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Solar power curves for water heater
solar panel array "Information applies to all PV panels that I know about.  Doesn't matter if they are mono-crystalline, poly-crystalline, or thin film." 
"Designer needs to make sure they have the Imp and Vmp specs for the panel in question so they can design the array and water heater element ohms to maximize output."

Images and content provided by and approved by author, and friend, NetworkNerdvana

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1) Maximizing solar-yield in ideal conditions
Check your PV modules Vmp and Amp specifications to calculate resistance required for maximum power point resistance to maximize power output.
Maximum power point resistance in ohms equals Vmp/Amp
In order to obtain 100% power output in ideal conditions this PV module must be loaded down with 7.1 Ohm load. 
Notice how there is actually a wide range of resistance (5.22 ohms to 10 ohms) where the module produces 90% to 100% of its rated output.

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2) Ideal solar conditions almost never happen
You will almost never see 100% ideal solar conditions because most of the time the sun hits the PV module at an angle. 
On a nice sunny day most of your summer harvest will occur at 60% to 90% of maximum solar.
Ideal resistance is great for that instant of ideal solar conditions and produces 100% output.
But look at what happens when that same resistance hits the two lower solar output curves. The output falls to 95% then 75%. This results in lost solar yield when measured on a daily basis.

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3) PV magic water heater formula:
Element ohms equal (Vmp/Imp) x 1.40

Match element load in ohms to approximately 40% more than Vmp/Imp.
Using a slightly higher resistance than the Max Power Point resistance for the element load will increase solar-year-old on a daily basis. 
The higher resistance will provide a better match for a longer time during a sunny day. 
This will boost power output when the sun is coming in at an angle, which is most of the time.

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4) Single element resistance means efficiency suffers on cloudy days
The magic formula for matching element resistance to panel (Vmp/Imp) x 1.4 works well for maximizing power on sunny days. 
However when clouds block the sun the efficiency will began to suffer and when combined with reduced solar output these affects multiply to result in minimal power harvest during cloudy days.
If the system is limited to a single element not much can be done to optimize further. Perhaps it isn't such a big deal since there may not be much solar to harvest on really cloudy days.

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5) Dual element system allows harvesting solar on cloudy days, mornings and evenings
Using two heater elements of the same wattage in series doubles the resistance seen by the PV array.
Solar adaptability can be obtained. 
If there is a mechanism that can sense low versus high so were conditions The system can shift gears between a single element (10 ohms in this example) during hi solar and two elements in series (20 ohms) during low solar.
Shows what happens over the course of a sunny day
Shows what happens over the course of a sunny day
Convert AC water heater to DC 11) How to convert AC water heater to DC
Using the power curves on this page as scientific basis, Network designed a solar water heater using DC power without needing to convert the power AC.
So now you can connect ordinary residential electric water heater to solar panels (with some alterations of course) and get full heating.
My role was illustrator, along with my general knowledge of AC water heaters, we put together the DC Solar water heater designs that show on following link:
Convert AC water heater to DC

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