Adding more panels to my existing system

[opfor]

Greetings folks,
Looking for some answers on adding more panels to a system I put together about seven years ago. This was my first solar project and built this for a small off grid cabin in the hills of East Tn. I only went 12v since this was my first build and here are my components. I'm using an Outback 80amp Flex max, purchased four 190 watt grape solar panels, 36.2V and 45.4Voc. I could not put all four panels in series as it would have been over the 150V max of the Outback so I put them in two series of two panels each, series/parallel. I have a total of 760W of panels and for a 12V system the max is 1000w, lucky to see about 500w being produced on average day being what it is here in E Tn. The panels run to a breaker box on the outside of the mount and so each series of panels has its own 20a breaker and I also have a 3rd slot and this is where I'm wanting to add two more panels and thus where my questions begin.

1. I've always heard that when adding additional panels you need to try to match within about 10% of the existing panels voltage, is this still the best rule of thumb?

2. I've read where it is very common to oversize a system for the very reasons I mentioned above, because you will hardly ever make 100% (especially in my lat/long). My question is how much can I go over and be safe? I have heard folks say that a system is only going to take in the amount of amps that it can use and so not to worry about this side as long as you keep your volts below your max. Would you agree?

I ask these questions because I have found 2 used panels that are extremely close on the volt side 37.3v, 46.8 Voc. They are poly whereas my original are mono, guessing this does not matter as volts are volts regardless or is it not a good idea to mix and max poly with mono panels. The panels are much larger in watts than I need being 335w ea which would make my total system 1430. And before anyone says I need to go to a 24v system I am aware and would agree however lets just say its not in the cards now as I'm not looking to invest in a new inverter and my current battery config will not allow with only three 12v 200AH ea batteries (LifeP04).

I have learned a ton from this my first project and would do things a lot differently now knowing what I know but just trying to see if I can add those two extra panels and be ok? The panels are 2 years old and a decent price at $275. My goal is just to hit that 1000w max input while I'm at that location.

TIA

 

[Mattb4]

So 4 panels original that are 2S2P giving a Voc of 90.8vDC, Vmp of 72.4vDC (total watts 760)
Want to add 2 panels to make a 2S3P setup. The 3rd string of the new (used ) would be 97.6Voc and 75.6Vmp (total watts 670).

Doing this will reduce your 3rd strings voltage to match the voltage of strings 1 and 2. The lower voltage will impact your total watts a small fraction (V X Imp=W or 72.4v X 8.86a = ~642w). Your total potential max wattage would be ~1400w. However as you note you are likely only going to see 75% or 1050w. I see no problems doing this.

 

[opfor]

This is encouraging to hear since if I understand it correctly my 80A Outback Flex maxes out at 1000w anyway since it is a 12V system. Someone mentioned that on those rare cold days where it could potentially make the max amount, say 1400w the controller has to get rid of the extra ( I guess in the form of heat inside the controller). I'm just concerned about burning this unit up, at the time the most expensive part of my system, think these sold for around $700 new back in 2016.

 

[opfor]

72.4v X 8.86a = ~642w
Curious how you came up with the 8.86 for Amps. Here are the specs from the two panels

 

[Mattb4]

This is encouraging to hear since if I understand it correctly my 80A Outback Flex maxes out at 1000w anyway since it is a 12V system. Someone mentioned that on those rare cold days where it could potentially make the max amount, say 1400w the controller has to get rid of the extra ( I guess in the form of heat inside the controller). I'm just concerned about burning this unit up, at the time the most expensive part of my system, think these sold for around $700 new back in 2016.

There is no extra. Your SCC only draws as much from the panels as it can charge to the battery. In your case 80a. Panels do not push watts.

In regards to the calculation, Since I only had the voltages you gave me in your first post I used Watts/Vmp=Amps 670w/75.6v=8.86a The actual number based on your labels is 37.33 X 2= 74.66Vmp and 8.97a I did not add correctly the first time around.

 

[opfor]

There is no extra. Your SCC only draws as much from the panels as it can charge to the battery. In your case 80a. Panels do not push watts.

Interesting, I did not know this, always assumed the power was being pushed into the SCC. My last conversation was with a reputable off grid solar company (will not call by name) and they made it a point to say I could fry my SCC if I had my config set up with too many panels. They also stated that the SCC would have to rid off the extra in the form of heat and would eventually damage the unit. I mentioned to them that I thought the system would ONLY take what it needed but they did not agree. Frustrating how you can hear different info from sources you think are telling you the right thing?

 

[ryankshaw]

They were right if they meant that if you put too many panels in series and go above the open current voltage your mppt can handle, you’d fry it. But going over amps from panels in parallel is not a problem

 

[Mattb4]

Interesting, I did not know this, always assumed the power was being pushed into the SCC. My last conversation was with a reputable off grid solar company (will not call by name) and they made it a point to say I could fry my SCC if I had my config set up with too many panels. They also stated that the SCC would have to rid off the extra in the form of heat and would eventually damage the unit. I mentioned to them that I thought the system would ONLY take what it needed but they did not agree. Frustrating how you can hear different info from sources you think are telling you the right thing?

Funny enough but they are correct. But not in the way of panels pushing power. Loads draw power. Now if you connect up too many panels so that you exceed the SCC max voltage you will fry it. Even if you are under max voltage cold weather might possible push you over depending on how close you are. Now as to heat. If your SCC is running at max because of sufficient loads, and you also have more than sufficient panels, it will get hotter than if you are not running it at full. Some of these SCC are rated based on limited hours of max operation.

Think of it like a 100w light bulb in a closed area. It will only draw 100w and not be any problem for a few hours being on. Now leave it on for 24 hours and watch the space it is in get hot. Eventually it will destroy itself. At no time did it draw more wattage from the 15a circuit it was wired to (15a at 120vAc = 1800w)

ETA: The concept is known as Duty cycle.

 

[opfor]

Sadly he was not referring to the units voltage side as I made it clear to him that I was aware that the SCC maxed out at 150v and thus why I had to do the panels in 2S2P as it was not possible in a 4S config. Appreciate you guys confirming all this and good to know I can add these into my system. I can pick up those two 335w for $275 and the cheapest way to max my system out without having to spend a lot more funds.

 

[Mattb4]

Incidentally I find this website handy for mixing solar panels. https://solarpanelsvenue.com/mixing-solar-panels/

 

[Fenachrone]

That's a great resource, thanks for posting it!

 

[opfor]

Yes, great link. Thank you for posting!