One of the most common ways that this method is used by households is through solar water heating. In a broad perspective, these systems involve well insulated tanks for storage and collectors, are either passive or active systems (active systems have pumps that continuously circulate water through the collectors and storage tank) and, in active systems, involve either directly heating the water that will be used or heating a non-freezing heat-transfer fluid that then heats the water that will be used. Passive systems are cheaper than active systems since they do not require a pumping system (instead, they take advantage of the natural movement of hot water rising above cold water to cycle the water being used through the collector and storage tank).[44]