it an attractive option for many coastal regions interested in harvesting renewable energy, in combination with the environmental advantages.The outflow of all rivers in the world combined is 1.13 million m3 per second. The theoretical potential of all of this water is about 2 TW (1 TW = 1 million MW), roughly equal to the total produced electricity of the entire planet. In theory, this means that the all current fossil fuel and nuclear power plants could be replaced by RED power plants. In practice, this is much more difficult. First, RED systems are not 100% efficient, and not all rivers are ideal locations for this type of power generation. Ideal locations for RED must take into account the local marine, environmental, and available infrastructure.