Looking for input on my current plan for full off grid solar

farmerc356

New Member
Joined
Sep 6, 2021
Messages
3
This is being typed on mobile, apologies for any errors.

The wife and I have bought a currently off grid home that has, for us, an undersized solar system. The system works perfectly as intended but does not meet the demands of work from home and a two and a half year old who wants to do what was normal life in the city just a month ago. It has a 24v 4000w magnasine inverter with a FLA battery bank and 800 watts of solar. The well is currently ran of of generator power, which we would like to get away from.

The planned upgrade:

I have in my possession 16 Trina 410 watt panels, and have ordered a 12kw growatt inverter as well as two of the 5kw batteries from signature solar. The lead time is what it is and I can deal with it as we already have a functioning system albeit one that isn’t quite large enough. We’ve began the construction of a new ground mount which will have 9 panels facing south, 3 panels facing southwest and three south east. According to what I believe is the correct math I should be able to make 5 strings of 3 panels and stay within safe limits while considering the potential for very cold temperatures in Michigan. Record low for this area is -40 which happened last year.

I’m currently on the hunt for a combiner box, which I’m leaning towards a pvwatts 8 into 2. Any advice on this would be greatly appreciated. I’m also curious if I can install the panels and then wire them into a combiner box and not put any fuses in until the rest of the equipment shows up. This would be in an effort to have the majority of the outside work completed prior to the weather turning cold.

I’m sure there will be more questions along the way. I’ve tried to do as much research as possible to be able to accomplish the majority of it without any major outside assistance, but I’m mostly trying to double check everything for my own sanity 🤣. I appreciate all of the knowledge I’ve gained here and hope to continue to learn some more.D17E6B22-44EC-4BC6-94DD-DC8E18B8A38C.jpeg
 

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FilterGuy

What, me worry?
Joined
Nov 26, 2019
Messages
4,738
Location
Los Gatos CA
We’ve began the construction of a new ground mount which will have 9 panels facing south, 3 panels facing southwest and three south east. According to what I believe is the correct math I should be able to make 5 strings of 3 panels and stay within safe limits while considering the potential for very cold temperatures in Michigan. Record low for this area is -40 which happened last year.
Ideally, you would have a separate controller for each grouping that is facing a different direction. With that, you would get *some* power out of at least two sets all the time the sun is up.... maybe even all three.

If, for whatever reason you don't go with different controllers, at least put blocking diodes in so the 'dominant' set of panels don't back-feed onto the other two and waste energy.
 

farmerc356

New Member
Joined
Sep 6, 2021
Messages
3
Ideally, you would have a separate controller for each grouping that is facing a different direction. With that, you would get *some* power out of at least two sets all the time the sun is up.... maybe even all three.

If, for whatever reason you don't go with different controllers, at least put blocking diodes in so the 'dominant' set of panels don't back-feed onto the other two and waste energy.
I was planning on using the charge controller built into the growatt, it has two inputs so I was hoping to have the main south facing array in one input, and the SE and SW array in the other. In this video
on this combiner box https://watts247.com/product/solar-panel-combiner-box-8-circuits-to-4/ will mentions it has blocking diodes built in. Would those perform the same task that you are talking about?
 

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
9,235
I think there is no backfeed when you parallel multiple PV strings of same length but different orientation.
If panels are completely in the dark, yes they will sink current. I was just doing that to try testing electroluminescence of bad panels, using an IR camera.
But with almost any amount of indirect light, the Voc of a panel will rise above Vmp of an illuminated panel. Therefore, it won't steal any current from the strings which do get direct sun.
I like multiple PV strings of different orientations into a single MPPT; it utilizes the MPPT over more hours. Penalty is about 2%, according to testing SMA did, compared to separate MPPT. That makes it a win in terms of kWh/$

Blocking diodes will consume more power during forward-biased operation than they save when reverse-biased. (give or take duration of each situation, and what applies voltage to them.)

You should have fuses or suitable breakers if 3 or more PV strings are connected in parallel. By suitable, I mean that polarized breakers aren't suitable for interrupting fault current that backfeeds a failed PV string. If all breakers are ganged, then it would work. There are non-polarized breakers which would be suitable.

3 panels in series is 154Voc under nominal conditions. That won't reach 200V at -40 (apparently C or F is the same at that number). That's 65 degrees below 25C ambient, so with panel's -0.29%/degree temperature coefficient, Voc would increase 19%. Get a data sheet to find the exact temperature coefficient. Four panels could reach 250.7V if I use that temperature coefficient and the +3% on top of Voc, as shown on label.

I'm seeing 50.4 Voc on this data sheet, different from label. Maybe it's that "(II)"

 
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