Can you recall why??
This is a low voltage element as well as being a PTC device.
I would have had to buy the specialized PTC element (I think it was not too cheap) and then have the effort to replace the existing element. With either of the Elons one uses the existing element. For me to improve efficiency I could replace the current 3kW element with a 4kW one but the current setup, at the moment, gets up to temperature so only would be useful in winter. A benefit I am finding with the Elon vs evacuated tube is that I have no possibility of over- heating of water when none is being drawn from the evacuated tube geyser. If none was drawn from that geyser the water would come close to boiling pointā¦ Quite dangerous as I did not have an anti-scald or mixing valve installed.
How does your setup perform in winter?
An MPPT can add value, but there might be other cheaper ways to do the same thing.
Let me explain. The job of the MPPT is to match the impedance of the panels to the battery (or the loads). For a battery this is easy to see: The impedance of the battery is much lower than the panels need to work optimally, which pulls the working voltage down to battery voltage causing the panels to lose as much as a third of their power capacity.
With a resistive element the resistance is practically constant (not really, it does vary a little bit with temperature, but close enough for all practical purposes). So you can get a lot of the same benefit by simply matching the panels yourself.
If you take (for example) the common 3kW element, which by the formula P = VI means it draws 13A, then simply picking a panel that makes 13 amps around its maximum power point is probably good enough. Then you simply put enough panels in series to get the open circuit voltage high enough (above 230V) and you are done. An MPPT will add no extra performance in this scenario.
Where an MPPT might help:
- You have more than 3kW of panels on, because you want to compensate for lower insolation levels in winter. Then you need something to limit the power a bit.
- You have panels that arenāt well matched to the element. Then if you can get the voltage higher than 230VDC, a buck converter can swap some volts for amps and get more power out, or a boost converter can boost the voltage a bit to make use of higher amps.
The only question then remains: Which costs less? It is probably a lot cheaper to buy a slightly smaller, but better matched panel, then to buy an MPPT.
Those ceramic PTC things also concern me a bit. As far as I know their resistance is not constant. So it is unclear to me how the two will interact without an MPPT.
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Interesting question. One thing to keep in mind, PV production in Gauteng is normally much better than Summer production. Cape town is the other way around. We tend to ask, how does your solar system perform in the winter, and actually it all depends on the area you stay in. If your panels were installed at the correct fixed angle, the rainy season will result in the worst PV production overall.
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In the dead of winter the temperatures got to about 55 deg C which is not bad at all considering the colder temperatures of the water supply and the fewer hours of sun.
Quite correct, summer to winter solar yields are similar in Gauteng whereas the fair Cape has a greater summer production compared to winter.
Very interesting point thanks.
I know many people underestimate the amount of overcast days we have during the rain season in Gauteng and the impact on PV production.
I read somewhere a while back, February is apparently the worst.
Old but useful - How winter affects solar output in every province in SA | News24
Another question, @plonkster mentioned you have to put enough panels in series to try match the VOC to the 230v AC.
@JacoDeJongh You installed 4 panels for your client, @Greenman you said you installed yours with 3 panels.
Not 3 or 4 panels will get the VOC close to the 230v AC or am I missing something?
Does it also depend on the power rating of your element?
Correct. I have 4 x 360W panels in 2S2P config and only get 80V under load.
And how is yours performing, wonāt you be better off connecting the 4 panels in a single series string?
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What size element do you have in the tank? Is it the original resistive element, or did you replace it with a low voltage PTC element? My argument is specifically for cases where you keep the original resistive element.
Since V=IR, and R is constant for such an element, the voltage it will operate at depends entirely on how much current it can push. If you match the current of the panel with the operating current of the element, and you have enough panels in series to get V high enough, it should automatically level out at the voltage given by this formula.
Current production of PV modules is proportional to the insolation, so on days with less sun, I will be lower and therefore it will also settle at a lower voltage.
The ideal is to get the voltage to be quite close to 230V (again, for the original resistive element), but the way you do that is to match the current, which for a 3kW element is around 13A.
Simpleā¦ they donāt. They shunt the current straight into the resistive element, and if you match it right, the voltage rises to whatever V=IR mandates.
Good question!
The reason I couldnāt connect the panels in series was because the MPPT couldnāt handle the voltage.
Yes. The PTC element looks like a single element but actually has 2 elements: one for 220V mains and a low resisistance one for the PV panels.
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Yes, for the Elons, 4x panels would obviously give more power and if you are in Cape Town that would probably be necessary due to the lower solar irradiation in winter. In my case, our hot water usage is moderate so the 3x panels is sufficient for our needs. The favorable price of panels also makes it attractive to install 4x panels.
The Elon Smart can handle max of 230V Voc, 15A.
The Elon 100 can handle max of 240V Voc, 20A.
So, you will need to work out your own series/parallel arrangement for the panels that you want to use.
One benefit of the dual element is that you can heat the water with DC simultaneously with AC from your grid tied inverter or from a hybrid such as Sunsynk/Deye.
The Elons use either DC or AC but not simultaneously.
OK! Well, 80V sounds about right as well. A 72-cell panel of around the 300W to 500W size should have a Vmp of around 36V per panel, and open circuit around 43V. Since you have a 2S setup, I would expect voltages between 72V and 86V.
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I presume you have the Elon 100?
What does it look like. (Power Optimal website is 
)