I have the same issue, after some research it came down to the Control Load 2 kicking in during the day. Apparently, AGL has this configured to be excluded from any solar production. A while ago I had this idea to turn on Control Load 2 which allows the hot water to heat during the day. Using a timer I could program it to come on during the day at the redued $$ rate. If the sun was out I was expecting free hot water.... not the case as it seems.