The extraction of useful energy from ocean waves requires that the waves act on some form of responsive mechanism able to resist the working force that the waves apply and hence transfer net power from the waves. This mechanism by which energy is transferred from the waves and into a useful form in the WEC is known as the power take-off (PTO).
The extent of a PTO system within a WEC is not always clear. In addition to motors, generators, actuators and other energy conversion components it could also be defined to include the bearing and sealing systems required to enable relative motion. Similarly, it could include transformers and other export equipment required to interface to a local electricity grid. For the purposes of this document, however, the boundaries of the PTO are taken to be the structural attachment points of the hydraulic rams at one end and the electrical export connection from the PTO at the other. The bearings, sealing systems are only discussed in limited detail to provide important context, as are the electrical export systems.