Convert AC water heater to DC using low voltage

Photovoltaic water heater
100 watt solar panel
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Example Renogy mono-crystalline panel
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One 100 watt solar panel at Amazon $149
One 250 watt panel at Amazon $350
Two 150 watt panels at Amazon $300
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Branch wires
Mounting brackets

Specifications of the 250 Watt RENOGY panels
Maximum Power: 250W
Optimum Operating Voltage (Vmp): 30.1V
Optimum Operating Current (Imp): 8.32A
Panel Ideal resistance at 1000W/m^2 solar isolation = Vmp / 1mp 30.1V / 8.32A = 3.617 Ohms (per panel)

Project: Connect 24 volt DC directly to AC water heater.
PV solar voltaic solar panels:
If panels are wired in series, the amperage increases, and voltage stays same.
If panels are wired in parallel, the amperages stays same and voltage increases.
So 2 Renogy 250 watt panels wired in parallel will produce 30.1 volts and 500 watt.
Parallel means both negative wires are connected together and both positive wires are connected together.
Series means negative wire on first solar panel is connected to positive wire of next panel... etc.

On water heater, change lower element to 900-1200 watt DC, and disconnect wiring from top of heater.
Leave upper element in place and connected to 240 volt.
Upper element works in ordinary fashion when water heater is ON.
Lower element is ON when solar panels produce electricity.
Drawback of low voltage: long time to heat.
Other option: use same plan using smaller 20 gallon heater/ read: Under counter water heaters
Reliance water heater
Reliance/ 6-10 gallon/ Side connections/ 6 year warranty
Reliance water heaters
12-3 extension cords
12-3 southwire cable
Reliance manual
State manual

Notes on DC elements
Single U and double U DC elements, 24 volt 500-900 watts.

The single U is ordinary residential water heater style with 2 screws to connect the wires. So your solar panels are connected directly to the element.
The double U element  has 4 screws for 4 wires and can accommodate two separate connections.... meaning you can have two separate sets of solar panels, and each set of solar panels can supply power to one of the U
3U elements are usually larger thread and will not fit a residential tank.

DC elements
Single U element at Amazon
Double U element at Amazon
single U element
double U element
Solar PV safety
How to set up array
How to replace water heater elements
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Convert AC water heater to DC water heater/ high voltage

Typically in residential water heating, small water heaters (20 gallon) use 1000-1500 watt elements. Ordinary shower uses 10 gallons hot, ordinary bath uses 20 gallons hot.
You can set thermostat to higher temperature to get more miles out of a tank of hot.

Larger water heaters use elements with more wattage in order to heat the water.... if the tank is full of unheated water, it will cool the water that is heated.
A 500 watt element would struggle to heat a 40 gallon tank

5.866689  Kwh to raise 40 gallons of water 60F (starting at 60- and ending at120F)
Typical shower might be 104F
1000 watts for 1 hour is 1 Kwh
Your 500 watt element produces 1 Kwh for each 2 hours operation
So you would need 5.867 x 2 hours or 11.73 hours to heat a tank ... and that is with ideal conditions, no consumption of hot during those hours, full sunshine, no cooling of tank etc etc ... except the tank would cool, and sun is not dependable each day.
.. or if you satisfied with less-than-thermostat-setting hot water, say 80?

If you have a double U element, the time would be less.

Conclusion: the direct solar PV using low voltage DC elements plan would work best with a smaller (20-30 gallon), well insulated tank.... and that was the project I was planning at my house, except hadn't gotten around to it yet.

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