To over Watt, or not

[Snowynorth]

Is there an upper limit to over paneling, watts, and amps?

I can buy 455 watt Longi, or 425 watt q cells, both 1/2 cut bifacial perc, at a reasonable price. The longi are less money. I'm going to install 12 panels for now 2s1p into one mppt on Deye7.6k. I only have 2 wire from the panels to inverter in the ground right now. Then expand next year with more home run wire, and another 12 to 18 panels, once I'm ready to tie in a shop, and 2 small kids/guest cabins. The scc on longi 455 is 11.65 x 2 =23.30a into 18a mppt. The q cell 10.78 x 2 =21.56a. Either way I'm over on watts and amps using 12, and under using 10. I think I'm good on volts with our cold winters. PV inpit voltage 100-500. (MPPT range is 125 to 425v. I'm assuming that's 125-425 per mppt) am I wrong? The PV input volatage and the MPPT range voltage have me confused somewhat. If 12 panels work, it boosts my voltage, and hopefully faster battery charging. I'm off grid.

 

[WYtreasure]

Yes there is a limit to how much overpaneling you may add and that info should be in your manual.

 

[time2roll]

20% to 50% should be fine. Although I would stick closer to 20%. Stay well under the controller Voc limit.

 

[BentleyJ]

The PV input voltage and the MPPT range voltage have me confused somewhat.

Typically a charge controller will have a minimum start-up (sometimes referred to as "wake up") voltage, an absolute Maximum voltage limit and an MPPT voltage range. In most cases the MPPT voltage range is narrower than the operating voltage limits of the charge controller. For example in your case lets say you decided to use 11 panels to keep the voltage below the 500V top limit but by doing so your array is operating at say 465V its above the MPPT range and its possible that the charger would be maximizing the panel output.
The upper voltage limit would apply to both MPPT inputs. However, the same is not necessarily true of the amperage. I've seen cases where each MPPT input is rated for say 20A but if both are used the unit total is limited to say 35Amps NOT 40A as may be expected.

 

[BentleyJ]

"Meant to say the charger would NOT be maximizing the panel output." if its operated above the MPPT voltage range.

 

[Snowynorth]

Thanks. It's starting to make sense to me. So the 2 mppt are not necessarily 2 charge controllers? Start up is 125v.

 

[Snowynorth]

"Meant to say the charger would NOT be maximizing the panel output." if its operated above the MPPT voltage range.

It gets flipping cold here -40 and lower. I did a panel calculator online. I'm staying well clear of max voltage

 

[BentleyJ]

Thanks. It's starting to make sense to me. So the 2 mppt are not necessarily 2 charge controllers? Start up is 125v.

I can't speak to the internal electronic architecture of all products. That said, each MPPT input most likely does go to a separate charge controller, its just that if both are being use together there is 2x heat and they both tie together at the battery + and - terminal so the total system current may be limited to a value less than the sum of each individual MPPT controller if one or the other were being used alone.

 

[Snowynorth]

The below sounds sketchy, but what do I know. Not much but more then I did when I started out, and I appreciate it.

One of the suppliers I contacted about panels said to make sure I have dc breakers and surge protection. This makes perfect sense we get lightning in the warmer months. I'm going to ground my ground mount too. He also said that I could run my 2 conductor 4 awg into a bus bar then run appropriate awg from the busbar to both mppt. He said it would be like wring my house panel, and it getting split up through the various breakers to plugs, lights etc.
Does this make sense? I think, he thinks I might buy more panels.

What I have for now is 4awg 2 conductor + ground direct bury already trenched in 250+ feet. I bought it and did it because the cable worked out to less then $1.00 per ft, and my buddy had a rented trencher right down the road. I'm going to expand panels for shop etc, with proper PV cable next year, but I want some power this year.

 

[WYtreasure]

It gets flipping cold here -40 and lower. I did a panel calculator online. I'm staying well clear of max voltage

That calculation might be worth repeating, a few times, while checking your work carefully.

 

[Bvillebob]

Frankly it's my opinion that the whole concept of "overpaneling" is a carryover from the days of cheap, dumb, charge controllers. It may still apply to the lower quality ones today, but modern quality controllers monitor their output current and limit it to what they can sustain, and the user can set the maximum output even lower. So long as the controller is smart enough to manage its output, I can't see any reason to limit the amount of panels attached to it.

In the real world, I live in the Pacific Northwest. In the winter the days are short and usually cloudy. I have a total of nearly 20 kw of panels feeding three Midnite Classic 150 controllers into a 24 volt battery system, 1800 ah at the moment, soon to be 2400 ah. At 6 kw output one of the arrays is capable of putting out over 200 amps at 24 volts, the max output of the controller is 96 amps. I manually limit the output to 85 amps just for more headroom and to keep the temps down. I have so many panels because I have many days in the winter when I generate a total of 2-3 kWh per day even with all those panels, they're obviously way more than I need in the summer, but panels are cheap these days.

The "worst case" scenario happened in reality last week, we had a week of clouds and the batteries were down to about 20 percent, then we got a couple of days of sun. That means the controllers all ran flat out for a couple of days refilling the batteries. The controller temperatures stayed well within the safe range, they didn't have any issues at all, and haven't in the last year or so they've been running.

Check your controller, so long as it's capable of measuring its output and limiting it, I can't see any reason to worry about adding panels and good reasons to do so in many cases.

 

[TorC]

Also, if you didn't already do so, look up your panel datasheet and use the exact temperature coefficient for calculating Voc. You're cold enough that a variation in that can make a noticeable difference. Where I am, I pretty much just take the label Voc and call it a day, but my record coldest is only ~50°F.

You can overpanel more with a lower voltage system. In Bvillebob's case, his controller is capable of handling the voltage and current required to feed ~90A± into 48V. That raises the controller input Isc high enough that it's very hard to hit with a 24V output. The Isc limit isn't quite as hard and normally dangerous as the Voc limit for exactly the reason he says, but if anything at all goes wrong you're outside what the controller can safely and reliably open the circuit on.

 

[WYtreasure]

Please continue with the discussion. I like it, and am learning.
This is what happens when you make a really stupid post and can't delete it. You have to make up something that is less stupid.

 

[wattmatters]

I can buy 455 watt Longi, or 425 watt q cells, both 1/2 cut bifacial perc, at a reasonable price. The longi are less money. I'm going to install 12 panels for now 2s1p into one mppt on Deye7.6k.

Do you mean 6S2P?

For -40°F (or -40°C) I would add 20% to Voc at STC. Voc for 455W 72 cell Longi LR4-72HPH is 49.5V, so add 20% for very cold ~= 59V.

6 x 59V = 354V

Check the rating spec for %Voc/°C. It's something like -0.27%/°C. STC is 25°C, which is 65° above -40°C, so -0.27%/°C * 65°C = -17.55%.

PV inpit voltage 100-500. (MPPT range is 125 to 425v. I'm assuming that's 125-425 per mppt)

If 500V is the upper limit per MPPT, then 500V / 59V = 8 panels max in series.

What is the MPPT input current limit for your inverter?

I've seen both 10A and 18A MPPT models. The former you should not parallel those Longis, with the latter you could.

 

[Snowynorth]

Do you mean 6S2P?

For -40°F (or -40°C) I would add 20% to Voc at STC. Voc for 455W 72 cell Longi LR4-72HPH is 49.5V, so add 20% for very cold ~= 59V.

6 x 59V = 354V

Check the rating spec for %Voc/°C. It's something like -0.27%/°C. STC is 25°C, which is 65° above -40°C, so -0.27%/°C * 65°C = -17.55%.


If 500V is the upper limit per MPPT, then 500V / 59V = 8 panels max in series.

What is the MPPT input current limit for your inverter?

I've seen both 10A and 18A MPPT models. The former you should not parallel those Longis, with the latter you could.

Deye/GSL 7.6k 2 mppt 18 +18 amp, Yes I meant 6S2P. I have a little side solar 12 volt battery charger project. That's why I mistakenly wrote 2s1p. It appears with Longi 455w I would be over watts by 520, under nicely on voltage, and over on amps @ 23.2 for 18a mppt =5.20 over. You think this is acceptable?

 

[Snowynorth]

Yes there is a limit to how much overpaneling you may add and that info should be in your manual.

Unfortunately there is nothing in the manual that refers to over paneling.

 

[Bud Martin]

The SCC pulls the power from panel as needed up to the maximum power the SCC can provide to charge the batteries, the panels do not push power into the SCC.
I.E. you US AC outlet can supply up to 1800W of power (120V x 15A), so if you plug in 100W lamp, the lamp will only draw 100W from the outlet.
Max PV input Voltage is the one you need to watch out for.

 

[TorC]

I think it's unwise to exceed the Isc of the charge controller, as the MPPT sweep may pass through that range. Probably not immediately destructive like exceeding Voc, which is likely to fry the SCC, but still a risky business.

 

[Hedges]

The below sounds sketchy, but what do I know. Not much but more then I did when I started out, and I appreciate it.

One of the suppliers ... also said that I could run my 2 conductor 4 awg into a bus bar then run appropriate awg from the busbar to both mppt. He said it would be like wring my house panel, and it getting split up through the various breakers to plugs, lights etc.
Does this make sense? I think, he thinks I might buy more panels.

I don't think two separate MPPT of two separate SCC (or inverter) will ever tolerate sharing one PV circuit, like both wired to a single busbar.
Some devices have multiple MPPT inputs and allow you to parallel them.

Page 46 of the following manual, Sunny Boy -41 model 5.0kW through 7.7kW have three MPPT inputs A, B, C. Inputs A and B can be connected in parallel (which might accommodate current of 3 PV strings, while each by itself could only handle 1, not 2.)

https://files.sma.de/downloads/SBxx-1SP-US-41-IA-xx_12.pdf

If the following is manual for your Deye 7.6k, I don't spot a mention of that in the PV section.
I think the_colorist is familiar with the brand; you could ask him.

I prefer multiple runs of wire for multiple PV strings. Smaller gauge easier to pull, and I can rearrange between multiple inverters.

https://www.deyeinverter.com/deyeinverter/2021/12/06/%E3%80%90b%E3%80%9130240301000366_502012859-%E4%BD%BF%E7%94%A8%E8%AF%B4%E6%98%8E%E4%B9%A6-%E5%82%A8%E8%83%BD%E5%8D%95%E7%9B%B8-sun-5-8k-sg01lp1-us-eu-ver2.2-%E5%BE%B7%E4%B8%9A%E8%8B%B1%E6%96%87-%E5%B0%81%E9%9D%A2%E5%BD%A9%E5%8D%B0-%E5%B0%81%E9%9D%A2%E5%B0%81%E5%BA%95157g-%E5%86%85%E9%A1%B570g%E9%93%9C%E7%89%88%E7%BA%B8-142.5x210mm-%E6%9C%80%E6%96%B0.pdf

 

[Snowynorth]

The SCC pulls the power from panel as needed up to the maximum power the SCC can provide to charge the batteries, the panels do not push power into the SCC.
I.E. you US AC outlet can supply up to 1800W of power (120V x 15A), so if you plug in 100W lamp, the lamp will only draw 100W from the outlet.
Max PV input Voltage is the one you need to watch out for.

I will definitely, stay well under the input voltage at even below our worst cold. I've calculated it numerous times, considering our crazy cold snaps.