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Convert AC water heater to DC using low voltage
<Photovoltaic water heater
100 watt solar panel
Back view                      Front view
Example Renogy mono-crystalline panel
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Buy from my affiliate links:
One 100 watt solar panel at Amazon $149
One 250 watt panel at Amazon $350
Two 150 watt panels at Amazon $300
Buy solar panels at Amazon
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:

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.

If panels are wired in series, the volts are higher, amperage is lower, and resistance is higher. Resource with calculation. This is good for harvesting maximum power in low solar conditions. Series = good for low solar conditions (after 3 pm on sunny day or overcast day).
If panels are wired in parallel, the volts are lower, amperage is higher, resistance is lower. This is good for harvesting maximum power in high solar conditions. Parallel = good for high solar conditions (mid-day sunny conditions)
How to optomize solar panel array

Solar panels are like flashlight batteries.
Total wattage or power stays the same no matter series or parallel.
If panels are wired in series, the amperages stays same and voltage increases.  
If panels are wired in parallel, the amperage increases, and voltage stays same.
So 2 Renogy 250 watt panels wired in series will produce 60.2 volts and 500 watts.
2 Renology 250 watt panels wired in parallel will produce 30.1 volts 16.4 amps and 500 watts

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

Choose element rated for your output:
Volts and watts are printed on end of each element.
Ordinary elements work fine for AC or DC.
You can connect 100-250 volt 30 amp solar installation directly to residential water heater... except alterations are needed, otherwise the thermostat will melt. Read about converting to high voltage DC

In 2017, most local hardware stores do not carry elements rated for low volt and watt ratings, so you need DC element for those applications.

The math shows it:
volts x amps = watts

Example: If your panels produce 91 volts x 16 amps = 1456 watts (the 91 volts 16 amps will probably burn up the thermostat when direct connected to water heater so use high voltage plan).
If your 91 volts drops to 60 volts in the afternoon, then the wattage drops and element produces less heat

If you connect 1456 watt output to an element rated for 120 volt 900 watts, then element will probably burn up.
If you connect 1456 watt output to an element rated for 120 volt  2500 watts, then element will still produce 1456 watts heat.
If you connect 1456 watt output to an element rated for 24 volt 600 watts, then element will burn up.

If your solar panels generate 145 volts x 15 amp = ?
240 volt x 15 amps = ?
What happens when you connect 240 volt 15 amp electricity to 120 1500 watt element?
The element burns up.

Choose element rated for your output:
Ordinary elements work fine for AC or DC, but are not often available in low volt and watt ratings, so you need DC element.

Single U and double U DC elements, 24 volt 500-900 watts.
The single U has ordinary element style with 2 screws for connecting 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 made for 3-phase boiler installations, have larger thread, and will not fit a residential tank.

DC elements
Single U 24 volt 600 watt element
Double U element
single U element
double U element
Solar PV safety
How to set up array
How to replace water heater elements
Figure volts amps watts
All solar links

Convert AC water heater to DC water heater/ high voltage

What size water heater?
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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E-mail: geno03245w@gmail.com
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