factors are usually set by the solar installer in relation to the particular installation, especially dependent on the efficiency of the heat exhchanger and production capacity of the panels. Controllers provide an overrun time to extract some of the heat energy left in interconnecting pipes after the panels cool off. They may also implement certain safety features such as cooling the store when it exceeds a preset temperature such as 65 °C, by sending excess heat back to the panels to be given off to the environment. Photovoltaic powered solar controller A photovoltaic (PV) powered solar controller uses solar electricity produced on-site to run the pump that delivers the solar-heated transfer fluid to the hot water store. One claimed advantage of PV power is that it reduces the overall carbon emissions associated with operating the system since it avoids the need to supply this energy from fossil sources. However, the energy required to operate the system is very small in comparison to the energy produced by the system and the carbon emissions reduction of adding PV power fractional. The most practical benefit of a PV powered controller is the resultant simplicity of the overall system. Rather than using complex algorithms based on store and panel temperatures, the pump is driven directly by the PV panel: when the sun shines, the pump runs. In practice this is nearly (90-99%) as efficient a practical control algorithm as most others achieve and has obvious advantages for reduced system complexity. A disadvantage to the PV powered approach is that the pump stops immediately after the sun is occluded. With vacuum tube and heat pipe solar panels, these can have an appreciable amount of energy stored in each tube at the moment the sun goes in. To avoid overheating the tubes it is necessary to either pump the circuit for a short time after the sun, or else to provide a large reservoir of fluid in the header above the tubes. Neither of these options is really compatible with the simple direct-PV pump approach and so such systems are limited to using the less efficient flat panel collectors. A PV powered controller may contain a small electricity store to allow the controller to remain powered and display temperatures at night when there is no sunlight. This electricity store is usually in the form of supercapacitors, since these have a much longer life than batteries. The