Have panels, need controller help

[Sold]

I have been preparing to erect a mobile PV system on a cargo conversion to toy hauler/camper I’m building. I was leaning toward 4ea 210w Newpowa panels but I got a killer deal from HD yesterday on 8ea 165w Nature Power panels for about .71/watt. I just haven’t been able to sort out truth from fiction on sizing the controller yet. Using Midnite Solars’ calculator I can easily arrange a 4s2p array in 24v and use a Epever Tracer mppt 150v 40a Controller. But I also see people throwing out the w/v=a (in my case 1320w/ 24v = 55a) as if that is the bottom line in my SCC choice, calcs be damned. I honestly don’t know. This may be part of the overpaneling discussion I haven’t entered into yet. Still busy trying to learn the basics! At any rate, my batteries are on a slow boat so I have plenty of time to figure it out. BTW they are (will be) 8s 280ah with a Daly 150a BMS. I’m leaning toward the 24v Giandel psw 2000 or 3000 inverter.
Applicable specs that I input to various calculators are imp=8.72a isc=9.41a Voc=22.31v Vmp=18.94v. Thanks,

 

[Rednecktek]

Well, you've got 1320 perfect world watts of panel and a 24v system, so the most your controller could use would be 55a, so round that to 60a. That seems to be a sweet spot in SCC's anyways and will have some overhead available for overpaneling.

But I also see people throwing out the w/v=a (in my case 1320w/ 24v = 55a) as if that is the bottom line in my SCC choice, calcs be damned.

There's a reason for that too. Yes, you could totally use a 40a but that's only 960w, so in the event that you got some place with really good sun and your panels were just working like crazy, anything between 960w and the 1320w of panel is just clipped and gone. Is the cost savings of using a 40a vs a 60a worth the 30% loss in capability of your panels? Is running your SCC at 100% of it's capacity going to do any damage to it?

Going to the 60a will gain you the ability to use 100% of your panels capacity, will give you head space so you're not running at 100% of your SCC's capability, and isn't going to cost that much more than a 40a model.

Just my thoughts, but why skimp out and nerf your panels?

 

[Sold]

Ok, thanks for explaining. I didn’t realize I was leaving power on the roof. Why is a calculator even necessary if everyone can just use ohms law? I definitely want to buy the right equipment to utilize my panels to their fullest, and I don’t want to overspend. But I do plan on making my rack tiltable so that I can maximize my Solar potential, so I will get a 60. BTW, what is overpaneling exactly?

 

[Rednecktek]

Why is a calculator even necessary if everyone can just use ohms law?

Because a lot of people think Ohm's Law is some sort of meditation technique? Ooohhhmmm... Oooohhhmmmm... Oooohhhhmmm..

BTW, what is overpaneling exactly?

Over-paneling is intentionally putting more panels on a SCC than it can actually use based on things like which way your panels face and such. Just to make up an example, your system can use up to 1300w in a perfect world. If you were to put up say an 800w string facing east and an 800w string facing west you'd be "over paneled" because you're putting 1600w on a 1400w controller. Because the sun would only be hitting 1 string at a time mostly (yes, there may be some clipping at mid-day) you'll get more overall generation because your panels will start earlier and end later.

The other type of over-paneling that happens is when you put 1600w of panel on a 1400w SCC assuming that because of your location or panel angle or whatever you'll never actually SEE 1600w so you're trying to compensate for that by putting in more panel and getting as close to that SCC's maximum as possible.

Did that make sense?

 

[rmaddy]

Why is a calculator even necessary if everyone can just use ohms law?

There's more than just the charge current calculation. You also need to make sure the temperature adjusted Voc of your panel arrangement will never exceed the max PV input voltage of the chosen charge controller. Many charge controllers also limit the input amps which is of course affected by the panel arrangement.

 

[Sold]

Yep, that makes sense Red. I picked up that there were other considerations like extreme temps, but I was overthinking it. And some of those calculators take the wrong road home. Anyway, I’m squared away for now. Thanks for the help.

 

[Rednecktek]

Woot! Glad we could help.

 

[Kornbread]

I will always have some shading, so overpaneling is not as much of an issue as one would suspect.

Building a small system for dad's camper and was talking to him about positioning the panels, near his camper, that would provide max sunlight, and wanted to be sure he realized how important it was to avoid as much shade as possible. Two 260w panels, in parallel, 30amp mppt controller. Dad watched the charge controller as I put my hat over one of the panels covering maybe a ~cell or three. The amperage from the charger to the battery dropped considerably, and depending on which direction the cells were covered, also made a difference. This system is overpaneled by ~20%, but due to there always being some shading, the controller rarely clips or sees 30amps.