Abstract: There is disclosed a roller drive unit formed of one or more drive rollers, and which receives input power from any suitable engine powered wheeled vehicle which is available, the driving wheel(s) of the vehicle engaging the drive rollers so as to provide input power. The drive unit includes a hydraulic pump arranged to obtain input power from the drive rollers, and which operates a hydraulic motor under the control of a flow regulator so as to generate output power available from the motor at a substantially constant speed. The roller drive unit therefore is particularly suitable for providing emergency power for operating an electrical generator, and which can obtain its power input from any suitable available vehicle. The roller drive unit will therefore be particularly suitable for use as an emergency or standby drive system for operating electrical generators, and especially in remote locations where mains or other electrical power is not available.