A device for supplying electricity to electrical equipment on a first element drivable for rotating operation with a predetermined rotation rate. A source of electricity includes a stator and a rotor that are rotatable in relation to each other. One of the rotor and stator is attached to the first element and another of the rotor and stator is attached to a second element including at least one braking member. In accordance with a predetermined drag coefficient, when interacting with a surrounding fluid medium in rotating operation a braking torque is induced, which results in a difference in rotation rate between the first element and the second element. An output to the source of electricity is based on a product of the braking torque and the rotation rate difference.