I am aware this thread is about using the extra solar electric
panel's energy to heat water, but for those interested in alternative solar heating
methods, like the black garden hose coiled on deck...loved that one, there is another neat little inexpensive system available that is relatively unknown that I plan to give a try. It's made by Heliatos. They make small solar water panels
(2'x2') that use a constantly circulating loop of water (that you can plumb through your water tank). The circulating water is controlled by a small pump with it's own stand alone "small" 10 watt solar electrical
panel. The dual water panel kit is a little over $500 bucks. (They probably don't charge enough since it's "for a boat"...Ha!) That's the one I may try as I only have a 6 gallon water heater onboard. I haven't done any calculations on heating
my 6 gallons, but I am hoping to be able to get by with only one panel...don't know. Anyway, the Heliatos is an automatic system. The sun comes up, the little stand-alone 10 watt solar panel makes electricity and starts the pump circulating water. The sun goes down...solar panel quits, the pump stops. There are no on/off switches, or electrical
connections to the boat system at all---simple.
You can check out the marine
kit versions of this system here >> Heliatos Solar
. Scroll to the bottom of the webpage for the boat versions and kits.
I do not have any affiliation with the company.