The principle behind tidal generation is similar to wind turbines, except that instead of wind turning the turbine blades, the process uses underwater current caused by tides. One of the benefits of tidal power over wind power is the predictability of tidal currents, enabling the developers to know exactly when the turbines will be producing power. Electricity can also be generated by harnessing the energy of waves. The aim is to capture the vertical movement in the water surface caused by waves and to convert that energy to electricity by turning a generator, Both tidal and wave power are in the early stage of commercial use in the UK. Currently they are more expensive than other renewable technologies such as wind and hydro, but it is hoped that costs will decrease in future.