A hybrid power system has an internal combustion engine, a gear box having an input connectable to the engine and an output shaft to drive a vehicle, a first power drive and take off releasably connectable to the gear box input, a second power drive and take off releasably connectable to the gear box output, at least one motor/generator connected the first and/or second power drive and take off, a connecting clutch releasably connecting the first and second power drive and take off, and a battery to power or be charged by the or each motor generator.