Off-grid Yurt Solar Setup

bryanthebold · May 2013

Hi everyone,

I've been lurking for a while, learning what I can. I am designing an off grid system for a yurt located in southern Utah. We have minimal power needs; lights, music, small appliance, and maybe fridge.

I calculated my needs to be about 44ah 528 watt a day MAX, and that's with foreseeable improvements added. (24 ah's max from fridge, 10 ah from lights 10 hours a day, 10 ah music 5 hours a day) I doubt we'd be running it that intensely except a few days a year. Currently all our needs are DC so I don't think we will have an inverter except maybe a small cheap one for occasional jobs. We also have an 8000 watt gasoline generator if we want to run power tools and such.

Starting with panels, I am drawn towards the DM Solar two packs from Amazon because they are cheap. Either that or two maybe three 100 watt panels from whoever.
DM Solar 145 watt x2
Vmp 18.7 V
Imp 7.75 A
Voc 22.3 V
Isc 8.37 A
$230 + a lot to ship, still cheap though!

Charge controller. Me like everyone else is drawn towards the MorningStar Sunsaver MPPT, but I have some questions...
The DM solar panels would max 290 watts into a 12 v battery system, does that mean the sunsaver is still maxed out to 200 watts and the other 90 is wasted? I realize that maximum output for panels isn't that common but considering the location and weather of the yurt, I think its quite a bit more likely that these will be running at full sun a lot. I could grab 2x 100 watt panels and stay within the rating. I also have considered going to a 24 v battery bank but since we run mostly 12 v I worry we may waste a lot of power stepping it down.

With the sunsaver I am assuming I should wire in series and up the voltage... With the DM solar it would be ~45 V input at 8.37 A. Sounds great. I haven't experienced the magic of MPPT but I am guessing the amps will be higher than 8 A at max sun? If I had 100 watt panels I would still want them in series correct?

My other option is to get a Morningstar ProStar 30 with meter and call it good. It would be much easier to modify my system with, allow for a bit more expansion, and nice meter to see whats up.

Battery wise I was thinking that around 200 aH would be sufficient. I may try to stay with sealed batteries as we often are gone for months at a time and I might not have time to perform maintenance. Will a battery temp cord work with sealed batteries?

Random stuff:
For grounding... I need to make a ground and run it from the panels frame to a grounding rod, but how do I ground inside the yurt? the actual wiring I guess

I want to mount the panels on top of the lattice seen in front with an adjustable angle.

I doubt we could utilize the generator to do much efficient charging as its about 50 ft away and too far for DC. You could use a high capacity battery charger and run a long AC stretch though? If the charger works at 15 A would that work with the AC run? That would make up some of the efficiency. I am referring to a standard automotive type battery charger.

Thank you for entertaining my pure nOOb mind. I took physics in college so I get most of the things but I have never been an electrician before. Its fun though. Here are some photos of the area to perk your curiosity.

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Cariboocoot · May 2013

Re: Off-grid Yurt Solar Setup

Welcome to the forum.

Would you believe you are about the third solar yurt project to come to the forum?

So let's see what we can sort out (caveat; at the moment I'm not the best choice for sorting things out so there may be mistakes made).

You're looking at an all DC 12 Volt system, right?

Okay one trouble spot: "maybe a 'frige". Refrigerators of any kind are big power users, no matter what the manufacturer claims. Check the numbers on this very carefully. You may even have to get it, run it through an Ammeter, and see what the results really are. Also know that they will vary from day to day.

Second, let's fix on using Watt hours for consumption. You can use Amp hours @ 12 VDC but it may confuse people if the Voltage is left out.

Next, never mind the panels/controller stuff; first up is battery capacity to supply the loads. Then you get the solar to recharge them. Without at least an estimated daily Watt hour consumption all you're doing is guessing at equipment, and there's a very high probability of guessing wrong.

For example: 200 Amp hour battery. Let's say you use the ol' standard dual GC2's for 220 Amp hours @ 12 Volts. In reality that's about 1200 Watt hours. Is that enough? Don't know; no usage numbers.

Take those 290 Watts of panel: on an MPPT controller they might put out 18 Amps of current max. Except if you put it through an Morningstar 15 Amp controller, then you only get 15 Amps. Either number comes up a bit shy for recharging the 220 Amp hours GC2's: 8% and 6% respectively. But either might work depending on how/when you use the power.

Right now we're all waiting for the introduction of a couple of new MPPT controllers in the 30 Amp range which will be a great solution for people who want to run a couple of GC2's.

The Rogue is due out any day now, but no production schedule on the MidNite as of yet.

But you do need a firm power usage number.

bryanthebold · May 2013

Re: Off-grid Yurt Solar Setup

Yes it would be a 12 v system,

Here is how I have been calculating my daily use...
4x lights at 3 watts each = 12 watts x 10 hours = 120 watts hours
lepai amp at 40 watts x 10 hours = 400 watt hrs
ARB or engel fridge, 50 quartish size. They claim .5-2.5 A draw. So at 2.5 A x 12 v = 30 watt x 24 = 720 watt hours

Total: 1240 watt hours use per day

Hmm... a little math sure does help

Cariboocoot · May 2013

Re: Off-grid Yurt Solar Setup

So you see the problem? At 50% Depth of Discharge the GC2's would just make that if you had a bit more panel and could rely on sunshine every day to supply some of the load.

What you'd really want: 1240 Watts hours / 12 Volts nominal = 104 Amp hours * 4 (for 25% DOD) = 416 Amp hours. Round up: two parallel strings of GC2's for 440 Amp hours @ 12 Volts.

What you'd want to recharge that: 44 Amps @ 12 Volts = 528 Watts, less typical 77% derating = 685 Watt array (round up to nearest available size).

Not saying it can't be done any other way. I just think you may be a bit disappointed with undersizing batteries and array.

mike95490 · May 2013

Re: Off-grid Yurt Solar Setup

Check out the power needed for a full size, Energy Star fridge. They are pricey, but not much more than 750Wh day.

Also, try the ROGUE MPPT controller, http://www.roguepowertech.com/ or go whole hog and get the midnight classic http://midnightsolar.com/

When you add the real power usage, you are looking at least a 24V system.
(4ea, 6V, 200ah batteries in series = 4800wh stored, 1,000 wh useable daily)

bryanthebold · May 2013

Re: Off-grid Yurt Solar Setup

So what I am getting at here is both my battery bank, and panels wont provide enough power for my daily use. I am hoping my estimate is higher than what I will be using. The fridge is going to be a smaller engel which I doubt will use the maximum output all the time.

2 packs of the DM Solar would get me 4 panels at 580 watts total, and I could double the batteries to around 400 aH capacity at 12 V.

The morningstar pro 30 will be over the rating so I think I will go with the Tristar. MPPT being preferred but the cost is considerably higher.

If I went 24 V on the battery bank how would I down convert it for 12 V appliances? Or would be be smarter to throw an inverter on it and run everything off AC?

Cariboocoot · May 2013

Re: Off-grid Yurt Solar Setup

bryanthebold wrote: »

So what I am getting at here is both my battery bank, and panels wont provide enough power for my daily use. I am hoping my estimate is higher than what I will be using. The fridge is going to be a smaller engel which I doubt will use the maximum output all the time.

Don't bet on it. Power usage estimates are almost always low, especially as once you have electricity you tend to use more.

2 packs of the DM Solar would get me 4 panels at 580 watts total, and I could double the batteries to around 400 aH capacity at 12 V.

On PWM controller you will get about 32 Amps max from four of those panels. It would work for 440 Amp hours: about 7% peak rate before loads.

What is the price per Watt on those DM panels now? When you get up above 400 Watts of array it is sometimes cheaper to buy "GT panels" and an MPPT controller; the difference in panel price makes up for the higher cost of MPPT.

The morningstar pro 30 will be over the rating so I think I will go with the Tristar. MPPT being preferred but the cost is considerably higher.

The Prostar 30 would just make it; might run at peak longer though. The thing is it would limit your expansion ability whereas a bigger controller wouldn't.

If I went 24 V on the battery bank how would I down convert it for 12 V appliances? Or would be be smarter to throw an inverter on it and run everything off AC?

You could use a DC to DC converter like this: http://www.solar-electric.com/12to24or24to.html
But only do that if you must have 12 VDC. Frankly, it's much easier to get a suitable inverter and standard buy-them-anywhere 120 VAC devices. Easier on the wire runs too; everything after the inverter is off-the-shelf household equipment.

As a rule I recommend 24 Volt systems for most installs. 12 Volt only if it is necessary (like an RV app) and 48 Volt if you need the big power.

Off-grid Yurt Solar Setup

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