Off grid cabin solar build using some freebies

[Offgridcabinman]

New member here. I have an off grid property in E TN where I’ve build a well-insulated 300 sf cabin (plus upstairs) that just has the basic necessities. I have a small 5k BTW window unit, coffee maker, tv+dvd player, few other small devices. I’ve been running everything off a 3900/5000 Westinghouse Gen with no problems, even able to add another small window ac when company comes over.
New to me components and not installed yet:
1500w pure sine wave inverter. 8-AGM batteries @ 125ah each. Been used in a back-up bank for long time and simply aged out so they replaced them. Probably never been under a load of any significance. Not sure if that’s good or bad.

My question: Since I only go here every 2 to three weekends, could I use up most of the battery storage and let a small solar system recharge as I’m gone? I don’t want to invest much in the solar panels and charge controller so I was hoping less that $500 but I’m flexible. I can still run the generator but I don’t want to charge the batteries with it all at once because of that amount of power needed and would rather let a solar system recharge while I’m gone over time. I don’t have much sunlight there in the woods so that will also be a challenge. But I think my roof will be the clearest spot and still only get maybe three good hours of sun and the rest iffy.

I have an older aluminum tool box I thought about placing the batteries in and storing partially under the ground under the cabin so that in case of a battery issue I wouldn’t lose the cabin but also it will keep the batteries warmer in the winter and cooler in the summer.

Any thoughts would be great. I know this is the place to go for people who know a lot on this subject and are passionate about it, so good or bad give me your thoughts and knowledge. As my entry into solar just want to keep it simple and learn as I go and add to the system in the future.

 

[Offgridcabinman]

Oh forgot, the inverter is a 48v so I’ll have to run batteries in series and parallel if I use all 8 instead of 4. I would like to toss the inverter and just get a 12v inverter but I hate to buy another one now.
I already have 12v rv lights ran off a single 100ah AGM inside the cabin that also runs the control panel of the RV frig. I charge with a regular battery charger via the gen.
I’m leaning toward ditching the inverter but for now I don’t think it matters much because I can always switch to 12v later and run batteries in parallel.

 

[socalryder77]

8 batteries in parallel and series would work just fine with your 48v inverter, bit I think it's output is too low for your loads. Also considering the batteries' age and the fact that you only have an 50% DOD on AMGs, I don't think they will last a full 24hrs with your loads without generator charging. To recharge those batteries while youre gone is easy, but to run no generator at all while you're there would be quite an investment. To run some numbers, I have a 5k BTU window AC and on high, it draws about 500Ws consistently, so if you have 2, that's 1000W minimum at all times. With all the other small items, your probably about 1350w at all times in summer, with a peak of 2850w while the coffee machine is on (this is assuming you run your AC at night). If you don't want to run the generator at all, you'll probably have to invest some money on a bigger inverter, more batteries and few thousand watts of panels to supply your daytime load, while also recharging your batteries from what they used the night before.

If i had your load and i wanted to run no generator at all, but had full sun (which you mentioned you dont have), I personally would get a minimum 5000w inverter (AC and coffee machine have a surge when they kick on) at least 3500w of panels and 20kW of usable battery capacity to provide enough power for both of the ACs through the night and long enough into the morning to where there is full sun on the panels.

Another note on panels, they don't ever get the full amount of watts they're rated for. So to get 3500w under full sun, you may actually need 5000W of panels. Right now, I'm getting half of my panels' rating because the sun is so much lower.

Perhaps someone can chime in, I'd love to be wrong about my thoughts, but I say this based on my experience the last several years off grid. And I started in a small cabin like yours.

I initially started with 2000w inverter and 2000w storage and 1200w panels, but I always had to recharge for about 1.5hrs in the AM and 1.5hrs in the PM. I could only run one AC (14k BTU inverter AC) and one kitchen appliance at a time, but whatever small electronics i wanted. In summer, as soon as it dropped below 80, AC went off and i was able to run off the battery till morning. Fridge, stove and water heater - propane. Heat was, and still is wood burning stove.

 

[Rednecktek]

Ok, let's look at what you've got to play with and ignore anything Blue because you have a budget.

8x 125ah lead batteries = 12v × 125ah = 1000wh ×8 = 8000wh ÷ 50% = 4000wh. That's actually not bad at all!

1500w inverter @ 48v is suprisingly small, but usable if you're careful.

Various 12v loads... that's a kicker...

$500 budget... not much but can be worked with. Let's call it $400 to account for cables, fuses, lumber, etc.

Check your local Craigslist/Facepage and see what's available in your area for solar panels. Around here a 250w panel is about $100. Pick up a pair of those. $200. You'll want to get this up over 1000w eventually so you can charge your batteries in a day, but 2 weeks of no load should still be plenty to fully charge them by the time you get back.

Vevor has a decent little MPPT controller 50a for about $100. That'll get you 750w worth of charging so you can add a panel later.

Vevor also has a 3500w 12v pure sine inverter for about $200. That puts you a little over budget though and really maxes out what 12v can do.

The other option is the same panels with a different controller ( the best budget option being the older PowMr metal box for about $85) and a 48v -> 12v step down converter to feed your 12v stuff. You'll be limited by the 1500w of your free inverter and you'll have to calculate your 120v loads because when a converter says 240w, it DOESN'T mean 242w. That keeps you in your $500 budget and gets you using what you have, but you'll have less than half the power your generator can provide.

The 3rd option, and this will put you over budget a bit, but a PowMr 3.2kw 24v AIO is $400 and a pair of panels. That's almost the same capacity inverter as your generator, has your MPPT, transfer switch, and AC Input from the generator all built into one box. Your batteries can easily be put in 2s4p for 24v. Even a single panel should get you on paper and worst case you fire up the generator for a couple hours when you first get there and coast all weekend. You'll still need a converter to take the 24v down to 12v.

Ok, I should have opened with that one.

A working system is better than no system and will give you the chance to learn what a solar system can and can't do. You can always modify later with larger inverter or LFP batteries or more panels and the like.

Sadly the paywall is real.

 

[Offgridcabinman]

Ok, let's look at what you've got to play with and ignore anything Blue because you have a budget.

8x 125ah lead batteries = 12v × 125ah = 1000wh ×8 = 8000wh ÷ 50% = 4000wh. That's actually not bad at all!

1500w inverter @ 48v is suprisingly small, but usable if you're careful.

Various 12v loads... that's a kicker...

$500 budget... not much but can be worked with. Let's call it $400 to account for cables, fuses, lumber, etc.

Check your local Craigslist/Facepage and see what's available in your area for solar panels. Around here a 250w panel is about $100. Pick up a pair of those. $200. You'll want to get this up over 1000w eventually so you can charge your batteries in a day, but 2 weeks of no load should still be plenty to fully charge them by the time you get back.

Vevor has a decent little MPPT controller 50a for about $100. That'll get you 750w worth of charging so you can add a panel later.

Vevor also has a 3500w 12v pure sine inverter for about $200. That puts you a little over budget though and really maxes out what 12v can do.

The other option is the same panels with a different controller ( the best budget option being the older PowMr metal box for about $85) and a 48v -> 12v step down converter to feed your 12v stuff. You'll be limited by the 1500w of your free inverter and you'll have to calculate your 120v loads because when a converter says 240w, it DOESN'T mean 242w. That keeps you in your $500 budget and gets you using what you have, but you'll have less than half the power your generator can provide.

The 3rd option, and this will put you over budget a bit, but a PowMr 3.2kw 24v AIO is $400 and a pair of panels. That's almost the same capacity inverter as your generator, has your MPPT, transfer switch, and AC Input from the generator all built into one box. Your batteries can easily be put in 2s4p for 24v. Even a single panel should get you on paper and worst case you fire up the generator for a couple hours when you first get there and coast all weekend. You'll still need a converter to take the 24v down to 12v.

Ok, I should have opened with that one.

A working system is better than no system and will give you the chance to learn what a solar system can and can't do. You can always modify later with larger inverter or LFP batteries or more panels and the like.

Sadly the paywall is real.

Great response! Interesting ideas. I will look at those items.
The largest unknown to me was if it was ok to recharge slowly over a longer period of time. I have read that AGMs like a fast and high charge. So that is great news to me that a slower charge will suffice. I noticed yesterday that Lowe’s has Renogy products on sale marked way down…200w panels with 20a MPPT for $229. May be a great starter kit?
I have access to scrap wire from work to make up connections and I have a crimper, so a lot of the small items and wiring I can make up myself. I think they even have the battery to battery connector wires made up that I may need able to scavenge through. We have some solar panels at work, so I need to find out who to talk to to see if they may have a few panels they can’t use. You never know, that’s how I got the batteries.
Appreciate the response and I’ll look into those options.

 

[Offgridcabinman]

8 batteries in parallel and series would work just fine with your 48v inverter, bit I think it's output is too low for your loads. Also considering the batteries' age and the fact that you only have an 50% DOD on AMGs, I don't think they will last a full 24hrs with your loads without generator charging. To recharge those batteries while youre gone is easy, but to run no generator at all while you're there would be quite an investment. To run some numbers, I have a 5k BTU window AC and on high, it draws about 500Ws consistently, so if you have 2, that's 1000W minimum at all times. With all the other small items, your probably about 1350w at all times in summer, with a peak of 2850w while the coffee machine is on (this is assuming you run your AC at night). If you don't want to run the generator at all, you'll probably have to invest some money on a bigger inverter, more batteries and few thousand watts of panels to supply your daytime load, while also recharging your batteries from what they used the night before.

If i had your load and i wanted to run no generator at all, but had full sun (which you mentioned you dont have), I personally would get a minimum 5000w inverter (AC and coffee machine have a surge when they kick on) at least 3500w of panels and 20kW of usable battery capacity to provide enough power for both of the ACs through the night and long enough into the morning to where there is full sun on the panels.

Another note on panels, they don't ever get the full amount of watts they're rated for. So to get 3500w under full sun, you may actually need 5000W of panels. Right now, I'm getting half of my panels' rating because the sun is so much lower.

Perhaps someone can chime in, I'd love to be wrong about my thoughts, but I say this based on my experience the last several years off grid. And I started in a small cabin like yours.

I initially started with 2000w inverter and 2000w storage and 1200w panels, but I always had to recharge for about 1.5hrs in the AM and 1.5hrs in the PM. I could only run one AC (14k BTU inverter AC) and one kitchen appliance at a time, but whatever small electronics i wanted. In summer, as soon as it dropped below 80, AC went off and i was able to run off the battery till morning. Fridge, stove and water heater - propane. Heat was, and still is wood burning stove.

I get it, you’re right on. Not going to run two ACs with the inverter and may not even be able to run one consistently was my thought, because I know ol Murphy all too well. I have a Gen switch I plan to install so I can switch between gen supply or another load supply, in this case solar/batteries. I don’t mind running the gen when there’s a heavy load.
If I can convince my wife (it always comes back to this lol) to let me install a simple kit I think she’ll be on board with later improving it… because why??? She hates the generator running! Even though it’s very quiet and I have it tucked away in a separate shed. She wants to hear the sound of nature. But for the time being, we really don’t want to sink thousands into a system we don’t use very often. In a few more years when the kids are on their own we’ll have more time and money to burn.

 

[Bluedog225]

Welcome. Renogy panels are good in my experience. But small and expensive. Lots of used 200-300 watt panels available very cheap around here. E.g.: $40 each.

I’d get a CO alarm and move the coffee maker off the batteries to propane or butane for $30 (or cheaper at the Chinese market). https://www.amazon.com/GS-3400P-Por...03299&sprefix=butane+stov,aps,167&sr=8-3&th=1

I suspect a lot of months the AC is optional.

Interestingly, folks calculate solar production based on a “good” 5-6 hours a day. So your shade may not be that much of a problem. Gets the good midday sun I suspect.

And yes, charging over the week is absolutely fine. When the batts are full, the charge controller can give them the higher voltage they need.

I run my cabin off 600 watts of panels, a 200 amp lfp battery, and a 1200 watt inverter. Just for the basics. The battery tops off while I’m away. Note, the lfp battery can be used down to 0%. The AGM can only be used down to 50%.

Putting the batteries in a vault is a good idea for temp control. Watch for flooding.

There may be more efficient “inverter” based air cons out now compared to an old fashioned window beater. Maybe Midea is worth looking at.

 

[socalryder77]

I get it, you’re right on. Not going to run two ACs with the inverter and may not even be able to run one consistently was my thought, because I know ol Murphy all too well. I have a Gen switch I plan to install so I can switch between gen supply or another load supply, in this case solar/batteries. I don’t mind running the gen when there’s a heavy load.
If I can convince my wife (it always comes back to this lol) to let me install a simple kit I think she’ll be on board with later improving it… because why??? She hates the generator running! Even though it’s very quiet and I have it tucked away in a separate shed. She wants to hear the sound of nature. But for the time being, we really don’t want to sink thousands into a system we don’t use very often. In a few more years when the kids are on their own we’ll have more time and money to burn.

Now, if i were to do it my way, I would keep the 2 ACs (assuming they do only draw 500w each) and ditch the electric coffee maker, and would keep the 1500w 48v inverter and 8 AGMs and wire up them up 4s2p and get 800w of panels and battery charger. That would let you run both ACs during the day with a very slow drain and you would use the generator just here and there. This way you'd only have to buy the least amount of new equipment and you wouldnt have to run the generator 24/7. I used to have to run the generator 24/7 for power and I hated it. This will also give you some real world data to plan an improved system.

 

[Offgridcabinman]

Appreciate all the replies and links! I'm learning a lot. Here's some of what I'm taking away, the main things anyway:

-It's ok to charge the batteries slow instead of a fast high charge.
-Looks like the coffee maker should go, or at least not be used on the batteries. I have alternative ways of making coffee so that's ok...but this is one of those small 2-3 cup units so maybe it would be ok as long as I don't leave the warmer on for a long time, maybe...
-More solar panels are better!
-Higher amp MPPT is better.
-If I need both window A/C's then run the generator. May even have to do that for prolonged use of 1 AC.

I can probably get by with the minimum approach that I asked about, but if it becomes used more often and well liked by the wife there could be some upgrades to improve the system in the future.

I think the setup I have with 8 - 125ah AGMs, 1500W/3000 inverter, and at least 200watt panels/20amp MPPT (even better 400W/40amp or even more!) would get me by for my infrequent visits as long as the generator is used when heavier appliances are used.
This would definitely suffice for mild temp times of the year, which is about ~60% of the year.

 

[littleharbor2]

The coffee maker shouldn't be a problem if you only use it to make the coffee and shut it off . Get an insulated carafe.

The 1500 watt inverter might be a bit light though, depending what other loads ar running.

 

[Q-Dog]

-It's ok to charge the batteries slow instead of a fast high charge.

Not just OK, slower is better. I always try to charge at the slowest rate that gets the job done.

 

[45North]

If budget & timing is an issue, mebbe you could put off the air conditioning till next spring or later
I don't know how hot it gets in Tennessee but seems to me air is a bit of a luxury for a cabin in the woods.

 

[Bluedog225]

I think it’s gotten up in the 110 eff range in Tennessee. (That’s 43 in Canadian ). Starts to feel a little warm down south.

 

[Offgridcabinman]

It's hot and humid here in the summer...AC is a wonderful luxury! I normally just run one 5000 BTU unit in the evening and through the night. It's about 2000' elevation and in a forest so it normally feel much cooler than at home, by 5 - 10 degrees. So yea I don't mind running the Gen...and in fact it's good to run it some to keep it working. But the times when AC is not needed, such as now, we should be able to do everything we want like watch a movie on the TV and charge phones, etc. Coffee in the morning could be done differently or as littleharbor2 said just run it into a thermos.
When I have four or five men staying there I will bring a second AC and run the generator more.

But in the winter, it wood stove time!
Gotta attach a few pics.

 

[Offgridcabinman]

One more question:
My frig is an RV newer style Dometic that requires power to operate the ignition and motherboard/controls. I have a single AGM under it to run that and my RV-style lights throughout the cabin (on a separate DC subpanel and switch box).
-Can I pull a 12V branch off a series bank? I don't know the fridge specs, but i'm certain 48V would blow it. The lights are probably 12/24V but I also highly doubt they could handle 48v.
-If not, I still need a way to charge the single AGM 12V. I could use the same battery charger I have, but that gets weird if i'm using an invertor mostly, lots of wasted energy.
Possible i could get a 48v to 12V transformer.