Recommended Solar Panels

[markrw]

You can produce more than the max of the panel

The panel will have a temperature coefficients (TCs), most often in terms of a percentage per degree Celsius (something like TC Voc = -0.35%/°C). This means that for every degree change in temperature of the panel, the Voc will change in the opposite direction by 0.35%. The Voc is normally measures and quoted at a panel temperature of 25 celsius. Therefore if there is a cold wind blowing, by example, but there are lots of sun, you can and will get a higher PV voltage than the quoted values

So I could get higher than the vmp of the panel?
So if my inverter operates at 30 to 80vdc and has max open circuit of 102vdc and I used 2 panels in series of 37.62 vmp and voc 46.24 would I be in danger of going over the max on cold sunny days?

 

[Koosvanwyk]

So I could get higher than the vmp of the panel?
So if my inverter operates at 30 to 80vdc and has max open circuit of 102vdc and I used 2 panels in series of 37.62 vmp and voc 46.24 would I be in danger of going over the max on cold sunny days?

Yes, that is the theory. Generally inverters can handle Amps that are to high by clipping it, but overvoltage would usually damage the MPPT / inverter. Their might be a safety marging already built into the max VOC of 102V.

 

[markrw]

Yes, that is the theory. Generally inverters can handle Amps that are to high by clipping it, but overvoltage would usually damage the MPPT / inverter. Their might be a safety marging already built into the max VOC of 102V.

I did not know that. So am I cutting it too close with the panels I had looked at? Would I be better off running 420w panels in parallel that have 40v vmp and 47v voc?

 

[TheChamp]

I did not know that. So am I cutting it too close with the panels I had looked at? Would I be better off running 420w panels in parallel that have 40v vmp and 47v voc?

No, you need to be well clear of the minimum Vmp, you are barely cutting it at 47V. I think you are worried about nothing but if you want to be at ease, get smaller panels than the 310W and still connect them in series, it will move you away from the Voc.

 

[RonSwanson]

So is highest voltage with lowest amp always preferable from an efficiency point of view ignoring the cost of the cabling?

the max amp is 8.78 so with 4 panels I would be under the max of 40A of my inverter.

When dealing with charge controllers, you need to be far more concerned with voltage than amps. Many charge controllers will reduce the amps because it's easy to do, but voltage is more complex to reduce. I am not saying that your charge controller is capable of doing this though, just something to bear in mind.
The optimal (most efficient) voltage for your charge controller is between 30 and 80V DC. It makes sense to use a higher voltage (like 75V) rather than a lower voltage (like 37V) because 1) there is less voltage drop at higher voltages and 2) you need to consider startup voltage in the early morning / late afternoon, as well as on cloudy days.

So I could get higher than the vmp of the panel?

Yes. I have seen evidence of an array rated at 5.88kWp pushing 7.79kW, with a cool coastal breeze blowing across it.

I guess that you missed this:

Always size the max voltage at VoC, remember that the specs are usually given at STC which is 25 degrees, and temperature drop will increase voltage.

I did not know that. So am I cutting it too close with the panels I had looked at? Would I be better off running 420w panels in parallel that have 40v vmp and 47v voc?

I am not sure, because the panel you linked to does not have a proper spec sheet. Most panels come with a spec sheet, like this:



Looking at the column for a 535W Longi panel, you will see that the Voc voltage is 49.35 volts. You will also see that the Temperature Coefficient of Voc is -0.27% (at STC). This means that for every degree below STC, the panel will produce 0.134232 volts more. If you live in a area with cold mornings / evenings, or with cooling coastal breezes, this could be about, say 20 degrees. That's about 2.55 volt more per panel, and if you have a string of 8 of them in series, 20.4 volts. Also be careful, because some panel specs give the temp coefficient at NOCT (45 degrees), not STC.

A good step by step string sizing guide is here.

 

[Speedster]

Sorry for my phone finger drawings , purplee blobs fuses

I would run ea string +- to near inverter and fuse ea end with a fuse
You get combiner boxes for this purpose, not hard to make your own tho

Can see internals in link

Takealot

View attachment 1282898

What cabling would one use inside the combiner box. Would "normal" AC cable work or would it need to be solar cable? And what size is needed for say 300V, 13A strings?

 

[TheChamp]

What cabling would one use inside the combiner box. Would "normal" AC cable work or would it need to be solar cable? And what size is needed for say 300V, 13A strings?

Normal AC cable will work, it's very short wires so the effect should be very minimal.

I would prefer using AC cables because I can bend and route them nicely because of the inflexibility, most of the ones you buy do use AC cables anyway.

 

[thehuman]

Normal AC cable will work, it's very short wires so the effect should be very minimal.

I would prefer using AC cables because I can bend and route them nicely because of the inflexibility, most of the ones you buy do use AC cables anyway.

Copper busbar linking fuses will look really neat too