The motion of the world’s oceans generate twice the energy that the U.S. currently generates from all sources at the very top of their capacity.  In September, the first commercial wind farm began producing electricity off the coast of Portugal.

Inhabitat explains how it works:

Pelamis Wave Energy Converters are tethered to the ocean floor by cables and are pointed perpendicular to the coastline. Each device is composed of several sections connected with articulated joints. As the waves roll in past the device, each section is driven up and down, while the hydraulic rams inside resist the motion. This resistance pumps high pressure fluid through hydraulic motors, which drive electric generators, thereby producing electricity. This electricity is then transmitted via underwater cables to the mainland.

Naturally, the amount of electricity generated depends upon the power of the waves at any given time, so like wind and solar energy, the electricity generated is not on demand.