diy solar

diy solar

Another beginner build - 12v RV off grid planning questions

lectric74

New Member
Joined
Aug 10, 2022
Messages
24
Hello everyone,

I'm new to doing this type of thing, but have run a simple 50 watt panel through a Harbor Freight controller, not that it matters in this design.

I have 2 projects coming up I'm trying to plan for, and I'd like to discuss the first and see if there's anything I'm not thinking of as I have a few months ahead to buy the parts and pieces needed for this build.

This project is personal and is for an off grid location that my dad's family has in a trust and I've camped at for years. I currently have a 91 Wilderness trailer (planning to upgrade next year), and it stays up there for months during the spring, summer and early fall. This year has been the first that my dad hasn't been up, and he has no plans to come back up. I've taken over his RV pad, and he's asked me to pour a pad for a propane tank he bought, so I plan to include a tower for solar. My plan is to eliminate the need for my 3650 watt gas generator, as well as reduce the propane use for the refrigerator. The trailer uses a 120 v 30 amp feed, so my calculations are based on that. I tried to be pretty conservative in my estimates, and ensure a good overhead to prevent ever firing up a generator (might be a bit wishful, but I plan to downsize the generator to a Honda 2000w inverter to charge batteries in dark times). Eventually I want to also add an irrigation pump to run a few heads around the property to help with fire prevention. I'm not going into brands at this time, just keeping it generic as I plan this out. As I start to buy I will come here for reviews and recommendations on specific brands of things. The listed specs are an idea of what I plan, not final part plans.

Currently I have planned:

4 240 watt mono panels
5 12v 200 aH batteries
8000 watt inverter
60 amp charge controller
6/3 wire
12/2 romex
Main battery disconnect switch
6 spot IP 65 breaker box
32 amp ground fault breaker
20 amp ground fault breaker x 2
Outdoor 30 amp RV box
1.5" PVC conduit
2 outdoor 120 v outlet boxes with outlets

So, plan is to have an enclosure at the bottom of the tower to house the batteries, controller, inverter, breaker box, a 120 v charger for the generator, and have one 120 v 20a duplex outlet. Charge coming from solar to batteries, batteries in turn running inverter, inverter hard wired directly to breaker box, 6/3 to the outdoor 30 amp plug box, and a second 120v 20a outdoor box on a pole mount with both outlets running 12/2.

Is there anything obvious I'm not thinking about that would be needed. I know the trailer peak draw (30a x 120v) is 3600 watts. When I did the math I figured at peak draw I would have about 20 hours total run time, which is obviously not going to be the case as even when I run the AC unit on the trailer it runs for 4-6 hours but isn't drawing peak power once it starts, and the fridge draws around 7 or amps when cooling, but only 1-2 once it reaches temp, so I calculate it at 300 watts average, so in practice it would be much longer. I assumed 6 hours of sunlight per day (typically see 8-10 but being conservative), and so replenishing the batteries daily would be around 840 watts for 6 hours. Might be very heavy for the majority of the time, and probably close to accurate when I'm actually up there while still leaving some cushion.

This being the first time I've tried to put a system like this together, is there anything that I'm doing wrong, things I'm not thinking of, or are my calculations way off? Am I under-sizing the system, or going way over needs? Any help or advice you can give is greatly appreciated. My hope is, not only not to run the generator, but to have the fridge running on 120v power instead of propane so I don't arrive to a dead fridge because of propane, even though it will be less likely once the 120 gallon tank is set up, not to mention the environmental benefit of it.

Currently I'm camping about every weekend from early May through mid-late September. I usually try to spend a week or two each summer where I don't leave and can do lots of clean up and such, and if I stay remote in a new position I may spend more time up there, but I still think this system is built enough to handle it, but I'd love second opinions and advice. In this location nearest utility power is a mile down the canyon, and unlikely to be moved up further in the foreseeable future.
 
welcome! you're on the right track. calculate loads, then size your system accordingly. where are you located? suncalc.org + PVwatts are both very important for getting actual PV production estimates. location and panel placement are important.

AC usage will be tough to accommodate. is it a "standard" overhead AC on the roof? if so, probably 13.5k btu unit and those things are not very efficient. i think you're significantly underestimating if you're planning on having AC.

an 8kw inverter is pretty large for what you're describing. 4kw is probably sufficient. remember, everything currently is running on a 3650w gas genset.
 
When you say AC usage will be tough, please elaborate. Am I not including enough storage? Also keep in mind, the trailer is in a shaded location, so AC usage is typically less than 6 hours a day, and yes it is the roof mounted unit which is currently run with the generator.

As for the inverter, I went oversized because the next trailer may actually be a 50 amp supply (6000 watts) and I have included the capability of my family connecting their trailers to the 20 amp outlets to maintain batteries on their trailers, and even run their fridge during the week. It is also large enough to run the future irrigation pump to help maintain the property. The rest of my family is not up often, but when they are I'd like to make things easier for them. I also plan to have a pellet grill, and want the ability to run that without a generator. Heavy cushion I realize, but the extra amperage should pay off in the long run. I will also go up this weekend and put an amp clamp on the incoming feed and see what my amp draws are for AC startup and run. I'll also see if I can't get the specs from the AC unit. It is older (31 years), so I'm sure a newer unit will be more efficient.

As for location, the property is in north central Utah. Todays data for the property shows just shy of 14 hours of daylight, so my thought is I have enough cushion, but I'd like to be sure. Panel placement will be solid mounted about 12' above ground with no trees nearby to interfere. Plan is around 35-40 degrees tilt with a south face. My current 50 watt panel maintains the 12 v battery on the trailer while sitting on the ground with a south face and about 20 degree tilt.
 
That’s a summer angle at your latitude isn’t it?
Yes it is, the property is basically inaccessible during the winter months, having 3-8 feet of snow on average on the ground. While the system will still run during the winter months, it won't be taxed and I need to have enough angle that I'm not up there weekly on snowshoes to clean the panels. During the summer is the only time the AC would be used, and even then it typically doesn't run for greater than 6 hours a day.

Edit: the charge controller will float the batteries during the winter months, I will not be removing them, although I will have a manual disconnect for the inverter to prevent unwanted discharge while nobody is on site.
 
Last edited:
I can see a couple of problems. I'm assuming you want to keep this 12V, and five 12V batteries in parallel is going to be hard to keep balanced. With 1000Ah of storage, you are going to need up to 125A of charging current for optimum battery life. 1000Ah/8 = 125A. To get that much current, you are going to need two charge controllers, and 125A X 13Vcharging X 1.175fudgefactor = 1909W, basically double what you are thinking about.

Really, for this size of system, you are really placing yourself at a disadvantage sticking with 12V. I'd say you're better off with a 24V or 48V system for these power levels.
 
assumed 6 hours of sunlight per day (typically see 8-10 but being conservative
That’s not conservative- it’s optimistic.

Even if not a northern latitude you won’t get full sun output over 4-5 hours/day.

I run 800W of panels positioned to maximize daily harvest and I’m sure that your 960W isn’t going to meet your expectations with A/C use - the generator will be needed often imho.
also see if I can't get the specs from the AC unit. It is older (31 years)
That’s not that old; not ancient history
included the capability of my family connecting their trailers
still think this system is built enough to handle it, but I'd love second opinions and advice. In this location nearest utility power is a mile down the canyon,
When you say AC usage will be tough, please elaborate. Am I not including enough storage? Also keep in mind, the trailer is in a shaded location, so AC usage is typically less than 6 hours a day
I think you need more battery storage and panels

Do the math on an energy audit and use a solar calculator and report back here with the results.
 
I can see a couple of problems. I'm assuming you want to keep this 12V, and five 12V batteries in parallel is going to be hard to keep balanced. With 1000Ah of storage, you are going to need up to 125A of charging current for optimum battery life. 1000Ah/8 = 125A. To get that much current, you are going to need two charge controllers, and 125A X 13Vcharging X 1.175fudgefactor = 1909W, basically double what you are thinking about.

Really, for this size of system, you are really placing yourself at a disadvantage sticking with 12V. I'd say you're better off with a 24V or 48V system for these power levels.
Nothing set in stone currently, and I have looked at some 240 watt panels that are 24 volt output so that is an option. I'm trying to ensure this will work without and happy to learn what's best to do. In that case, would I want to do multiple banks with multiple charge controllers, or how is best to keep things balanced and configure the batteries? I'm not sure how I'd do 48 volts unless I do 2 banks of 4 batteries in series.
 
That’s not conservative- it’s optimistic.

Even if not a northern latitude you won’t get full sun output over 4-5 hours/day.

I run 800W of panels positioned to maximize daily harvest and I’m sure that your 960W isn’t going to meet your expectations with A/C use - the generator will be needed often imho.

That’s not that old; not ancient history



I think you need more battery storage and panels

Do the math on an energy audit and use a solar calculator and report back here with the results.
That's what I'm trying to figure out, I'm already thinking closer to 2400 watts solar, but as mentioned, I'll do an energy audit and solar calc to see what it says. I am definitely northern so I used the suncalc listing which shows 14 hours currently, then assumed that the majority of those hours would not be full output.

As for the family, when I say they are rarely up there, I mean that since my parents don't go up, they might spend 2-3 days up there, the rest of the time the outlets are going to be vacant, or running something like a small pellet grill. But, I do appreciate the advice, it's what I'm looking for to try to make this system make sense financially and ensure it will meet the needs.

I realize the 91 trailer isn't old old, but it's still got some wear, and is unlikely anywhere near as efficient as it was when it was new, nor is it as efficient as a newer unit is going to be.
 
Ok, energy audit done and calculations done:

12,780 watt hours/day
1 day without sun
48 volt LiFePO need 389 amp hours (4 banks of (2) 24 v 100 amp hour for a total of 8 batteries)
6 hours sun (I know but I'll go into detail) need 3277 watts solar
350 watt panels need 10

Obviously much bigger than I had originally planned which is why I came here for advice and to learn. I have a concern with the panel count and size, but using the same panels I just bought for my father in law, 10 360 watt (33v x 10.8a) panels takes about 12'x17.5' to mount. I am working on the math for the snow loads to ensure the tower I build can handle the size, it may be too much and I'm not sure I can find more wattage in a more compact layout to lower that load planning.

I mentioned that the sun hours are probably high, this is because of timing. Assuming a newer trailer, actual hours running AC will be reduced (newer trailers have better insulation). But, in a typical weekend, I arrive sometime between 6-10 PM, typically just open some windows on the trailer if it's hot, and might run the fan function on the rooftop AC. Saturday, most of the day is spent outside the trailer, mowing lawns, weed eating, running sprinklers, and general work around the property. I've been tracking my AC usage this summer, at most 3-4 hours of AC is typical, and 3-5 total days per summer that it is needed or used. Sunday, make breakfast and load up to go home, no AC usage. I think it might be safer to accept some loss of battery capacity on Saturday, if I can find a happy medium with array size and battery capacity. I think I should have just planned without AC, but I know it will occasionally be used so I'm doing my best to plan for it. In addition, any newer trailer, even if using a 50 amp feed, will be connected to the 30 amp circuit. I intentionally planned my AC a little heavy by using 6 hours in the calculations, although it is extremely unlikely I'd ever use that much. The refrigerator is also calculated heavily, as it's hard to say how often it actually runs, so I calculated it at 240 watts for 12 hours a day, I think that is probably pretty heavy once temp is reached, but wanted to build some cushion.

These things in mind, what would be the thoughts on panels? Could I potentially be ok if I went with ~2400 watts? Are there higher wattage panels in a smaller size (current plan using those panels they measure ~71"x41")? If I can't find a way to get the array size down I might just think about keeping the generator for when I need AC and then I can have a significantly smaller system to run the fridge and grill and supply some small stuff for family when they are on site.

I do believe I'm currently overthinking the battery array, originally thinking each group would need to be isolated, but now I believe that is unnecessary, but I would love to know for sure if there's something special needed.
 
Last edited:
Keep the generator / small generator, you can use it to charge up your batteries as needed if the solar panels aren't enough. It's likely more efficient to run a generator to charge your batteries than running the generator directly to loads for hours. You can run it before bedtime to catch up on any battery shortage and then turn it off to go to sleep.

If you go the charge the batteries with a generator route, you only need enough batteries and solar to run your loads through the day and night when you aren't their to benefit from ac or supervise the generator.

It doesn't seem like you are going to be there enough to justify going huge on solar and batteries vs using a bit of fossil fuel as necessary. The benefit of having enough batteries to get you through the night so you don't have listen to a generator at night is the bees knees. During the day, a quick top off isn't so bad though.

The bad part about generators is they waste fuel and make noise even when they aren't powering much in the way of load. They work great to dump energy into batteries and then go away until needed again however. Especially if you find the sweet spot on your chargers / generator where you are really in the maximum power to fuel usage zone.
 
I did find the MagnaSine hybrid inverters as well (wanted to get one for my FIL's system but they are on backorder currently), that will use a hybrid supply from a generator and the batteries. Might be better option and lower how big the system needs to be. Running a generator isn't the end of the world if I can use a small inverter generator that's quiet.
 
Last edited:
Back
Top