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Mixing solar panel strings


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Hi guys. I know you must not mix solar panels.But what about 2 different strings?  1 string of 5 panels (37 vmp and 8.8 amp) and another stting of 3 panels ( 54.6 vmp and 6.5 amp)

Edited by vulgrim
Wrong spelling
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20 minutes ago, vulgrim said:

1 string of 5 panels (37 vmp and 8.8 amp) and another stting of 3 panels ( 54.6 vmp and 6.5 amp)

Presumably, you are asking if it's OK to parallel these two strings.

This is acceptable IF the total Vmp of the two strings is fairly close. I've never heard of panels with a Vmp of 54.6 V, but let's say they exist. These two strings add to 185 V and 164 V respectively, a difference of some 11%. I have no direct experience with this, but it seems a little too different for me. If they were say 5% different, no problem. 11% would probably be OK, only pushing one of the strings (hopefully the weaker one, which may change depending on the time of day) only a short distance away from its maximum power point.

The different maximum power current (Imp) points (8.8 A versus 6.5 A) are not a problem, provided that the strings are suitably fused, as they always should be.

BTW, a quick search turned up panels with a Voc of 54.6 V (88 cells); of course Voc and Vmp (open circuit voltage and maximum power point voltage) are quite different things.

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My first thinking is NO

No matter how you match disparate panels you will always end up with lost of losses that are normally not worth it. That said, you can get close, but you have to understand that there will be losses, and it will never add up as before. Ideally you should replace all panels at the same time. Financially we know you can't, so read below:

Here is a generic (based on an idiot who financially can't afford to replace all panels AKA me):

  • Know the Voltage from your panels (real, not just the Vm displayed)
  • Know the physical size of your panels (sounds wierd, but but many of the calcs are based on m2, so if a panel is a different size then the Voltages and Amps may not be accurate for your use)
  • Work in percentages difference overall - It is too easy to say that a panel is only 2V difference, but also understand the current difference... If the current is higher than a previous panel, then that panel may heat up and provide less current.
  • Now let me give you a horribly rough example:
    • 6 x Panels with rating of 38.4V and 7A in Series (approx 260W in brochure) = 1450W real Max input
    • Add 6 x Panels with 300W in brochure, but with differences, 51.1V and 5.8A = in theory - 1778W added
    • Reality works out that instead of 3228W expected, you will most likely end up with closer to 1558W - Only a few 100W difference. There is a huge technical explanation and ways to improve this calc, but it is almost impossible to discuss here. Understand that I have taken very worst case based on the the imaginative values I provided, I am just trying to prove a point.
  • The closer you can get the Voltage values for additional panels is the first prize, but you have to understand that the current values are also relevant. Mixing panels is not a good idea in many situations unless you completely understand all sorts of electronic and electrical values that impact. But more importantly, understand that you will get the worst of the worst from each set. Don't do it unless you are damn sure that they are as close as possible, and do not expect full service as they will be electronically fighting with each other.
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8 hours ago, KLEVA said:
  • Now let me give you a horribly rough example:
    • 6 x Panels with rating of 38.4V and 7A in Series (approx 260W in brochure) = 1450W real Max input
    • Add 6 x Panels with 300W in brochure, but with differences, 51.1V and 5.8A = in theory - 1778W added
    • Reality works out that instead of 3228W expected, you will most likely end up with closer to 1558W - Only a few 100W difference. There is a huge technical explanation and ways to improve this calc, but it is almost impossible to discuss here. Understand that I have taken very worst case based on the the imaginative values I provided, I am just trying to prove a point.

Just reread your question - apologies, but most of what I discussed was about putting panels in Series together, not 2 strings in parallel (and for some reason I couldn't edit my post). Coulomb's response above is more accurate for your scenario.

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