diy solar

diy solar

Roof mount and side mount solar array questions

koho

New Member
Joined
Aug 27, 2021
Messages
9
Hi folks. I have 960 watts in 3 panels on the south facing sloping roof of the tiny house and one 320 watt panel also facing south mounted vertically high on the back wall. Nothing is hooked up yet. A Growatt 24V 3000TL inverter is mounted inside.. A 200AH 24V lifepo4 battery bank(EG4 from signature solar). I chose the two panel locations because we are in the north in the mts with very heavy snowfall(4'-6'). The roof panels will shed snow in the first sunny day or if I get up on the roof and shovel them!. The sun angle is low in the winter, hence the vertical panel but it will never be shaded or covered with snow like the roof panels. My question is should I do a series install with the roof panels and run the back panel in by parallel or do 2 strings in series. Note the back vertical panel will aways be in sun albeit at a poor angle in the summer. The roof panels are aways in sun 9 months of the year. The Growatt can accept no more than 145Watt DC PV input, hence the first 3 panels in series. Should I put in two solar array disconnects so I can shut down the roof panels temporarily until I can get them shoveled? That would leave the back 320 watt panel still working. Thank you in advance!
 
I think the best answer would be two solar charge controllers- one for the roof and one for the wall. Since you already have one - just get another (plain SCC).

That way the panels are always charging the battery at the optimal power.
 
To answer your question of how to wire the panels… it depends…
Overall I think it would be best to wire the three panels on the roof in series to one SCC(Solar charge controller).

you need to make sure the SCC can handle the voltage of the three panels in series + the added voltage when it is cold (solar panels produce extra voltage the colder it gets). The last thing you want is to fry your SCC one morning when it is cold.

You also need to make sure the solar charge controllers will NOT charge your cells if they are too cold.
 
The growatt has an 80 amp MPPT in its construction and should handle the 3 in series fine.. Adding the back vertical panel in parallel to feed the Growatt's MPPT is my question was the second string. Your idea to avoid that string and feed it to another mppt is interesting and had not occured to me. I was thinking about having separate dc breakers for each string and try them independently and combined to see what happens esp with snow covering the top panels. I do like your suggestion and that might be a better idea than manually shutting down the top panels. The open circuit voltage of my panels is 46.7. X3 is 140 V. Growatt takes 145 as the max. Thank you.
 
You are probably in trouble with the cold on that 3s setup with the Growatt. On your panels there is an adjustment to voltage for temperature. You need to calculate it with the lowest winter temperature. It will raise the voltage of the panels.

Now if you are buying another SCC, then just buy one that can handle the voltage and use the Growatt for the wall panels.
 
thanks Rocketman. I was not aware there was a cold "adjustment" on the panels. None that I can see but I willcontact Mission Solar in San Antonio. When its cold here the sun is very low and the roof of the tiny slopes down to the north making the incident light even lower so I assume that will attenuate voltage gain even more but I will watch output. both strings will have dc cutoffs of course. We are normally below zero most the winter unless it is snowing. -30--35 is as cold as it usually gets but -10- to -20 is a common temp in dec-Feb.. I would like to find the coefficient of voltage with temp. I will report back to the forum on any interesting findings. I built this tinyhaus with R30-45 spray foam insulation and a 750 watt htr keeps it at 45 on the coldest nights and we don;t drain our pipes. I will leave it hooked up to utility power but I am curious to see its performance off grid. My first tinyhaus is also fully off grid with roof panels but it is shut down in winter.Both have state of the art BMS of course with their LiFePO4 battery banks.
 
Here is a post that explains how to figure out the temperature coefficient - (temperature adjustment).

 
Here are some alternatives to consider:

1. Can you put four panels on the roof (probably not because you said tiny house) wire those four up in 2s2p. Then buy a new panel and a small SCC for wall mount.

2. Wire three roof panels up in parallel- hint- use larger wire to help with voltage drop. And small SCC for wall panel.

3.Buy a expensive high voltage (200v) SCC to wire three roof in series use Growatt for wall mount (financially this idea is the most expensive)

Three is the most expensive and probably does not make sense. Two is the cheapest, and one will get you even more solar power.
 
Here are some alternatives to consider:

1. Can you put four panels on the roof (probably not because you said tiny house) wire those four up in 2s2p. Then buy a new panel and a small SCC for wall mount.

2. Wire three roof panels up in parallel- hint- use larger wire to help with voltage drop. And small SCC for wall panel.

3.Buy a expensive high voltage (200v) SCC to wire three roof in series use Growatt for wall mount (financially this idea is the most expensive)

Three is the most expensive and probably does not make sense. Two is the cheapest, and one will get you even more solar power.
Rocketman, you have been very helpful. I found tables for output with temperature and was startled to find that my voltage could increase to over 20% with temps around -20-30. Whew! Had no idea. My Voc of my mission solar panels are 46.7 which could soar to over 160 V on cold days if the 3 are in series.We can get near zero in Sept and Oct so it does appear my Voltages of 3 in series will exceed Growatt's max of 145 V. I have already ordered another MPPT(Renogy) of 20 amp. Its max VOC is 100V but I would exceed that with a cold day(46.7 VOC X2=93.4 X20% =112 V. Too high or can these MPPT units run above their rated amperage? The alternative is 2 strings in series +2 in series which may be unbalanced on days with snow on the top panels. Wiring 3 in parallel on the roof would generate 25.8 amps.. which is pushing the limit of #12 wire run about 15 feet.The panel wiring is #12. I guess i am not sure which route to go and would like another suggestion from you. Thank you very much BTW
 
You do not want unbalanced solar panels.

If you parallel the three panels (which is what I would do in your situation), you will need fuses on each paralleled string. But then use #10 or #8 wire from where they parallel down to the SCC. (If the connectors can handle #8 - I would use that). Eight or ten gauge wire should handle the amps - double check that.

you don’t want to run more current through a mppt that they are rated for. Really good high quality units will just stop charging - all others will just get fried.
 
Thanks again RM. After thinking about the options I decided to do 2 strings, both parallel. That keeps the voltage down and max amps @ 16X2 or a bit over 34 amps which should work fine for # 8 OR 10 WIRE. I will monitor it all of course and plan to put in a shunt and wifi to the cell phone. If there is xtra power I will try to feed a chest freezer nearby on the porch. it may work or not. Thanks again.
 
You want the panels the same - so if three are on the roof - I would put those three in parallel. I would not combine one on the roof with one on the wall.
#10 can handle 30amps - so it would be fine running your three panels, but I would still use #8 if it is an option.
 
thanks. RM. Why not a wall and a roof together? In case the roof panel is partially covered the wall will still feed electrons..... I might also ask when you connect a battery disconnect switch, does it matter whether you use the pos or the negative cable? It would seem either would work
 
I was told and have always disconnected the positive line. Don’t know why… just disconnect the positive first. (I think it has something do with the negative can be connected to ground - so there can be alternative paths which you do not want the power flowing).

As far as connecting a roof panel (with usually more power flowing through it) with a wall panel (with usually less power flowing through it). I can’t say for sure why it seems wrong to me. I think you will get less power when both are operating. Look up connecting different sized panels together. As I remember, in parallel, the panel with the lower voltage will drag the other panels voltage down. With good sun on the roof, it’s voltage will be a bit higher than the wall so you will be getting less overall power. When the roof is covered- it’s diodes cut it off - so there will not be any advantage or disadvantage to having them connected.

You can safely connect the three roof panels in parallel with #10 wire.

Don’t just take my thoughts on it - research it - it may be just fine. I know people connect panels in parallel with different facings- but I believe that is usually over-panelling and using like east- west orientations.
 
fine detailed response and thanks again. I have battery disconnects on some of our farm vehicles becauise they tend to be used seasonally and one has the neg disonnected and another two have the pos broke, Doesn't seem to make any difference at least in trucks and tractors but I have always wondered. This solar stuff is new to me but I am a good carpenter and mechanic and I had to start from scratch. I did have a minor in physics so I can tell an electron from a neutron............then again maybe I can't!. Since I have splitters I think I may try both 2 and 2 and 3 and 1 depending upon the season. I can access all the cables for the tests to monitor current and voltage. I tilted the roof panels a bit yesterday to encourage snow sliding but time will tell. PITA to try to keep the roofs from loading and they do load here. local code is 127-170 lb/ft sq which is enormous of course. All the weak roofs are long gone. cheers
 
Back
Top