Photovoltaic water heater Convert AC water heater to DC water heater/  240 volt
The future of water heating for much of the world is the sun
You can do it yourself using ordinary electric water heater and 4-6 solar panels
Conversions are needed
Convert AC water heater to DC using low voltage
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100 watt solar panel
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Back view                      Front view
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

Buy Bundles;
4 panels 250 watt at Amazon $1275
6 panels 250 watt at Amazon
10 panels 250 watt at Amazon
20 panels 250 watt at Amazon

Solar panel connectors
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"90% or so of their original performance after 25 years.  They can easily last longer than that, slowly decreasing output as time goes on."
Plan below uses solar panels only, and requires no kit
1000 watt system: four 250 watt panels
3000 watt system: twelve 250 watt panels

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)

Element Wattage = 4500W
Element Resistance: 240 * 240 / 4500 = 12.8 Ohms
Ideal number of panels per single string assuming 1000W/m^2 solar isolation:
12.8 Ohms / 3.617 Ohms = 3.538 panels, round down to 3 panels per string.

System A) 750 watt system, 3 panels, 1 string, Ideal Resistance 30.1 * 3 / 8.32 = 10.85 Ohms
System B) 3000 watt system, 12 panels, 6 panels per string,  2 strings, Ideal Resistance (30.1 * 6) / (8.32 * 2) = 10.85 Ohms.
A string of 3 panels is like putting 3 D-cell batteries in a flashlight. They are in series, so voltage increases
Calculate volts and amps for array/ How to set up array
All costs and decisions are energy
Invest in things that make energy! Solar panels are warranted to deliver 80% rated value over 25 years.
Everything is on sale today... prices only get higher... for wire, solar panels, fuel, aluminum, copper

Oil predicted to run out 6-20 years/ natural gas 65 years/ coal 200 years.
35% of world's electricity generated by coal.
So question is... how much fossil-fuel is needed to make a solar panel?

To answer the value of solar technology requires fossil-fuel-value of human activity needed to make the panel at today's standard of technology...
,,, vrs less-fossil-intensive technology that produces a lesser quality solar panel
Estimated right now
DC water heater Best option for small system/ 6 panels, 1 or 2 strings
This wiring diagram does NOT by-pass water heater safety features: (ECO thermal cut off located on upper element, 600 volt wire insulation, wire size, voltage, tank hull ... unless human being does NOT install wire connections inside enclosure with cover or wires the circuit while the Power is turned ON.)
Simultaneous DC water heater/ direct connect solar panel to heater
Upper part of tank can heat using 240 volts whenever upper thermostat turns ON. Plus lower part of tank can heat with solar whenever lower thermostat turns ON. Plus option for tank to run on solar only by switching off power to upper thermostat
How it works:
This DIY solar water heater probably performs similar to Techluck, except off-the-shelf parts are more reliable since DIY repairs and tweaks and variables can be made. Long term cost will be lower than Techluck. Solar output can be higher than Techluck. The basic parts can be expanded to make full DC solar water heater.

Number of solar panels undetermined. You can run full maximum 240V DC 26 amps through the DC-DC relay... and that will match 240V 4500 watt element See DC power relays
Recommend buying DC circuit breaker that is rated below the max rating for DC relay.
With this illustration, showing simultaneous operation, the power supply is ON at all times, unless ECO reset button on upper thermostat is tripped.
Lower thermostat has 24 volt DC power available at all times, unless ECO on upper thermostat is tripped.
Water heater thermostats are mechanical and will operate fine with 24V DC. Thermostats will not melt with 24 VDC.

Ordinary lower element is resistive load, and will work fine with high voltage DC power
When lower part of tank is cool, solar power turns ON.
When lower part of tank is hot, then upper part of tank is also heated (because hot water rises to top of tank)
When lower part of tank is hot, DC-DC relay turns OFF, and solar power cannot reach lower element.
Upper part of tank heats normally using 240V household power. Or add 30 amp switch or timer to the black wire that feeds voltage into upper thermostat.. Adding switch or timer means tank is heated using only solar. Adding timer or switch on black wire, as described, does not by-pass ECO, and ECO will turn off solar if tank overheats.

ECO (reset button) on lower thermostat is not needed. Use ordinary lower thermostat because the upper ECO controls entire water heater, just like any ordinary water heater.
This design voids water heater warranty.
Hot water from this design can be recirculated another tank, gas or electric See recirculation
Safety: This illustration requires DC circuit breaker or fused safety switch (as shown in drawing). DC breaker must be clearly marked and located next to water heater.
Also requires safety switch for regular 240V AC household power also located next to tank.
All safety features apply to this diagram, including conduit, safety switch, circuit breaker, clear labeling, solid copper wire only, no stranded wire, and all electrical connections covered inside contained space, proper wire size and grounding as per local code. etc.
DC water heater wiring
Larger image
Less-than-best design because upper part of tank must be heated by 240 volts before solar can heat lower part of tank 
Non-simultaneous DC water heater
This wiring diagram does NOT by-pass water heater safety features

Non-Simultaneous shown here ... upper part of tank heats fully before power supply turns ON.
Solar is NOT available unless top of tank is heated
Lower element is only ON after upper part of tank is heated by 240 Volt

Upper thermostat receives 240 Volt in usual fashion.
When tank is cold, Upper thermostat sends power to upper element.

After upper thermostat reaches temperature set point, upper element is off, and 240Volt power is sent to lower element and thermostat.
This wiring varies from normal.

Upper thermostat is turned off, and 240V power is sent to power supply.

Power supply activates and produces 12-24 Volt DC
When lower part of tank is cold, the lower thermostat activates, sending power to DC-DC relay.
When DC-DC relay activates, the lower element receives DC power from PV panels

Power supply is ECO protected

How to choose element for DC water heater
Low watt element is better for low solar output.
__ Rule of thumb: measure PV resistance at high solar output... then multiply that resistance by two... and that is best element wattage for low solar.

Higher watt element is good for high solar output... measure PV resistance and match element wattage to that resistance.
Resistance of element is based on rating printed on end of element. End of element shows volts and watts. Square the rated voltage and divide by rated watts to calculate ohms or resistance.

See element ohm chart

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250V x 25 amp = 6250 watt
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When 1st pole burns out, move wires to 2nd pole, etc
Siemens 3RT10/ 460 volt AC 5 Hp/ 24VDC coil
3-pole 90 amp contactor/ 120 volt coil
Ordinary 3-pole AC contactor can be wired for DC
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Example shows 6 break points on positive wire

Use a 4-pole contactor and add double break point on negative wire
Power supply Power supply
Input 100-240 Volt AC / output 24 volt DC

120-220 Volt AC input/ 24 volt DC / 72 watt output/ Amazon
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Indicates when temperature in lower part of tank falls below temperature set point
Indicates when solar power is available on lower element
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12 volt dc hour meter
Add 12 Volt DC hour meter
Indicates how many hours the lower part of tank falls below temperature set point
Indicates number of hours when solar power is flowing to lower element
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Install electric meter on water heater
surface mount fuseholder Surface mount ANL fuseholder
ANL fuseholder and fuses
photovoltaic fuseholderDC fuse
>Fuseholder                         Fuse
Photovoltaic fuseholder/ DC fuseholder/ 1000 volts DC/ 
DC fuse/ photovoltaic fuse: use 20-25 amp fast-acting for designs on this page
Not waterproof... mount inside Nema 3R enclosure
Mount on din rail/ change fuse with flick of finger
Install 25-30 amp fuses
Use 10 x 38 DC fuses>
Photovoltaic fuseholder / DC fuseholder/ ultrasafe/ at Amazon
Ultrasafe fuseholders at Amazon
600 volt DC fuses at Amazon
Mersen 600 volt DC fuse
waterproof DC fuse holder Waterproof Solar fuseholder/ 1000 VDC/ 2 amp to 30 amp
Solar fuseholder at Amazon
Fuse holder plus 20 amp DC fuse
Fuse holder plus 30 amp DC fuse
DC circuit breaker

DC circuit breaker
DC circuit breakers for Solar/ use 20-25 amp for designs on this page
Buy from my affiliate links:
DC circuit breakers
High voltage DC circuit breakers at Amazon
Combiner boxes at Amazon

How to set up solar array
Din rail Din rail for mounting Breakers, fuse-holders, timers, relays
Make your own breaker box, combiner box 


Din rails for mounting
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How to set up solar array
30 amp DC safety switch 240 volt 30-100 amp DC safety switch
Heavy Duty 250/600 V Dc Voltage; 30-100 Ampere

Square D 30 amp DC/ indoor/ fusable/ HU361RB
Square D 60 amp DC/ indoor/ fusable H222AWK
100 amp non-fusable/ outdoor/ at Amazon

DC safety switches
Generic: How to wire safety switch

175 Amp Power Distribution block
Gather several smaller wires so they can supply larger wire
Distribution block at Amazon
Power distribution blocks
Marathon water heater is lightweight Marathon water heaters are rated for high temperatures from solar heating up to 170F
Marathon heaters have 1/2 plastic conduit for adding additional wires

Marathon elements
Camco 120F to 180F thermostat
Camco 180F lower thermostat

Marathon parts sheet/ pdf
Review Marathon heater
wire water heater thermostatSolar connections          How to remove water heater
Void product warranty
How to change/add wiring / foam-insulated tank
Rewire foam insulated  tank
Drill through top of tank near wiring cavities
Keep drill bit near outer edge of tank shell

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36" drill bits
12" drill bits

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How to wire thermostats

>Junction boxes can be located at top or side
6x6x4 box
Bud industries 11x7x5 box without knockouts
Boxes from Bud Industries
Boxes from Attabox
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Wiegmann boxes
Cantex boxes
Carlon boxes
Raco boxes
Hoffman boxes
Pull boxes
Water heater cover
Larger image
Thermostat and element must be covered
Cover, insulation and protector are necessary so thermostat can read correct temperature. Thermostats exposed to room temperature will misread temperature and cause overheating.
Thermostat must sit flat against tank.

Camco 90F to 150F ordinary thermostats at Amazon
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Camco 180F lower thermostat (commercial grade only)

Therm-o-disc upper element plastic terminal protector
Therm-o-disc lower element plastic terminal protector
Thermostats and covers
Solar water heaterLarger image
Best design for DC water heater

Maximize solar output
This wiring diagram does NOT by-pass water heater safety features
How this diagram works: 120 volt AC required.
Power supply receives 120 volt AC
Power supply is ON when ECO reset button is ON.
If ECO trips, then power supply is OFF.
Power supply sends 12-24 Volt DC to both upper and lower thermostats as illustrated.
The 12-24 volt DC supplies power to DC-DC relays that switch solar power On-OFF.
If power supply is OFF... then no solar power flows to elements
Photo eye receives 120 volt AC
Photo eye controls NO-NC relay with 120V AC coil
NO-NC relay controls DC-DC relay1
Lower thermostat controls DC-DC relay2.

Solar power has appropriate circuit breaker, safety shut off switch located at water heater, and correct grounding.
Solar panels and water heater are correctly grounded.
All electrical connections are inside enclosed box.
Water heater warranty is voided with any DC solar connection
Tank starts out cold, and lower thermostat turns ON.

When solar is good, and maximum PV power is available, then the photo eye is ON and DC-DC relay1 and relay2 are both ON

High solar output Result: DC-DC relay1 ON + DC-DC relay2 ON = Result solar power flows to lower element, and lower element is ON.
Lower element is 4500 watt, and meets estimated correct resistance for high solar output.

When conditions change, and less solar output is available, you want to raise the resistance.

The photo eye detects when solar output is lower.
Choose best location for photo eye / in a shaded area to control the on-off operation throughout day.

The photo eye turns off DC-DC relay1
DC-DC relay2 is still ON if lower thermostat is ON

Low solar output Result: DC-DC relay1 OFF + DC-DC relay2 ON = Result solar power routed through both elements in series and resistance is higher.

Optional 120 VAC Delay timer can be installed between NO-NC relay and DC-DC relay1. Purpose to prevent frequent on and off caused by frequent solar intensity changes.
Solar water heater
Larger image
Same as above 
... maybe this diagram uses unecessary extra relay to do same job as above
This wiring diagram does NOT by-pass water heater safety features

Specifications for DC water heater:
For this diagram and diagram above, the specs call for:
Two or three G9EB1 DC-DC relays with 24 VDC coil/ rated 30,000 operations at 240 Volt 25 Amp DC
One 250 Volt 25 amp circuit breaker to match maximum rating for relays
One NO-NO relay with 120 Volt AC coil
One power supply
One 120 volt photoeye
One stock electric water heater with ordinary upper and lower thermostats and ordinary 240 volt elements

How to add thermostat to gas or electric water heater
How to make heating tub
How to wire thermostats
Commercial 180 degree thermostats
Convert water heater to DC Purpose of this illustration is inspire experiment by folks who can avoid killing themselves and damaging property
This wiring diagram does NOT by-pass water heater safety features
Ordinary elements work fine with DC
Ordinary thermostats burn out with DC

This wiring violates water heater safety code and voids water heater warranty.
This wiring by-passes ECO thermal reset button located on upper thermostat,,, because Solid state relays fail closed and not open.... so in event of run-away heating caused by failed relay... the ECO will not turn off the solid state relay.
This wiring is not guaranteed to work... it should work... but solid state relays are never completely OFF.

Solid state relays can get very hot, and require heat sink.
Water heater thermostats must be covered with insulation to correctly read water temperature through tank wall. Exposure to room air can cause thermostat to misread temperature.
Relay must be located outside of tank because of high heat.
Thermostats are mechanical bi-metal switches... thermostats will work with power coming from 12 volt DC transformer
ECO protected thermostat located at top of tank can be substituted in place of lower thermostat... giving two ECO protected-thermostats.
Thermostats can be wired many ways. Illustration shows typical non-simultaneous, where either upper element is ON or lower element is ON or both elements are OFF. Both elements never ON at same time with this illustration.

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Turn off transformer at night when no solar is available
This wiring diagram does NOT by-pass water heater safety features
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Transformer: 120-240 Volt to selectable DC voltage

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When voltage drops, then wattage of element drops. Element voltage and wattage printed on end of each element. Element rated for 240 volt 4500 watt will operate at 1/4th wattage at 120 volt.
At 480 volts, the wattage is multiplied by factor of 4.
Overvoltage above element rating will burn the element out.
4500 watt @ 240 Volt will heat about 21 gallons per hour
DC DC solid state relay

DC-DC Solid state relays
3-32 volt input > choose 60-220 volt DC output
No arc. Quiet operation.
Turns power off by reducing voltage/ power is never completely stopped
Buy from my associate links
60 volt 25 amp DC-DC solid state relay
100 volt 25 amp DC-DC solid state Relay
200 volt 25 amp DC-DC solid state relay at Amazon

Relay safety cover
Heat sink

PV water heater discussion of elements/ link
PV water heater discussion of elements/ pdf
Use Hour meter
240 volt hour meter connected to upper element.
12 VDC hour meter connected to lower thermostat
Requires one 240Volt hour meter and one 12-24 volt hour meter

UWZ48E-240V AC hour meter at Amazon
Intermatic bezel to support UWZ hour meter
Intermatic ac hour meters at Amazon
240 volt hour meter
Intermatic GZ series 10-80 VDC hour meter
Intermatic FWZ53-24U
12 volt DC hour meters

How to read hour-meter results:
Wattage of element must be determined
Use multimeter to check voltage of water heater
Look at label on side of tank for different wattages at different voltages
Lower voltage means lower wattage than shows on element.
Confirm element wattage printed on end of each element

For example 4500 watt element and 240 volts
Hour meter reads 6 hours
4500 watts x 6 hours = 27,000 watt hours
27,0000 divided by 1000 = 27 kilowatt hours
With photovoltaic DC... the actual Kwh may not be calculable, and the data less useful
Thermostats, elements and wiring resources
Camco 90F to 150F ordinary thermostats at Amazon
Camco 120F to 180F thermostat (commercial grade only)
Camco 180F lower thermostat (commercial grade only)

Therm-o-disc upper element plastic terminal protector
Therm-o-disc lower element plastic terminal protector

How to wire water heater thermostats:
How to wire off-peak water heater thermostats
How to wire two water heaters

How to replace thermostats
How to replace elements
How to test elements

How to select right wire and breaker
Ordinary water heater elements will work with DC power
Quote: "Here is another way to look at it:

If your element is designed for 240 volts AC, then applying 120 AC or DC volts to it is unconditionally not a problem (unless it is not in water, of course.)
At 240 volts, it will pull about 4.17amps.
Successfully applying 480 volts AC to it will destroy it.
This means you can use AC-rated element

But if you have a panel whose Vmp is 480 volts and whose output power is 1000 watts, its Imp will be 2.28 amps, and its Isc will not be much higher, lets say 2.5 amps.
Now if you connect that panel to the heating element, you will not be in any danger of burning out the element, because it will be able to pull at most 2.5 amps from the panel, and the panel voltage will be less than 240 volts at that time.

It is a somewhat odd statement, but absolutely correct that if the panel maximum output power does not exceed the power rating of the heating element, then there is no combination of panel current and panel voltage that can damage that element. However, different combinations of panel specifications and heater specifications will determine whether most of that potential maximum power actually ends up in the heater or whether it is wasted because the panels are running far from their MPP.

Just don't try it with batteries, generators, or power supplies because, unlike solar panels, they can be damaged by attempts to draw more than their rated power from them and they could damage the heating element while trying!!!
Previous emails contain the calculations of what I expected to be the ideal number of series PV panels (Unisolar 64Watt) for the 4500W element in order to maximize yield aka (MPP) maximum power point.   That number of panels (and thus PV impedance) for this experiment equipment was calculated to be very close to 3 panels in single series string (or 2 strings of 6 each).   Other PV panel models have various Vmp and Imp specifications (and thus different impedance).  Other water heaters may have different heater element wattage (resistance) and thus obtain different results. 

Procedure: Array was put into various configurations and then the DC amps and volts under element load were measured.
Vm = Volts Measured
Am = Amps Measured
Wc = Watts Calculated = Vm * Am
Wpv = Watts PV name plate rating of the array
EF = Efficiency Percentage = Wc / Wpv

PV panels in single series string.

1 pv panel  Vm=19.41 * Am=1.0,  19.41Wc / 64Wpv     = 30.3% EF

2 pv panels Vm=34.75 * Am=2.5, 86.87Wc/ 128Wpv     = 67.8% EF

3 pv panels Vm=44.30 * Am=3.9,  172.77Wc / 192Wpv = 89.9% EF

4 pv panels Vm=49.03 * Am=3.6,  176.50Wc / 256Wpv = 69.9% EF

5 pv panels Vm=52.62 * Am=3.9,  205.21Wc / 320Wpv = 64.1% EF

6 pv panels Vm=53.97 * Am=4.1,  221.27Wc / 384Wpv = 57.6% EF
6 pv panels (2 parallel strings, 3 panels per string)
                   Vm=53.55 * Am=4.2,   224.91Wc / 384Wpv = 58.5% EF


3 panels of US64 is the ideal number in single series string for 4500W element as per calculations:

Its looking possible to calculate the appropriate number of panels in series to match the resistance of heater element.

Paralleling ideal number in series does NOT double output.
There are lots of different types of PV systems. Type 2 and 3 are most common.
1) Off-Grid DC Loads.
Sun -> PV Array -> Charge controller -> Battery Bank -> DC Loads

2) Off-Grid AC Loads (this is what my system is, Outback VFX).
Sun -> PV Array -> Charge controller -> Battery Bank -> Inverter -> AC Loads

3) Grid Tie Battery-Less
Sun -> PV Array -> GridTie Inverter -> AC Mains/Grid

4) Grid Tie with Battery Backup (example Outback GFX)
Sun -> PV Array -> Charge controller -> Battery Bank -> Grid-Tie Inverter -> AC Mains/Grid and AC Loads

Techluck would also be a separate type in my opinion.
Type 5) Techluck, Battery-Less
Sun -> PV Array -> DC to AC Converter Box (non 60Hz) -> Resistive Load
Adapter kit can be used or better idea is to use PV combiner.  But the combiner is more money.

Our DC Water heater system is Type 6:

Type 6)  Direct Power DC Loads, Battery-Less
Sun -> PV Array -> DC Relay -> Resistive Load

The main advantage to type 6 is:

No electronics to fail
No Heat sink to waste power
No Batteries
No Inverter
No Lightning protection needed
No Charge controller

This eliminates much of the upfront equipment cost and also the maintenance cost / time of that equipment.   No moving parts for PV or electric water heater, they go well together
Rules of thumb Here is the first rule of thumb:

Match element load in Ohms to approximate 40% more than Vmp divided by Imp. 

( Vmp and Imp are found on the spec sheets of the solar module) 
Simple answer: 

Start with 4500 Watt element.  Why? 
a) It comes with most heaters so you already have it
b) It has good power handling ability (at least 4500 watts!)
c) Have the option to work down one step (5500W with lower ohms) or up a few steps (3800, 3000W with higher ohms) in resistance IF NEED BE to match your designed PV array impedance.

 Element size (resistance) needs to have an approximate match to the PV array (Vmp/Imp) * 1.40 so there are a few ways to do this:

1) Jigger the number of panels in the PV array.  If you keep adding more and more individual panels per string you increase Vmp and therefore increase PV impedance.

2) Jigger the configuration (series/parallel) in the PV array.  If the PV target resistance is too high make more strings with what you have.  This will increase Imp and decrease the PV impedance.  

Example: 12 panels can be put into 6 different configurations:

one string of 12 panels
two strings of 6 panels
3 strings of 4 panels
4 strings of 3 panels
6 strings of 2 panels
12 strings of 1 panel. 

These different configuration give VERY different PV impedances.

3) Use a different element wattage (resistance).  Make sure that PV array wattage < element wattage.

4) If you haven't purchased the panels yet and your design isn't working out consider buying a different model of PV panel that has different Vmp and Imp specifications.

I have TWO favorites.

For a small system for a user that does not require a lot of on demand stand-by hot water the "Simultaneous DC water heater" will work well.
Its the one with the Grid powered upper tank,  PV powered lower tank.  The grid does not turn off the PV.  Relays are high powered DC-DC

-Uses one tank

-Limited grid-powered hot water on demand (upper part of tank)
-Somewhat limited pv-power storage capability (just lower part of tank)
-One relay only so some wasted power on cloudy days compared to two element model.
-Using NG or Propane to heat water on cloudy days not an option.

I like the option of adding timer so it just runs on PV during the day
For a large system or one that has space for two tanks the "Series wiring diagram" with TWO relays is my favorite.
There needs to be some way to show that this electric water heater is fed into a second NG, Propane or Grid Electric water heater. 

-Solar adaptive to have high efficiency during sunny and cloudy weather.
-Two water heaters means lots of gas or grid powered stand-by hot water in the times of bad solar.
-Separate water heaters means lots of PV hot water storage ability.

-Two tanks needed
-More complicated wiring
-Need to adjust photo eye so it switches system between high / low properly.

Only problem I see with the diagram is that both elements should be same wattage/resistance.  Don't need the wattage, Low watt, High watt labels.  System can be designed with any number of different elements according to PV size, etc, so doesn't need to be on the diagram.
There needs to be some way to show that this electric water heater is fed into a second NG, Propane or Grid Electric water heater.
     Ok will make drawing... page edit and this illustration are next
Jan 2015
Omron relay discussion:
Our target for this water heater is
250VDC * 18 amps = 4500 watts.   250VDC/18amps = 13.8 ohms
Conclusion: Relays should last many years of daily use/ 5 to 10 years
    Lightweight DC-DC relay: in stock $129

    Heavy-duty: 13 week delivery / in stock $286

    $129 DC relay is cheaper than spending $170 for 90 amp 3-pole AC contactor

And rated for DC so its way better.

    The full-solar water heater has two relays/
    Can you give rough estimate what you think lifespan of the relay will be?
    .... spec sheet shows minimum 30,000 operations at 250 VDC at 25 amp?

So 30,000 operations at 250VDC * 25 amps = 6250 watts.     250VDC/25amps = Impedance 10 ohms.

Starting to get in the ball park.  6250 watts of pv power is a little high to be dissipated from the 4500 watt element.
Perhaps if we use the example of 250VDC * 18 amps = 4500 watts.   250VDC/18amps = 13.8 ohms
would be a realistic large PV direct powered water heater example.

So I guessing we will get about 40,000 operations instead of only 30,000.  Not a big increase but whatever.

    .... can carry short term 40 amp for 10 minutes, 50 amp for 5 minutes
Yes.  Probably the max rating on the lugs of the dc relay.   Not a big deal for our use.
    .... our solar water heater might turn on-off 30 times per day on a party-cloudy day.
I disagree and agree.  The full model of our design uses two relays  One relay turns off the array when the
water heater reaches max temp.  On my solar powered water heater the on/of relay#1 may only operate typically maybe twice in a day.
It comes up to temp on a sunny day.  Turns off.  Maybe it bounces once more as water is used in the afternoon.
Lets assume 2 open operations per day.   40,000 / 2 / 365 = 54 years.
Relay#2 that controls the shift between high and low power may only change state once or twice (sunny day: low sun->high sun->low sun).
or many many many times on a cloudy day when clouds pass by.   This is where a delay timer comes in to prevent the flapping between high and low sun states.
Lets be less than optimistic and assume the average is 20 times a day.   5.4 years.

But!   The longevity of the second relay is helped out by one thing.

  In our design the second relay that controls the high / low solar state does an open operation when going from low solar state to high solar state.
In the "low solar state" both upper and lower elements are in series.  When they are in series in high sun the yield is not going to be very good because its not going to hit the power curve at the right spot.... Thus we want to shift to "high solar state" with only a single element in the circuit.  But what is the current and voltage flowing through the circuit?  Crappy! and because its crappy its electron flow is easier to break.  I would expect maybe a 2x longevity of the relay.   Take the 5.4 years and double that to 11 years.
Now it doesn't look so bad does it?

    .... will the solar water heater exceed 250 VDC at 25 amp?
No thats 6250 watts.  I don't think someone should build an array that big and try to dissipate the power into a 4500 watt element.

   .... how often will the solar water heater run at 30 amp instead 26 amp
I don't think the amps will go that high.  18 amps for a 4500 watts of PV array would be more typical.

    If the relays burn out once per year, then the cost of heating water is $260 per year.... 65% cost of using ordinary 240 volt electric
    If we use the heavy duty DC-DC relay, the cost to replace relays could be $600 per year.... 150% cost of using 240 volt electric

I dont think they will burn out nearly that fast.   I would still get the biggest relay I could afford because a relay is cheap compared to PV panels.   4500 watts of PV is $4500 give or take.

    Will the solar water heater fill tank with hot water if constrained by 25 amp 240 volt breaker?
I would derate the breaker by 0.80 because the load is considered continuous.   So 25amps will handle a  20 amps load.
I would also size the breaker up by a factor of 50% to handle the edge of cloud effect.  Then once the calculations are done take the next highest breaker size (round up).
Some of my panels put out 8 amps.  I have 15 amp breakers on those strings.   The wire is 12awg so everything is well protected by the 15amp breakers.

    Do you think the breaker will sufficiently protect the DC-DC relay from overclocking?
What do you mean by overclocking?
     Running more amps than rating on DC relay

     I'm trying to recommend a circuit breaker, based on what is available.
     I see 150 volt 20 amp ....

    If we recommend 150 volt 20 amp breaker....
    Will this heat enough hot water?

Depends on use and size of PV array.   Amps depend on Ohms of heating element.
Maximum size system would be about 4500 watts PV
4500 watts of PV on a sunny day will give about 4500 * 5 rule o thumb = 22.5KWh. 
Thats a lot of hot water.   Like 150 gallons or so  ....   ON A SUNNY DAY.
    What size tank do you estimate?
Depends on use and size of PV array.   Bigger = better as  long as the tank is cheap.
Storage is not wasted energy.
Even if I had a small 1500 watt array I would still buy the biggest tank I could afford.

    If we have 150 volt DC @ 20 amp max///
    How many 250 watt solar panels do you think?
    Still 2 strings with 7 panels each
Oh I see what you are saying.  Using a 4500 watt array with 4500 watt heater elements (12.8ohms) we will be higher than 150 volt rating on breaker.

Assuming 4500 watts PV with 4500 watt element.
12.8ohms * 1.4 = 18ohms target PV impedance
 285VDC/15.8A = 18ohms impedance
 285VDC * 15.8A = 4512 Watts PV
15.8A * 1.25 for continuous.  20 amp breaker is ok but .....

We need a breaker that can handle 285VDC.  Hmm  Maybe *gasp* fuses would be ok.
Damn.  I thought we had all the parts figured out :)

Are you trying to present and example to people?  What is the size of the array you are targeting.  Certainly not one size fits all.  I recommend we figure out parts for a large 4500 watt system and if user doesn't want something that big they just down scale the size of the array.  Everything else will still work.   Otherwise you will have to spec out small (1500) medium (3000) large (4500W) sized systems.
In our design the second relay that controls the high / low solar state does an open operation when going from high solar state to low solar state.
In the "high solar state" only the lower element is in series.  The upper element is bypassed by the upper relay.   When we want to shift to "low solar state" we open the upper DC relay to put the upper relay in series.  We only do this when the solar is low.   Maybe 300 mW/m2 or less.   This results in less current to break and less wear and tear on the DC relay.  I would expect maybe 3x the longevity of the upper relay or more.   Take the 5.4 years and triple that to 15 years.

Yes good point.
I will add that detail.
My estimate is still $4-5000 to build... and $260-$215 per year operation, repair and depreciation
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