System design review

[Reef Whooligan]

Hi! This is my first solar setup and was looking for some opinions on it. The use case is an off-grid cabin that is already wired to use a generator through a circuit breaker box. I would like to convert it to run on solar/batteries with the option of using the generator as needed. I think the biggest issue / unknown I have is that my roof faces east/west so I am planning on vertically mounting the panels on the south facing wall. There is limited open width on that wall, because of a large picture window in the middle, so the square-ish Grape Solar 100W panels will fit the best. The MPP 1012LV-MS is an all-in-one inverter/charger. My goal is to be able to run a mini fridge (295kwh/yr), a few lights at a time, and charge mobile devices like phones for our average 2-3 day stay without using the generator. For longer stays I am OK with using the generator to top off the batteries as needed.

I currently have only one of the 200AH batteries, so I can do some load testing, and have not purchased any panels yet. I do not know if I need the 2nd battery for my use case or if my solar system is over/under sized based on the vertical mounting. Any tips you have would be appreciated!

 

[John Frum]

Hi! This is my first solar setup and was looking for some opinions on it. The use case is an off-grid cabin that is already wired to use a generator through a circuit breaker box. I would like to convert it to run on solar/batteries with the option of using the generator as needed. I think the biggest issue / unknown I have is that my roof faces east/west so I am planning on vertically mounting the panels on the south facing wall. There is limited open width on that wall, because of a large picture window in the middle, so the square-ish Grape Solar 100W panels will fit the best. The MPP 1012LV-MS is an all-in-one inverter/charger. My goal is to be able to run a mini fridge (295kwh/yr), a few lights at a time, and charge mobile devices like phones for our average 2-3 day stay without using the generator. For longer stays I am OK with using the generator to top off the batteries as needed.

I currently have only one of the 200AH batteries, so I can do some load testing, and have not purchased any panels yet. I do not know if I need the 2nd battery for my use case or if my solar system is over/under sized based on the vertical mounting. Any tips you have would be appreciated!


View attachment 87776

Assuming all wires are pure copper with insulation rated for 105c.

2 strings of panels don't need over current protection
Won't hurt though.
Suggest you give each string a 30 amp fuse which is about the maximum for 10 awg wire.
That way its easy to add a 3rd string.
Also easy to add another panel to each string as long as you stay below 24 amps per string

For the battery wire
1000 ac watts / .85 conversion factor / 12 volts = 98.039215686 service amps
98.039215686 service amps / .8 fuse headroom = 122.549019608 fault amps
That means the smallest fuse you should use is 125 amps.
I suggest a 200 amp fuse which is the practical maximum for 2 awg wire.
The battery fuse should be as close as to the battery postive terminal as possible.

Assmuming the feeder box has a master breaker no extra overcurrent protection is required.
For 14 awg the master breaker should be 15 amps.
For 12 awg the master breaker should be 20 amps.
For 10 awg the master breaker should be 30 amps.

 

[Reef Whooligan]

Thanks for that information! Good catch on the fuses, I will adjust that. I do have a master circuit breaker in the box already but will need to verify what exactly it is when I am there next and will adjust accordingly.

 

[MichaelK]

There is no law that says your panels have to be facing south. I could suggest at least two different options for you.

Before that, I'd first recommend skipping 12V panels completely. You'll get far better bang for your buck going with cheaper/W grid-tie panels. Shop locally for them off of Craigslist or some other local venue.

Instead of an array facing south, have two arrays, one on the East roof, and one on the West. You can basically double what the south-facing limit of your inverter would be, because both arrays could never to at full output at the same time.

Alternatively, don't use the roof at all, and instead install a ground mount. Here's a pic of the array frame I designed and built, which can rotate East to West. I can hold as many as six full-size grid-tie panels.

BTW, unless your mini-frig is 12VDC, my recommendation is you give up 12V and move up to at least a 24V system. That will give you greater capacity for expansion in the future.

Let's look at your loads. Assume your inverter itself is going to consume about 360Wh/day. Your refrigerator will be consuming ~800Wh/day. Just to round out the numbers, let's say the little stuff consumes 40Wh/day. That works out to be 1200Wh, or 1.2kWh of power. I'm not sure that 600W of panels is going to replace that each and every day in the winter even with perfect panel alignment to the sun. I would question as to whether 600W is enough?

By upgrading to the 2424LV model, you could quadruple solar capacity with just the doubling of system voltage. You could implement three strings of 250W panels instead of two strings of 100W panels. 250W panels are dirt-cheap right now. Just bought 260W REC panels a month ago for 65$ each. That was with local pickup, cash and carry.

 

[Reef Whooligan]

I would question as to whether 600W is enough?

Thanks for all the feedback. I am not sure of the feasibility of the fridge either and will be testing run times with a similar mini-fridge before proceeding with the full system. Currently I run straight 12v, just for lights, and use small battery packs or my vehicle to recharge everything else. Even if the fridge doesn't work out I will still be happy to have 120v available inside for the small devices. I've been pretty stuck on the current design because of space constraints but I will re-evaluate next time I'm there for your suggestions.