▁turning ▁the ▁p ump ▁on ▁when ▁the ▁store ▁is ▁hot ter ▁than ▁the ▁pan els . ▁ ▁Most ▁commercial ▁controllers ▁display ▁the ▁temperature ▁of ▁the ▁hot ▁water ▁in ▁the ▁store ▁and ▁provide ▁general ▁status ▁information ▁about ▁the ▁system , ▁including ▁overall ▁energy ▁production . ▁ ▁Com ponents ▁The ▁simplest ▁solar ▁controller ▁circuit ▁uses ▁a ▁compar ator ▁with ▁two ▁temperature ▁inputs , ▁one ▁at ▁the ▁solar ▁panel ▁and ▁one ▁at ▁the ▁thermal ▁store ' s ▁heat ▁ex ch anger , ▁and ▁an ▁output ▁to ▁control ▁the ▁p ump . ▁Com mer cial ▁controllers ▁use ▁a ▁micro processor ▁usually ▁with ▁a ▁L CD ▁display ▁and ▁simple ▁user ▁interface ▁with ▁a ▁few ▁push but tons . ▁Power ▁for ▁the ▁controller ▁and ▁the ▁p ump ▁can ▁come ▁from ▁a ▁main s ▁electric ▁supply ▁or ▁from ▁a ▁phot ov olta ic ▁( P V ) ▁module . ▁ ▁Function ▁The ▁controller ' s ▁main ▁function ▁is ▁to ▁switch ▁the ▁circul ating ▁p ump ▁on ▁or ▁off . ▁ ▁The ▁p ump ▁is ▁usually ▁switched ▁on ▁when ▁the ▁solar ▁panel ▁is ▁hot ter ▁than ▁the ▁water ▁in ▁the ▁store ' s ▁heat ▁ex ch anger ▁and ▁off ▁when ▁the ▁panel ▁is ▁col der . ▁Switch ing ▁the ▁p ump ▁on ▁trans fers ▁the ▁heat ▁in ▁the ▁panel ▁to ▁the ▁store . ▁Switch ing ▁it ▁off ▁when ▁the ▁pan els ▁cool ▁prevents ▁a ▁revers al ▁of ▁the ▁process ▁and ▁loss ▁of ▁heat ▁from ▁the ▁store . ▁The ▁controller ▁measures ▁and ▁comp ares ▁the ▁temper atures ▁in ▁the ▁panel ▁and ▁the