1600w PV array help

[BirdWatcher803]

Hello all, I hope I'm not asking this in the wrong area or that I'm asking too much, but the more and more i look into what PV panels to get the more overwhelmed i've become. I have 4 x 50ah 24v batteries, a victron multiplus 24/3000/70 inverter, and a victron mppt 150/60, all to run power for an off grid cabin. I understand 1720w is the max, but have no idea the best route to get there. The Victron person I talked to recommended 8 x 200w RICH SOLAR monocrystalline panels in a string, is that fine? If so, would I need to put inline fuses between every X amount of panels? I just found out combiner boxes are a thing, are they necessary for something that size or just for making cable management sleeker? Or am I completely going about reaching the 1720w in a wrong / inefficient / dumb way? I know I am asking a lot so if this is too big a favor would you happen to know any design type service companies online that could help me? I've tried every local solar company but they all say they don't do arrays this small. I've been in construction/carpentry my whole adult life so the hands-on, hooking things together aspect isn't a road block, but electrical has always been witchcraft to me, and math is a foreign language (hence construction worker).

Thank you for any help or input.

[littleharbor2]

Do you understand how MPPT works? There are a number of ways to wire your PV array to achieve the voltage you need to charge your battery bank. Your controller can take up to 150 Voc. input regardless of your system voltage. This might help....

Matching solar modules to MPPT charge controllers - Victron Energy

[krez]

Hi Birdwatcher
I'm assuming the panels you have suggested are '12v' panels. They can have an open circuit voltage of around 22-24v which means 8 together (8x24+cold weather= blown up charge controller) you must not exceed your charge controllers upper voltage limit (150v) and with enough headroom to allow the voltage increases that are inversely proportional to temperature.

More details are required. Will your panels be in full sun, have long wire runs? In order to keep voltage as high as possible (most efficient) In your system with the items you described you would place 4 of your panels in series and parallel that 'bank' with another 4 in series or 4S 2P. So you'll have 100ish volts and 20amps ish max going to your charge controller. You'll need to fuse your charge controller to just above it's rated current. Solar doesn't need fuses for reasons that will become apparent as you learn more about this topic. Understanding fusing and why and where it is used and terminology such as series, parallel are absolutely vital before attempting these projects yourself. There are alot great sticky threads to sift through throughout the site that I and hopefully you will find very helpful with these basics, it should all be there as far as I know.

Also, was your battery bank a mistake? As your battery bank will be severely undersized for the power you may potentially throw at it with that charge controller aaand there may be cheaper solar panels you can purchase to reach your desires array size. '12v' panels usually come at a premium... You could get many different panels, as an mppt controller is very flexible with its input voltage. As long as what you end up with is greater than you bank voltage (atleast 5-10v) and lower than your charge controller limit (atleast 10v), you are fine...but yes more explanation helps alot.

[MichaelK!]

Hello BW
First question, are your 50Ah batteries Lithium? If they are LiFePO4, that makes up some for the small size. As a general rule, Li batteries can be discharged more then lead-acid, so you have some more "useable watts". That being said, 50Ah is not that big, and will require four parallel strings, which is NOT good for battery balance. Any chance they can be returned or exchanged for a larger capacity battery?

I am not a fan of 12V panels, so I would not recommend getting the Rich Solar brand. I would not mail order at all, because you will find much better deals on Craigslist, with local cash and carry pickup. Grid-tie panels are dirt-cheap right now, and you are likely to find 4W/$ panels depending on where you are located.

The key to designing your solar strings is that the voltage can never, ever exceed 150V, or the controller gets fried. So, when you buy your panels, you pay attention to specs on the back of the panel. There's Vmp and Voc. Vmp is the voltage at maximal power, or more or less what the panels will put out under load. The Voc is the open circuit voltage, that is the voltage when the panel is disconnected and no power is flowing. Voc is the number to pay attention to when you are trying to stay below 150V.

The important thing about solar panel voltage is that the voltage goes up as the temperature goes down. At freezing (0C) the voltage raises by about 1.12X. At minus 40C that goes up to 1.25X. So, what are your winter lows like? Do you worry about frost on your oranges, or getting the car stuck in deep snow?

Let's say you find a great deal on 250W grid-tie panels and you pick them up for 65$ each. You buy 6 of them. The sticker on the back says Vmp = 30.0V and the Voc = 38V. If you wired all six in series, you'd get 228Voc and kill your controller. How about three panels in series? 38V X 3 = 114Voc. That looks OK, but what about in the cold? At freezing, their Voc would be 114V X 1.12X = 128V. That is OK for your 150V controller. But what about this system in Montana? At minus 40C that 114V becomes 114V X 1.25X = 143V. That's getting close to the limit of your controller. So, three in series in Texas is likely to be OK, but maybe not up in Montana.

If you really want an accurate determination of what your solar string will output, use a string calculator like Midnight's. https://www.midnitesolar.com/sizingTool/index.php

Fill us in with more details, and we'll be of more help to you. BTW, combiners may be a code requirement in some areas. In general, only two parallel strings of panels can be strung without fuses or breakers to stop short circuit current flow. With three or more parallel strings of panels, fusing/breakers is a requirement. You don't have to have a combiner, but there has to be protection, and the combiner is the most convenient way of doing that.