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Help! Putting together a new system...

Kyfarmgirl

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I am trying to put together a solar system for a fully off-grid cabin on my farm. I have Will’s book (and have worked my way through it) as well as watching numerous videos and reading more books. I live in a rural area and the closest supplier/installer is 1.5 hours away (and they really focus on large/commercial builds). I am pretty handy but.....

The system will be totally off grid. I plan to run a refrigerator/freezer that is a new model ac (unless I use an older camper unit), a DC air conditioner, water pump, propane H2O water heater, LED lights, TV/phone/laptop/speaker charging, cell signal booster, and occasionally a whole house fan. Heat is wood.

I currently have the following:
10) 250 watt used Canadian solar panels
1) 12000 BTU 48 volt DC air conditioner.
1) Giandel power inverter (2000/4000): not sure if this will be big enough?

I have several lead acid marine batteries but hope to switch to lithium ASAP. Maybe build my own power wall?

Questions:
1. Do I need to use all 10 solar panels or could I get by with 8 (saving the other two for additional projects).

2. What would be the best solar panel array? 4 or 5in series, 4 or 5 in parallel or...?

3. What size MPPT controller do I need? I have done the math but am still really unsure.

4. Do you recommend roof mounting (roof is southwest facing and the pitch is near perfect) or a ground mount? I know it is a personal preference but I could go either way. Just can’t decide.

5. Any special considerations with a 48 volt system. Most everything deals with 12 volt or 24 volt.
 
Ten solar panels is an inflexible number that won't be easy to wire. About the only configuration you can safely wire them would be 2S5P, and it they are putting out 60VDC, they might not have enough voltage to fully charge your battery bank. Wired in 5S2P, you are looking at an Voc of around 187V, and maybe 200+Voc when the temperature drops down to freezing.

A 3S3P configuration would work well, with a Vmp of around 90V and Voc around 120V at freezing. You can't use the cheapest Epever controllers with the 100Voc limit, but they make a nicer higher capacity model, the Tracer 5415 and 5420 for about 100$ more. Assuming you are bulk charging around 50VDC, the math would be (250W panel X 9 panels)/50V charging = 45A. Assuming you get some loss and you use a 85% fudgefactor, that's 45A X 0.85FF = 38A. The Tracer 50A 5415 would handle that just fine.

Could you get another 2 panels? With 12, you would get [(250W X 12 panels)/50V] X .85FF = 51A. Look at the 60A Tracer (6420AN). You could wire them in either 3S4P or 4S3P, with the 200V controller.

I've made both roof mounts, and rotating ground mounts, and I'm partial to the ground mounts. I made mine out of welded unistruts. Here's a pic of one of my mounts that would give you an idea of the contruction. If you position the panels in landscape orientation, you could fit 6 panels on one mount. With 12 panels you can accomodate everything on two arrays. I rotate them east in the morning and west in the afternoon when I am running my 240VAC well-pump.

Is you Giandel inverter the modified sine-wave one? Personally, I would get rid of it. It's a low-budget economy brand that I wouldn't touch. Here is the 48V version of the model I myself am using. Please note that this unit is designed to be hard-wired directly into your mail electrical panel, so it won't have NEMA sockets. The best inverters are designed to be hard-wired. This inverter has a built in charger function, which means you can directly connect a generator to it's ACin terminals, and directly charge your batteries with the generator in stormy weather. It's also split-phase 120/240VAC.

Marine batteries are not the best choice. They are known as a combination battery that is for both engine starting, and some backup, but is optimal for neither. I would take a serious look at 6V Trojan L-16s. A somewhat cheaper route would be the 6V golfcart batteries that Costco sells for 99$ right now. https://www.costco.com/interstate-6-volt-golf-cart-battery.product.100476406.html
Two parallel strings of these Costco batteries would be about equal to one string of L-16s.

In terms of available power, that's hard to answer for YOUR needs. At my own cabin, with typical daytime loads, I might run 3.0kWh of power per day. That skyrockets to 20+kWh per day on days I am pumping water.

With nine panels, you are likely to produce 6-7 kWh of power in the winter, and maybe 10+ kWh in summer. In addition to running your daily loads, another thing you should factor in is your days of autonomy. That is, how long your battery will last with NO incoming solar. This is to account for stormy weather when the panels might be snow covered or really cloudy.
 

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Thank you so much for all of the great information!!! I am researching and configuring.



your solar panel mounts look great!!!! I would love to ground mount but I have cattle and do worry that they might get out (fence would be 20 yards away) and they would head straight over and start rubbing/scratching it over/step on panels.

I really like the all in one inverter also and thanks so much for the battery tip!
 
The main source of power consumption would be the DC mini split air conditioner (it is DC only), which would primarily be needed on sunny days. The water pump would be pumping water from a cistern for showers/dishwashing etc. the other main user would be the fridge/freezer. I have estimated 2kwh need for everything (not including the air conditioner). The air conditioner is harder to estimate and he could limit use (ie keep the house a little warmer) if draw was too great. Right now he is running the air conditioner (using a generator) for 2 to 5 hours a day in the summer. The mini split air conditioner is larger and more efficient so I estimate 650w x 5 hours = 3250w.

All of my math is being based on a 5k - 6 k system. My panels are rated at 250 watts but seem to be throwing just over 310 watts? Do I need to use 310 in my calculations?
 
I really like the all in one inverter also and thanks so much for the battery tip!
BTW, the Conext is NOT an AiO. It's a standard off-grid low-frequency inverter that needs a separate charge controller to charge the batteries. The built in charger I was referring to was the alternative charger when there's no solar coming in. I coupled mine to a Midnight 200 controller. I am very happy with it though. I'm using this for my workshop and wanted it for running my power tools.
 
All of my math is being based on a 5k - 6 k system. My panels are rated at 250 watts but seem to be throwing just over 310 watts? Do I need to use 310 in my calculations?
How are you coming up with the 310 number? Are you measuring the voltage of the panel while disconnected? If so, that is the Voc, which is the voltage when NO power is flowing. As a general rule of thumb, what the panels put out at noon under real sun is typically only 85-90% what the nameplate power is. That is because the panels are rated in a controlled temperature chamber with standardized artifical sunlight at exactly 1000W per meter squared.

What every solar producer should have is a clamp meter like this one.
You clamp the jaws around one (not both) wires, and you can read either DC or AC amperage. Then you have real-world reading of what exactly you are producing or consuming. This particular meter can also read AC inrush current, which is the surge spike when a motor driven unit turns on. The starting surge of my well-pump for example is 3.8X the running amperage. A cheaper "running only" meter is not fast enough to capture the inrush.

BTW, the neighbor's cattle occasionally get on my land, and they've never gone near the panels.
 
I am ordering the clamp meter right now! So much great info. Thank you
Here's a cheaper version that'll do the trick too. Clamp meters are extremely handy to figure out what's going on with your system.

All of my math is being based on a 5k - 6 k system. My panels are rated at 250 watts but seem to be throwing just over 310 watts? Do I need to use 310 in my calculations?
The only way you can measure the actual output from a panel is via a dedicated tester, using a decent MPPT with a display or app, or by manually measuring both voltage and current at the same time, and then do the math. A clamp meter makes that a lot easier to do.

If you measure Voc and Isc independently, the only thing you do is find out how far off the spec the panels are. A significantly lowered Voc or Isc on a clear, sunny day generally means something is wrong with the panel.
 
I have looked at the aio unit attached. Any thoughts? Again this is a 48 volt system.

I am still having a lot of trouble drawing out a 9 panel 3s3p configuration for my panels as suggested. my brain is having a lot of trouble. My panels have a VOC rating of 37.4 with an ISC of 9. I took a pic of specs and added it below.
 

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Looking at the specifications of this system, I see one glaring omission, the time the unit can surge. A low-frequency inverter can typically surge for 5-60 seconds to about 200% of rated capacity. A lot of high-frequency inverters can also surge to 200% but only in the range of 8-20 milliseconds. The fact that there's no time listed for the surge makes me suspect that the manufacturer purposely left it out because it is less than stellar. Maybe they just left it out, or maybe they don't want you to know till after you've paid your money?

In the real-world, what that means is that these high-frequency inverters can not start motor-driven equipment like circular saws, refrigerators, air-conditioners, and well-pumps that typically have high starting surges. That's why I recommended the clamp meter I mentioned above. It is designed to measure starting surge. Without that, you are just guessing.

What I would suggest is seek out other individuals here that have this or that inverter and find out first-hand what they have and have not started with their inverters.

In terms of wiring your panels, keep in mind that voltage goes up as the temperatures go down. Your panel has a Voc of 37.5 at room temp, but more like 40.0Voc at freezing. What are your winter lows like at your location? Use this convenient solar string calculator to predict how high your January voltage will go.
 
FWIW if the Giandel isn’t a “true sine” inverter I wouldn’t use it. Get a pure sine inverter
It’s not the Giandel brand I’m opposed to (I’ve been using a 1200W for three years+) but the modified sine annoys some electronics, kills refrigerators, and can burn up TVs and motors.
 
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