Ray's PV water heating

 https://rmenke.blogspot.com/2019/07/diy-solar-water-heater.html

Ray is now heating water with DC electric power from a string of three serial panels.  His electricity has a cost of about $.03/kwh.  The three panels are dedicated to heating water and can not be used to sell power to the grid (or otherwise used) without reconfiguring.   The $.03 is the amount that he would  be paid for excess power converted to AC and sold to the grid.

If Ray needed or wanted more water heating power, he could go to four or more panels in series (instead of the three he is using) or maybe put in two parallel strings (total of 6 panels).  To avoid attempting to switch DC power, which is much more arc prone than AC, Ray depends on not using enough power to over heat the water in the old electric heater.  The higher the voltage, the more likely is arcing, so it is possible that Ray could use the AC switch in his heater, designed to handle 240VAC, to switch the ~100VDC from his three panels.

My water is now heated with a standard AC water heater but retrofitted with about 3kw heating elements to replace the original 4kw elements.    The smaller elements allow my heater to have a chance of operating from my PowerWall battery if the grid is down; my single battery PW has a maximum power of 5kw.  My water heater is on a timer that starts about 9am in order to prevent night time heating from the grid.  My cost is about $.06/kwh which is the amount that I would be paid if the power was sold to the grid.  Night time water heating would cost me about $.10/kwh if I allowed heating at night.

My water heater daily power is about 3kw for less than an hour.  So, 3kwh or about $.18.  Ray's power/energy consumption should be about the same and should cost about $.06 whether from panels dedicated to heating water or not.

If Ray wanted to consider how to automatically switch his DC power (via built in thermostat), he could look at fewer panels in series.  One panel for 30-35 VDC or two for 60-70VDC.