I needed a way to heat water when the sun was shining brightly. Our solar array (Peak 15KW) is able to provide enough power to do this providing that the brightness is more than 40,000 Lux. I have set this as the level to turn on a SONOFF-TH16 switch to boost the electric water heater. when the Lux level drops below 38,000 I turn off the heater. This saves drawing grid power costing 38c (Australian cents) which is what I am charged.
This system is remarkably good at avoiding expensive grid power.
THANKS for IFTTT-Tempest app which makes this all possible
Obviously someone else will have to find the best Lux settings depending on their Solar array and water heating element. Yes we also have Solar heating tubes. In summer they are great but they are useless in winter where we live.