03-19-2023, 04:47 PM
I am drawn to the simple glazed box water heater. I have to fight the impulse that since a solar thermal collector is something like 80% efficient and solar PV is something like 20% efficient solar thermal has to be something like 4x as good, right? For a stand-alone system it probably is. I, however, have already bought all the MPPT charge controllers, batteries, inverters, propane water heaters, and generators. My job now is to keep all these assets profitably employed. I have a large 119 gallon water heater with 4,500 watt elements. I have bought 1,500 watt elements to replace them. If I now buy 1,500 - 2,000 watts of extra PV capacity the system as a whole will easily run that water heater for 4 hrs a day on any decent day, more on exceptional days. That will keep me in hot water most of the time. On bad days I will burn a little propane heating water and a little gas running the generator, but the savings in gas and generator maintenance from the extra solar PV will more than make up for burning some propane.
I selected 1,500 watt elements because I anticipate it working well with my 4,000 watt inverter. I bought a current switch that will shut off the water heater when it sees a pre-set A/C current draw (other than the water heater). All the A/C load wires other than the water heater go through the hole in that switch. So the charge controller turns on a 12 vdc control circuit when the array voltage goes above something like 95 volts which it does when the batteries are satisfied. This control circuit powers a 240 vac relay that turns the water heater on but the normally closed current switch is part of the 12vdc circuit so if the inverter has other loads (I am thinking around 1,000 watts worth) the water heater gets turned off until those other loads go below the threshold.
If this all works I should rack up several hours per day of water heating without me having to pay attention or do anything except on cloudy days when I would have been crying anyways. Even then I will burn less gas in the generator and no more propane in the water heater.
I selected 1,500 watt elements because I anticipate it working well with my 4,000 watt inverter. I bought a current switch that will shut off the water heater when it sees a pre-set A/C current draw (other than the water heater). All the A/C load wires other than the water heater go through the hole in that switch. So the charge controller turns on a 12 vdc control circuit when the array voltage goes above something like 95 volts which it does when the batteries are satisfied. This control circuit powers a 240 vac relay that turns the water heater on but the normally closed current switch is part of the 12vdc circuit so if the inverter has other loads (I am thinking around 1,000 watts worth) the water heater gets turned off until those other loads go below the threshold.
If this all works I should rack up several hours per day of water heating without me having to pay attention or do anything except on cloudy days when I would have been crying anyways. Even then I will burn less gas in the generator and no more propane in the water heater.