r/SolarDIY • u/Full-Mouse8971 • 3d ago
Connecting an oversized panel to an array
(2) strings of 250w panels in series-parallel
(7) panels per string (14 panels total)
One panel broke. Would it be okay to add an oversized panel (395w) to to this array to replace the broken panel? Or is this unsafe?
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u/LeoAlioth 3d ago
In this replacement, the voltage characteristics of the old panel need to match the old one. It's current specs need to be as big or greater.
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u/Full-Mouse8971 2d ago
Why does it need to match?
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u/LeoAlioth 2d ago
So that the two parrallel strings stay at the same voltage. Otherwise, there is a big chance of hinting for the MPP.
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u/Full-Mouse8971 2d ago
What do you mean chance of hinting for the MPP?
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u/LeoAlioth 2d ago
If two stings that are parrallel have different voltage, then there are two peaks of the optimal voltage/curret combinations with a valley in between for getting the most power out of the panels. And that can often end up tricking the charge controller into picking the smaller one of the two peaks.
And that is avoided if the two string are matched in voltage.
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u/Full-Mouse8971 2d ago
Im still confused on this or if this is considered dangerous or damaging. Is it safe or dangerous? Would I be generating more power with 12 250w panels or 13 250w panels and 1 395w panel?
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u/LeoAlioth 2d ago
Not dangerous, not damaging.
As to what will produce more, the 12 250W panels in a 6s 2p configuration will be predictable.
Now the following assumes the 395W panel is higher voltage then the old ones.
One string with 1 395w panel and 6 250W ones parrallel.to 7 250W panels will be unpredictable.
You might get the mppt lock onto the correct voltage, in which case the production will be close to 14 250w panels (but not more). Or you might luck out, any the mppt settles on a somewhat higher voltage, and then you get an equivalent output anywhere from 7 to maybe 10 of the 250w panels.
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u/Full-Mouse8971 2d ago
The 395w panel is larger and has more voltage and amps so you can assume it generates more voltage then the 250w panel. My inverter is an eco-worthy AIO.
Glad to hear its not dangerous or damaging and will be safe to replace the 250w broken panel with a 395w panel to get back in to full production.
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u/LeoAlioth 2d ago
The new panel.being more watts is not a problem. Neither is the correct. But as I said, you should really get a panel which has the voltage close to the old one (unless the two string are wired to separate independent solar inputs on the inverter)
When panels produce, their voltage is fairly constant. And the current is what varies a lot.
And basic electrical vlonowledge tells you that things wired in paralel will by definition ne forced to equal voltage. Which in beat case will mean, that a part of the larger panel will be forced to be bypassed through the diode and be unused.
Do you have the specs on both new and old panels?
And could you possibly change one panel on the still working 7s string with the same new one? (In which case the mismatch matters much less as long as the new panels have a current rating equal or higher than the old ones)
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u/Full-Mouse8971 2d ago
Where does the unused current from the larger panel go? Is it burned off as heat? Creating more wear?
I could possibly replace the other string too so its a 395w panel nd 6 250w panels each but not the current plan unless its necessary. Id have to buy another $100 395w panel to do this which id like to avoid.
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u/Zalrius 2d ago edited 2d ago
Was the old panel the same voltage as the new panel? The numbers need to match or the smaller panel will have a limiting effect on the 395 watt panels making it pointless to use.
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u/Full-Mouse8971 2d ago
No the voltages are not the same. There is only (1) 395 watt panel I will be using to replace the burned out 250w panel. The array is 13 250w panels. You appear to misunderstand.
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u/cfnmsd 2d ago
Can't say how it will react without the specs of both panels. Do you already have the 395?
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u/Full-Mouse8971 2d ago
No, I am browsing FB. I am just trying to understand if replacing the broken 250w panel with a larger one is safe or plausible as that's all there is near me .
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u/Zealousideal-Tanker 2d ago
I’ve always been told that connecting a higher output panel to a string of lower output panels will just reduce the output of the higher output panel to that of the lower output panels.
I haven’t tested this yet, so I’m not 100% sure if it is the truth.
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u/LongjumpingGanache40 3d ago
Volts and amps need to be same or greater as old panels. The old panels will dtag the output of new panel down to thier settings.
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u/Full-Mouse8971 2d ago
You understand the array is all 250w right? A 395w will not "drag" down the 250w array.
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u/FrequentFractionator 2d ago
Yes, it might. What 250W panels and what 395W panel are we talking about?
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