There are also boost systems known as kinetic energy recovery systems (KERS). These devices recover the kinetic energy created by the car's braking process. They store that energy and convert it into power that can be called upon to boost acceleration. KERS typically adds and weighs . There are principally two types of systems: electrical and mechanical flywheel. Electrical systems use a motor-generator incorporated in the car's transmission which converts mechanical energy into electrical energy and vice versa. Once the energy has been harnessed, it is stored in a battery and released at will. Mechanical systems capture braking energy and use it to turn a small flywheel which can spin at up to 80,000 rpm. When extra power is required, the flywheel is connected to the car's rear wheels. In contrast to electrical KERS, mechanical energy does not change state and is, therefore, more efficient. There is one other option available, hydraulic KERS, where braking energy is used to accumulate hydraulic pressure which is then sent to the wheels when required.