At the moment, offshore wind energy is more expensive to install than onshore wind energy. This is mainly because of the high cost of carrying out engineering work at sea. However, offshore wind turbines are significantly larger than onshore wind turbines and can deliver much larger amounts of electricity than onshore turbines. But this picture is changing rapidly. A cost reduction programme, being carried out by the European wind turbine industry in conjunction with the European Commission, has already delivered significant reductions in the cost of installing offshore wind turbines.
IEA report on offshore wind
The IEA has produced an informative report on the current (March 2017) state of play in the offshore wind sector.