▁heat ▁ex ch anger ▁every ▁few ▁seconds . ▁ ▁Com mer cial ▁controllers ▁do ▁not ▁turn ▁on ▁the ▁p ump ▁until ▁the ▁difference ▁in ▁temperature ▁between ▁the ▁pan els ▁and ▁the ▁water ▁in ▁the ▁heat ▁ex ch anger ▁is ▁sufficient ▁to ▁provide ▁significantly ▁more ▁energy ▁than ▁is ▁consum ed ▁by ▁the ▁p ump . ▁This ▁temperature ▁difference ▁is ▁called ▁the ▁on ▁differential ▁( us ually ▁ 4 – 1 5   ° C . ▁They ▁turn ▁off ▁the ▁p ump ▁when ▁the ▁pan els ▁no ▁longer ▁are ▁hot ▁enough ▁to ▁provide ▁significant ▁heat ▁to ▁the ▁store ▁( the ▁off ▁differential ). ▁The ▁wider ▁the ▁difference ▁between ▁these ▁differential s , ▁the ▁fewer ▁p ump ▁on - off ▁cycles ▁will ▁take ▁place . ▁These ▁factors ▁are ▁usually ▁set ▁by ▁the ▁solar ▁installer ▁in ▁relation ▁to ▁the ▁particular ▁installation , ▁especially ▁dependent ▁on ▁the ▁efficiency ▁of ▁the ▁heat ▁ex h ch anger ▁and ▁production ▁capacity ▁of ▁the ▁pan els . ▁ ▁Cont rollers ▁provide ▁an ▁over run ▁time ▁to ▁extract ▁some ▁of ▁the ▁heat ▁energy ▁left ▁in ▁inter connect ing ▁pip es ▁after ▁the ▁pan els ▁cool ▁off . ▁They ▁may ▁also ▁implement ▁certain ▁safety ▁features ▁such ▁as ▁cool ing ▁the ▁store ▁when ▁it ▁exceed s ▁a ▁pres et ▁temperature ▁such ▁as ▁ 6 5   ° C , ▁by ▁sending ▁excess ▁heat ▁back ▁to ▁the ▁pan els ▁to ▁be ▁given ▁off ▁to ▁the ▁environment . ▁ ▁Phot ov olta ic ▁power ed ▁solar ▁controller ▁ ▁A ▁phot ov olta ic ▁( P V )