Well finally got the cabin now I need power lol looking for a small off the grid system

dmanjay · October 2014

Well I finally got my small cabin located in the middle of nowhere (the best place ever lol) here in NE PA and I'm looking to get an off the grid set up. I have an idea of what I need for power:

These are watts per day (avgs of course)
Small fridge= 950
Lights= 325
TV= 200
PS3=200
total= 1675

I'm thinking at going with a 12v system then switching to 24v later on and expanding, but here's what i'm looking at right now

2-100 watt poly panels (read some where they're better for places with mostly partly cloudy skies instead alot of direct sun)
40a MPPT charge controller
2- 200ah 12v eep cell sealed batteries

now i'm not sure what size inverter i should get thinking maybe a 3000 watt 6000 peak.

Let me know if i'm near what i need to be, now alot of this stuff won't be run all the time except the fridge. I'd only be there maybe 12 hrs a day total which 8-9 i'll be sleeping lol. Any help/input would be much appreciated!!

Mountain Don · October 2014

Re: Well finally got the cabin now I need power lol looking for a small off the grid sys

Am I correct in interpreting those watt values to actually be watt-hours?

I always wonder why anyone considers building a 12 volt battery system with an eye to going to 24 volts in the future. That guarantees the need to buy a second inverter. Better to put the money into a quality pure sine wave inverter that runs off 24 VDC and size the battery bank appropriately, IMO. Pure sine wave because the refrigerator motor will like that better. Depending on the charge controller, many can be used on a 12, 24 or even 48 volt system. But inverters have a fixed input voltage.

Look at the maximum watts that could be consumed at any one point in the day, not the daily watt hours total for the day, when sizing an inverter. Look at the daily watt hours total when sizing the battery bank. I doubt there is any reason to use a 3000 watt rated inverter with the items you have listed, but that will hinge on the start up loads for the refrigerator. Yes, you need a good sized cushion to permit starting of the refrigerator. If this is a part time use cabin you might be better off with a propane refrigerator. Depends. Then you could probably get away with a 12 VDC system, unless there are other large loads such as a microwave in the future.

Cariboocoot · October 2014

Re: Well finally got the cabin now I need power lol looking for a small off the grid sys

Welcome to the forum.

Okay now before you make the mistake of buying random equipment (which is where you seem to be headed) let's take a critical look at planning.

Basically you're in for a 2 kW hour system, which is quite normal size for small off-grid cabin use.

You do not want a 12 Volt system to begin with. There's just no point. Switching to 24 Volt later will just cost you an extra inverter.

Two 100 Watt panels aren't going to do much for your power needs. Look at it like this: 200 Watts * 4 hours of good sun * 0.52 efficiency = 416 Watt hours AC. Pretty far off the mark.

So let's look at 2kW hours on 24 Volts and see how much battery that needs:
2000 Watt hours / 0.85 efficiency (inverter conversion) = 2352 Watt hours DC / 24 = 100 Amp hours used. Minimum battery bank: 200 Amp hours @ 24 Volts. Your two 200 Amp hour 12 Volt batteries (if you can find such a think) would just meet this. What you can find (inexpensively) are GC2 6 Volt 220 Amp hour batteries. Four of them gets you 220 Amp hours @ 24 Volts, and with a little luck of some daytime load running should work - especially if you 1675 Watt hour estimate is accurate.

Now when it comes to charging that much battery if you use the peak current rule-of-thumb you'll see you need a 30 Amp controller and about 686 Watts of PV. As I said far off the 200 Watt mark. At that point you're looking at the cheaper-per-Watt GT style panels & a MidNite Kid 30 Amp MPPT controller.

Your inverter size is based on the maximum power draw at any one time. The biggest demand here is likely to be refrigerator start surge + whatever else is running. A good 2kW inverter ought to handle it. If the weather is inclined to be inclement you may want to go whole-hog on an inverter-generator (keep future expansion in mind when considering this) or else take the less expensive stand-alone charger route. Do not skip the generator unless you don't care what happens should the power fail.

mike95490 · October 2014

Re: Well finally got the cabin now I need power lol looking for a small off the grid sys

welcome.

12V systems are a bit marginal, for the surge needed to start a fridge. It has to be just right. So, maybe look at a 24V system from the start ?

200Ah @ 24V = 4800watt hours - same as the 2 batteries in parallel, but less copper wire (smaller gauge) needed. And AGM batteries in parallel is asking for trouble....

I would think a 1,000 24V inverter would be able to start the fridge, once it's running, it would only draw about 130watts.

And we are talking pure sine wave here, otherwise, the fridge motor will complain and heat up more than normal.

200w of pv is marginal I think

Well finally got the cabin now I need power lol looking for a small off the grid system

Comments

Categories