While a reefer will have an integral refrigeration unit, they rely on external power, from electrical power points at a land based site, a container ship or on quay. When being transported over the road on a trailer they can be powered from diesel powered generators ("gen sets") which attach to the container whilst on road journeys. Some reefers are equipped with a water cooling system, which can be used if the reefer is stored below deck on a vessel without adequate ventilation to remove the heat generated.