Connecting 39 volt panels to create 24 volt system

[Glennman]

I've heard that a person needs a 20 volt difference (minimum) between the output of the panels and the system. For example, I have 8, 39 volt (305 watt) panels and I wish to create a 24 volt system (24 volt inverter). Will 39 volt panels be enough voltage to power a 24 volt inverter system? If the voltages are too close, would there be any advantage to connect the panels in series/pairs (78 volts)? I was thinking if that was too much of a spread, then I could probably install a 48 volt system with 78 volts of solar. Any ideas on this? Thank you in advance! Glennman

 

[rmaddy]

If you use an MPPT solar charge controller then you only need the Voc of your panel arrangement to be 1V-5V above system voltage. But more is better. A few panels in series makes this trivial. Just don't exceed the max input voltage of your SCC. I have 3 panels in series that result in 120V and I have a 24V system. It works great.

 

[Bud Martin]

OP: What kind and the spec of the SCC?

 

[12VoltInstalls]

would there be any advantage to connect the panels in series/pairs (78 volts)? I was thinking if that was too much of a spread, then I could probably install a 48 volt system with 78 volts of solar. Any ideas on this?

I agree with this:

more is better. A few panels in series makes this trivial. Just don't exceed the max input voltage of your SCC. I have 3 panels in series that result in 120V and I have a 24V system. It works great.

Am mppt charge controller will output 12V or 24V charging profiles appropriate to the batteries you have. No need to go 48V just to run 78V open circuit.
One of the advantages to running panels in series (for 78V in your case) is that earlier or later in the day - or cloudy periods- when single panels may not make substantial voltage enough to give the SCC ability to make charging amps you can get some charging that you otherwise would not.
This effect is probably more noticeable in higher latitudes; it will work everywhere but the sun angle in northern regions is less optimal. My static panel voltage is 88V with panels in series - it made a noticeable difference for my situation.

 

[Glennman]

Thanks 12VoltInstalls. Sounds like I can go with a 24V system then with 78V open circuit (4 groups of panels wired in series of 2 each, 8 panels total). It makes a lot of sense what you say about getting some extra charging. Much appreciated. One person I spoke with suggested going with one additional panel (9 total) wired 3 in series, 3 parallel to achieve 117 volts. If I was to do that, would there still be a good efficiency with a 24V system? Of course, my MPPT SCC would need to be the correct one (whichever that would be).

 

[Brassburnz]

I have a 24v system (SCC 100v/40amps), four 12v batteries 2S2P, and four 37.8v VOC panels, 2S2P. Before the panels get full sun, they may register 28v and produce 5-10w. Today in full sun I saw something like 60v and 420w.

 

[pollenface]

I have 3s panels to make 66voc and a 100v mppt

Epever mppts require 3v higher than battery voltage. Victron require 5v.

 

[12VoltInstalls]

wired 3 in series, 3 parallel to achieve 117 volts. If I was to do that, would there still be a good efficiency with a 24V system?

Yes. The general rule is the higher the panel volts (within the boundaries of max voltage adjusted for temperature) the better.
I am of the opinion that facing SE and SW offers some advantages (in my situation and latitude) so if I were you I’d face the three series strings SE, S, and SW. You won’t harvest as much at midday as a single facing array might, but you’ll be closer to the max harvest earlier and later. If your panels are ground mounted you can experiment like that- I did and it worked good for me.