A dual rotor arrangement 100 for a wind or water turbine has a planetary gearbox with two inputs and an output. One rotor 1 is connected to one of the inputs 3 and the other rotor 8 is connected to the other input 6. A generator is connected to the high speed shaft 11. The rotational speed of the output is determined by the relative rotational speeds of the rotors which means that the generator produces electricity of a desired frequency. The arrangement enables electricity of constant frequency to be generated under conditions where the wind or water speed is variable.