Off Grid Cabin Advice

[Flumes]

I recently purchased a remote cabin (20x20) in the bush. Its not finished, and the previous owner ran the wiring for a 120V system. (lights and receptacles) The electrical panel has yet to be installed.

I am not very electrically savvy...and even less with solar so any guidance would be appreciated.

The cabin will be used for 3-4, days maybe a dozen times a year. Electrical load expectations are some led lights, a water pump and a fridge. (The fridge I hope to use is a Magic Chef 4.2 cuft with freezer. It apparently draws 1.5 amp and 150 watts when running.
I haven't put much thought into a water pump but it will be something that runs only on occasion to fill a 40 gal storage tank. (Once per visit)
I may put a tv with satellite dish in it for the kids.
I dont think theres any other load that will run...(maybe power tools occasionally)

So far, I bought a Honda 2000i generator. Im thinking it should be enough power to run the fridge and water pump and charge batteries.
I also snagged 7 batteries from work....(PowerSafe 12V105F) from a battery system that was upgraded.

I hope to buy about a couple of solar panels that would put out 250W @ 36V. They are made locally and sell for $1/w

Apparently I also need a MPPT charge controller and inverter.


So....my questions.

Will I have enough solar installation if its augmented with generator when needed?
Is it better if I go 12v Inverter or 24v and how big shold I go?

Recommendations for inverter or charge controller?


Thanks in advance.....really appreciate a the help

[MichaelK!]

In general, the higher the system voltage the better, and you're best off forgetting about 12V completely. The Powersafe batteries you want to apply may be a potential stumbling block. They don't appear to be designed for off-grid use, but as short-term emergency backup.

PowerSafe 12V105FS batteries feature advanced thin plate, pure lead technology and unique manufacturing methods to provide high cycling and fast recharge performance in applications where the power supply is erratic.

The problem with the Powersafe's is that being 12V already, their capacity is rather low at 105AH. That means parallel strings of batteries that is never a good thing. Just how used are they? You may plan on implementing them on a short-term basis, and plan on replacing them with better off-grid batteries in a couple of years.

I would say that a system of the size you're describing might need to supply on the order of 1000-2000wh (1-2 kwh), and a 24V system would be the best choice. You could also chose 36V, but you'd be limited to using Outback products only. They are the only major manufacturer that supports 36V equipment. I'll design this around T-105 batteries, which might be better choice than your Powersafes. T-105s are an 225 ah battery, and at 24VDC you'd need about watts of panels to charge them.

225ah X 24V =5400watthours.
5400wh X 50% depletion = 2700 watthours of daily capacity you shouldn't excede
5400wh X 15% depletion = 810watthours which you normally would expect to use for long battery life

(5400wh/C12) / 0.8 panel derating ~ 560watts minimum
(5400wh/C8) / 0.8 panel derating ~ 850 watt upper limit

For charging T-105s you'll need at least 560 watts to prevent sulfanation. Use NOTC ratings instead of STC ratings to determine wattage. If only STC is available, STC wattage X 0.8 ~ NOTC wattage

In the real world working at the upper limit (or past it) usually gives real power levels. So, if for example you purchased two 320 watt grid-tie panels you'd be at the lower limit. If you bought three 320 watts you'd be around the upper limit (320 X 3 X 0.8) = 768 watts.

Let's plug in the numbers appropriate for the Powersafes, which I think are less than ideal.

105ah X 24V =2500watthours. (per string)
2520wh X 50% depletion = 1260 watthours of daily capacity you shouldn't excede (per string)
2520wh X 15% depletion = 380 watthours which you normally would expect to use for long battery life (per string)

(2520wh/C12) / 0.8 panel derating ~ 260watts minimum (per string) (you could potentially make three 24V strings of Powersafes with 6 batteries)
(2520wh/C8) / 0.8 panel derating ~ 395 watt upper limit (per string)

What you could do in short term is wire your Powersafes in 2series3parallel configuration (very less than ideal!)

Buy a MPPT controller. Midnight's 150 or 200 might be a good choice for expandability.

Buy 750 to 1000 watts worth of grid-tie panels. You could wire these in either 3S1P or 2S2P or 4S1P configuration depending what open circuit voltage they run at.

Buy a 24VDC to 120VAC inverter. A cheap choice is Samlex's 1500-24

https://ressupply.com/inverters/samlex-pst-1500-24-pure-sine-wave-inverter

This choice is Samlex's smallest inverter designed to be hard-wired into a house panel

A higher quality choice is Schneider's SW2524

https://ressupply.com/inverters/schneider-electric-conext-sw2524-120240-invertercharger

It's hard-wired, built-in charger, and dual 120/240VAC output. And, multiple units can be paralleled for increased power. Top of the line inverter in it's class!

I recommend Schneider because that's what I use myself, which as demonstrated to me to be very, very rubust.

Here is what I would do

Wire the Powersafes into three 24V strings.
Wire 4 300 watt panels in a 2S2P configuration
Feed ~ 75VDC from the panels into a MPPT controller (need breakers for 2+ strings)
Wire the 24V battery bank into a 24V inverter that's hard-wired into the cabin's 120V panel (They are cheap. You can get one at Home Depot with breaker already installed for 50$.)
Plan on replacing the Powersafes into 2 years with real off-grid batteries after you've trashed them. Buy larger AH batteries so you only have 1 24V string (four T-105s in series).

You could later add more panels if you decide to upgrade to larger batteries like L-16s.

[ppbwpg]

I would add just a few things to a very good detailed post from MichaelK. I am building a similar system (not fully finsihed but system already in use for more than a year) for a cottage with similar usage (in terms of number of days per year). I would confirm that yes - 24V system IMHO is the best bet for your planned system. My recommendations (based on personal experience):

- Plan your system for real expectations (in terms of power usage) but spend some more money for better equipment (inverter, charger, etc.) with expansion in mind. There are great chances you may need more power than you think now, and if you use inverter/charger that can handle more then you feel you need now it will be easy and much cheaper to expand.

- How to wire panels pretty much depends on your particular location. If location shady it's a one design, if not then another.In shady or/and snowy locations it even matters how to place panels - in portrait or landscape position. Can be a subject for more detailed talk.

- Water pump. If your water supply is out of a water storage tank then you can run a pump by a generator. Most of water pumps you see in stores like HomeDepot, Lowes are AC pumps. They are not friendly with solar systems as since they draw a lot of power to start up. For example, I have 1/2 Hp JetPump and my 2000Wt generator cannot handle it well to start. Such pump attached to a solar system will drain batteries quickly. However, if you use it occasionally just use the generator and a regular pump. That's it. Just make sure the generator can handle start-up current. In my experience 3100Wt generator handles 1/2 Hp jetpump but 2000 - cannot.

(If you haven't bought a water pump yet consider a DC water pump designed for solar applications. Such pumps do not draw much of ampers to start and can be used along with solar systems. There are not big difference in price but you can eventually use it with solar system and keep your water tank full all the time automatically)

- do not buy a cheap inverter but get a one that can charge your battery bank from generator. In this case when you need to run a generator for any reason (rainy days, need extra power for anything else (water pump. power tools, etc)) you will charge the batteries at the same time. It can be wired to your Cabin electric panel that way that it will automatically power up cabin from solar power or from generator (and charge batteries when generator runs). I use for this Magnum 4K inverter (and happy with it) but you can find whatever you prefer.

- you definitely need proper batteries designed for off-greed solar system usage.

- there are some other concerns depends of your geo location, are you in such warm area as California and never seen snow on your roof or more toward North with 4 feet snow on a roof...

When I started with idea I thought that I need just 400Wt system and cheap equipment. After doing a lot of research and design and practical experiments it was realized that my initial expectations were wrong. MichaelK is right about sizing - this is also what I use for similar system: 4x265Wt panels and 24V battery bank. My daily power consumption within 1300 - 1600 WtH. Here is real-life info to compare expected consumption: I use 24H/day a cellular signal booster (powerfull one) and cellphone/charger with active hotspot constantly running; about 6 hours of a few light bulbs, from 8 to 12+ hours a day for workstation and big monitor, the rest of time workstation in sleep mode (still consuming some power). Even such modest usage uses minimum of 1300Wth up to 1600Wth per 24 hour cycle (measured with a meter multiple times, so this is a real avg number). If I followed my original plan and would buy cheap equipment then system could work but it could be costly to expand. In reality I see now that 4 panels can supply only what I just mentioned but for some comfort, for additional visitors I would like to have more power so I am adding 6 more panels. As since I spent some money initially for better equipment now I need just to buy 6 panels and 4 batteries and add them to the existing system, that's all (no equipment replacement, no re-wiring, etc.).

Good luck with your project.

[PNPmacnab]

I am not very electrically savvy..

And that is a problem. There is nothing really wrong with those systems described. I just gag every time I hear one being suggested. I have a fondness for camp life. I live off grid five months a year. And that is just summer which is a big qualifier. But, I have only a car battery and run a chest fridge, TV, lights, a medical device, a small pump and get all my hot water from PV. And the wife even has a dishwasher. I do have a generator that runs a clothes washer, that's all. Just bought a LG direct drive washer. With electronic drive, wash mode is only 280W. That is easy to do on solar.

I find you need a 2,000W inverter just for startup load on the fridge. Anything else will gag. I run the fridge only during the day and "store cold" for night. The fridge won't come on unless the battery is 13.5V. Startup current only, no battery storage needed. I am heavy on panels with over 1500W so I can operate real time. Panels are cheap, batteries are just money down the toilet. Too bad you can't buy this system,

[Sunking]

Too bad you can't buy this system,

Be gald you cannot duplicate his system. He has a perfect recipe for a fire. Surprised he has not been banned or at least warned for giving dangerous advice.

OK you have done one thing right, the generator. You need to follow that up with a good heavy charger for whatever battery voltage you decide on. Eliminate 12 volt right now and never say 12 volts again. You want either 24 or 48 volts for a cabin. 12 volt would be extremely foolish and expensive. More on that later.

However you have made one very expensive mistake and it is going to bite you on the arse. You bought used Telecom Batteries, and they already biten you once and you just do not know it yet. Those batteries are made for Telecom Standby Emergency Float Service, not cycle service. They are not made to be cycled. If pushed into cycle service, you get 50 to 150 cycle before they are useless.

The second surprise is how much money it will cost you to dispose of them. See I am in Telecom and what we do is sell used batteries to suckers. Instead of paying a contractor to come and remove and properly disposed of them, several thousand dollars, we turn a liability into an asset and sell them.
You also messed up buying a prime number of batteries. What the hell are you going to do with 7 batteries? You can only wire them all in series or all in parallel. Neither will work. Series is OK, parallel is a huge No-No. So congrats on 7 new boat anchors.

OK for you, 24 volts should be a good fit. If you anticipate ever going over 1500 to 2000 watts of panels in the future, then 48 volts. FORGET 12 VOLTS. Most likely you will need a good 2000 watt Inverrter. Not a cheap arrse mobile inverrter these jokers around here would recommend, bu tone made for home wiring like you use in your cabin where you will use 120 VAC wiring. A mobile inverrter like most people use i snot made, nor can they be used in any code compliant manner in a structure like a house or cabin. Iff the Inverter you are looking at has 120 VAC receptacles on it, keep on looking because Inverters with just Receptacles are Mobile Inverters. That is why they have receptacles. A Inverter made for a home will be hard wired and inaccessible.

Now with a 2000 watt Inverter comes with a few catches with respect to panel wattage and battery minimum sizes. A 2000 watt Inverter demands as much as 2400 watts from the battery. If that battery voltage is say 24 volts is 100 amps of current or 50 amps @ 48 volts. The battery must be capable of delivering the full current without voltage sag. Otherwise voltage collapses, and you Inverter trips off line from undervoltage despite the battery being fully charged and healthy. Easy enough to do, just make sure the battery meets the minimum requirement. That is easy to figure out. Depends on what type of battery you use but as a general rule:

FLA is good to C/6 where C = the battery Amp Hour Capacity, and the number 6 is Hours. So with a 100 Amp load, you would need at least 6 hours x 100 amps = 600 Amp Hours
AGM can handle higher loads of C/4 and some like Telecom Batteries can handle 4C or 15 minute discharge times as that is what they are made to do. C/4 is a safe bet and many can do C/2. So a 200 to 400 AH battery can work.

Second catch is meeting the batteries minimum charge current requirement. Let's make this super simple. All batteries need at least C/10. So if you had a 400 AH batteries would be 400 AH/ 10 Hours = 40 Amps. On a 24 volt battery would require 24 volts x 40 amps = 960 watts, just make it 1000 watts.

So watch out and be careful what you ask for. A 2000 watt Inverter requires at least a 1000 watt panel and a 800 pound battery. Or you could use a 12 volt car battery with a 1500 watt panel like some fools would use. That would require two very expensive MPPT controllers like two 60 amp controllers @ $500 each. If you know what you are doing, you would run 48 volts with a single 30 Amp Controller for $350

[Flumes]

Thank you for the replies. I really appreciate the time and effort and even more so the wealth of information you offered.

I don't think I can reply to each post individually so I just wanted to say thanks and hope you give me a some time to digest it all.

Couple of things I can comment on.
I will def have at least a 24 volt DC system.
The batteries I got didnt cost me a nickel. They were from a battery back up oil system for a large steam turbine generator. They rarely cycled...so I was hoping they will be good to start with.
I will definitely by a quality mppt charger and a quality sine wave inverter. And I will take the battery recomendations with me also when I change them.

I would like to wire the 120V electrical panel in the cottage with a transfer switch that allows me to run directly from generator to panel OR from inverter to panel. That way if my solar side fails for whatever reason, I can run directly from generator.

Im thinking of 600W of panels only out of inexperience. If I find I need more, I will add.

Ok...now I will reread everything again and again and again