The rotor on a variable speed WT will rotate at a speed calculated to make it as efficient as possible at the prevailing wind speed (i.e. at λ_max – see Tip Speed Ratio). Variable speed WTs will produce less tip noise at low wind speeds as they will be rotating more slowly, and can be up to 10% more efficient than fixed speed WTs. However, they are more complex than fixed speed WTs and require power electronic converters (PECs) to convert their output to grid frequency. A variety of partially-variable speed WTs are available, bringing most of the efficiency gains with only some added complexity and no PEC.