Author Topic: Off Grid 1500W per day 12v system  (Read 628 times)


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Off Grid 1500W per day 12v system
« on: April 15, 2019, 05:34:29 PM »
I was recently given the following equipment that was removed from an RV -
2 - Xantrex Freedom SW 815-3012 inverter/chargers (3 years old)
2 - Xantrex TrueCharge2 Series 60amp battery chargers (3 years old)
4 - 8275 series 12 volt GEL Batteries that have a 20 Hr. rate /225Ah (13 months old)
I also have the remote SCP panels that operate the devices.

I have an off-grid cabin (no power currently) in southern KY that I would love to have power in.
From what I've learned, a 12 volt system is better suited to mobile, RV and marine. Higher voltage setups (24/36/48v) are much preferred for things like cabins.
With that said, I considered selling these inverters and chargers and using the proceeds to by a higher voltage system. I gave eBay and Craigslist a shot but no luck. I got a couple of crazy low offers but nothing I wanted to consider.

If I don't sell the inverters, I think I will just use one in the cabin.
Using calculators and research from other sites and help from other folks on different forums, I have come up with the following system requirements:
A 1500W per day usage - adjusted up using a 1.5 fudge factor gives me a 2.25KW adjusted per day usage.
For 2.5 days of storage, factoring in 50% DOD, I come up with an adjusted 11.25KWh reserve.
Because I have a 12 volt inverter, my calculations tell me I need 930+ AHs of storage (I have 900AHs with my 4 batteries).
Using 900 watts of panels, I calculate that I need a CC that can handle 75 amps. I think the Midnite Classic 150 should do the job.
(some of you may have seen a similar post to this on some other forums. Some of the good folks on those forums led me to this one...)
I have attached a simple line diagram that I'm considering.
« Last Edit: April 15, 2019, 05:36:07 PM by MrDjango »


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Re: Off Grid 1500W per day 12v system
« Reply #1 on: April 15, 2019, 09:32:23 PM »
I would use what you have.
Nothing wrong with 12v  - the main reasons to go higher voltage is the wiring size is smaller as are breaker sizes. It is also easier to have larger battery banks with higher voltage.

Not sure if the wiring  panel you have has the breakers already but if not consider getting a Midnite Mini DC box - it comes with big breaker for the inverter and also has spots for the PV and Battery breakers and another spot for DC loads ( you will need to use the larger breakers with the studs if you go with that instead of the din rail breakers and then that box only has three breakers plus the inverter breaker.  It also has the place to put the shunt and Whizbang - this box keeps all the wiring short and easy to follow the schematic that it comes with. The  Classic will mount to the top knockout .
I would also recommend getting a Midnite combiner box like the PV6 size so you can add more PV in the future if you want - and having breakers out near the PV makes switching them off easy and convenient.
Wire your 12v batteries in parallel carefully so they are more balanced to share charging and loads.
Look up the midnite string calculator to see which way to configure your PV for the Classic is best. Usually the closer you keep the PV voltage to the battery voltage the more efficient the Classic will run . But if you need to go series on the PV to use smaller wire that is no problem either .

system one
Classic 150 , 5s4p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system two
Classic 150 ,5s 135 watt Kyocero , 16s Calb 100AH LiFePO4 , Outback VFX 3648 inverter
system three
Midnite KID MPPT 24 DC in to  12 volt AGM batterie


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Re: Off Grid 1500W per day 12v system
« Reply #2 on: April 16, 2019, 11:37:36 AM »
Thanks Larry.
"Method 4" of the Smart Gauge" site is exactly how I was planning to wire the batteries.
I used the "string calculator" and it recommends the Classic 150 as the best option. I don't have a panel box yet so I will look at the Midnite Mini DC box.

Thanks again!


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Re: Off Grid 1500W per day 12v system
« Reply #3 on: April 19, 2019, 08:13:56 AM »
The lower voltage of a 12 volt system exists fine where wire/cable lengths are within a (large) vehical footprint.
However, if removed outside of that confine and wire/cable lengths are increased, one must pay attention to voltage drop.
Big or over-sized wire can help...but there are limitations, as the voltage (12) is similar to pressure, and can be hindered quite quickly if asked to go extra distances. Every layout is different, so keep this in mind.

There exits charts and tables that spell out 12 VDC voltage drop by distace and conductor size, No distance or sizes have referenced yet, pay attention to these given values for no regrets...


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Re: Off Grid 1500W per day 12v system
« Reply #4 on: April 19, 2019, 11:00:47 AM »
Thanks for the feedback.

My intention is to design the system so that the 12 volt cables running from the batteries to the inverter should only be about 30" long.
I was going to run 2/0 cable (because I have some) through a 200 amp 12v switched breaker from the batteries to the inverter. 
All the cables that connect the batteries together are 4/0. They came with the system.

I will only be using about half the power of the 3000 watt inverter so I feel like the 2/0 200amp setup should be plenty (keep in mind that this is a small hunting cabin with no large wattage appliances).


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Re: Off Grid 1500W per day 12v system
« Reply #5 on: April 20, 2019, 09:58:21 AM »
Good that you have all that covered.

How about the solar panels? Roof or ground mounted?
No shading?
What is calculated voltage of panel arrangement to push current flow through he charge controller? voltage drop counts here too.                         


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Re: Off Grid 1500W per day 12v system
« Reply #6 on: April 22, 2019, 08:51:51 AM »
My plan is to use 3 roof mounted panels in parallel that total somewhere in the vicinity of 900 watts. No shading.
I've attached a wiring diagram that I mocked up using a simple line drawing tool in Excel.
Here is the info that was calculated using the "Classic sizing tool" -
Power 310 Watts
VOC 39.6 Volts
VMP 32.67 Volts
ISC 9.98 Amps
IMP 9.5 Amps
VOC Temp Coef % 0.37 C
VMP Temp Coef % 0.4
Amps Environmental Data
Coldest Ambient Temperature -0 F
Hottest Ambient Temperature 104 F
Nominal Battery Volts 12 Volts
PV Array
Number Of Modules In Series 1
Number Of Parallel Strings 3
Total Modules 3
Rated PV Array Power 930 Watts
Anticipated Array Power @ 104 F 874
Watts Rated PV Array Current 28.5 Amps
Battery Charging Current @ 14.4V 64.6 Amps
VMP (Maximum Power Point Voltage 32.67 Volts
VOC (Open Circuit Voltage) 39.6 Volts
VMP @ -0 F 38.3 Volts
VOC @ -0 F 45.9 Volts


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Re: Off Grid 1500W per day 12v system
« Reply #7 on: May 02, 2019, 11:03:37 AM »
Looks good Steve, that should work just right.