A fixed speed WT will, once started, always rotate at the same speed, regardless of the wind speed. As power must be injected at a constant frequency onto the grid, operating at a fixed speed simplifies the generator and power electronic requirements for the WT considerably, making it cheaper. However, as the WT will only be able to operate at maximum efficiency at one particular wind speed, fixed speed WTs are around 10% less efficient than variable ones. A compromise is to use a WT that can operate at two different fixed speeds.