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Please Help - System Basics

Collin E.

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Aug 27, 2020
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Hi - I'm a newbie with no solar experience and looking to get some basics to move my project forward.

I found a deal on the Jinko 315w (JKM315M-60HBL) panel and bought 8 for a tiny home project. However, I don't really know where to go from here. Any help would be much appreciated!

Some specific questions:
* 12v or 24v?
* Charge controller configuration?
* Inverter size?
* Lithium battery bank size?
* Anything else to consider before integration?

System Requirements:
1598537817361.png
 
6.5kWh/day.

How are you heating water?

Note the irony that supposed energy saving devices like ceiling fans are your biggest energy hog. They are an off-grid nightmare. They're great for what they do, but even your 6 hours "on per day" may be low. People just don't turn them off unless they're very disciplined. Note that ceiling fans may use notably different power levels based on size and speed. My small ceiling fan in my RV uses 65W on high.

A "1000W" Microwave uses 1600W of energy.

I'm in AZ, and "peak sun hours" of 6/day isn't something I can count on. Did you get your sun hours from something like link #5 in my sig?

  • For flexibility, 24V is almost mandatory vs. 12V
  • You have 8 panels producing 2520W of peak power. 2520W/28.8V (peak charge voltage) = 87.5A - you need 1 or 2 SCC capable of at least this much current combined. 2X 50A charge controllers or 1X 100A charge controller.
  • Your inverter should be sized for your maximum anticipated concurrent loads AND surge. Your pump and your microwave are your biggest surge devices. A quality 4kW inverter with a GENUINE surge rating (measured in several seconds, not milliseconds) should do it. Assuming you want your morning kettle, microwave and toaster oven all running for breakfast time, and then your water pump comes one and POP! Inverter overloads. If you are conscience about not running higher power items concurrently, you can get away with less. Note that an inverter has a background drain of about 30W (depends on unit and size) continuously, so that affects your energy audit by 0.72kWh in this example (>10% increase).
  • LFP bank for 24 hours would be your daily kWh/0.8 = 8.1kWh
  • Be forward thinking. What do you anticipate using in the future, and can you easily design for that now or upgrade in the future?
 
This is great, thanks for the reply. For more context, this is a weekend tiny home at my ranch in Leakey, Texas on a ridge that receives a lot of sun/no shade. I'll probably use propane to heat water and got my sun hours from a different site, but this is what your link is showing:
1598545455674.png

I also have a 2350W inverter generator and a 4375W generator for tools and other needs.

System Recap:
* 8 panels producing 2520w peak
* 24v system for flexibility
* 100A charge controller capacity (evaluating OOYCYOO)
* 4kW Inverter (any recommendations?)
* 8.1kWh LFP bank (may build DYI bank based on this video to save $$$)
 
Do you intent to lay your panels flat on the ground or tilt for optimal year-round production?

If you plan to tilt them:

1598546120691.png

Now the question becomes, are you okay with under-producing some months, or do you want to be confident that you can supply your power year-round? If year-round, you should use 3.62 in December. If you plan to supplement with generator by default, you can use a higher number.

I'm a bit of a snob when it comes to equipment. I have all Victron stuff, which is spendy. I personally wouldn't consider the lower cost Growatt or MPP Solar stuff for something I want to count on. I'm looking at Victron, Outback, Schneider, Magnum, Xantrex (last choice).

A very flexible unit that also gives the option for 240V split phase is the Magnasine MS4024PAE. My neighbor just bought one to upgrade his old Trace (after 25 years). Had I not already been committed to Victron, I likely would have gone that route.

Same with the charge controller. I'm not trusting a $300 100A unit. I'm going with a Victron 150/100 unit or a midnite solar or outback unit(s).

HOWEVER, if you want to spend less, bumping your bank up to 48V and going with a MPP Solar LV5048 unit, you'd have more power (with less surge), it has a built dual 80A charge controllers for a mega-array, but at 48V, you only need 50A, and it's probably pretty cheap relative to separate high-end units. As I understand, purchasing units from ebay get little to no support. It's important to get it from the official US distributor in UT.
 
I'll definitely tilt them and don't mind some underproduction, given it's a weekend spot. Perhaps I'll be able to use a transfer switch to supplement with the generator. I'm generally a snob too, so appreciate you pointing toward the higher-quality options. I'll do some more research and report back. Thanks again for the help!
 
A benefit of a higher end units (and many of the MPP Solar/Growatt units) is direct feed via generator. You can attach the generator to the AC input, and it will charge the batteries and provide system power.
 
Hi - I'm reviving this thread with my updated plan. I'm going to use a generator for primary power for the foreseeable future to bring my entry costs down, but want to keep a few appliances on a solar circuit while I'm away from the cabin. I understand that I'll spend more in the long run if I have to up-size components later, but I'm trying to protect today's cashflow.

Any advice on this plan is much appreciated!

Load
* Fridge - 353w @ 4hrs = 1412w (assuming the compressor runs < 3.5hrs per day)
* Router - 7w @ 24hrs = 168w
* Wireless Security Cam - 9w @ 24hrs = 216w
* TOTAL LOAD = 1,796wh
* PEAK LOAD = 369w


Battery Bank (24v)
* capacity requirement is 1,796wh x 24v = 74.83wh
* TOTAL BANK CAPACITY @ 80% battery efficiency = 74.83wh x 1.8 = 93.54ah
* Using 2x Interstate deep cycle 12v 110ah batteries in series for 24v 110ah. <--$500

Array
* Using 4x Jinko 315w 120 half-cell panels
* Open Circuit Voltage = 40.7v
* Total Series Voltage = 40.7v x 4 = 162.8v
* Should I configure in series or parallel?
* Peak Watts = 1,260w

Charge Controller

* Sizing is a total mystery to me!

Inverter
* Giandel 24v 1200w Pure Sine Inverter (allows for future expansion) <--$240

Fuse Blocks
* Do I need fuse blocks between components? What size?

Breaker Panel
* Advice on a panel to supplement my main (generator) panel?

Anything else besides wires, connectors, etc.?
 
Check out this page to show you series vs parallel panel wiring. Simply enter the info for your panels.

If you wire 4 panels in series, you will need a charge controller that can handle 162.8V and 10.1 amps
If you wire 4 panels in parallel, you will need a charge controller that can handle 40.7V and 40.4 amps
If you wire 4 panels with 2 in series and 2 in parallel, you will need a charge controller than can handle 81.4V and 20.2 amps
 
I usually try to get as many panels in series to lower the amperage. That allows me to use smaller wiring. For the system you are building, that isn't a big deal. I also know that there are some integrated charge controller/inverters on amazon. I don't know anything about them, but some of them have been reviewed on youtube.
 
capacity requirement is 1,796wh x 24v = 74.83wh
I think for clarity this should say "1,796Wh / 24V = 74.83Ah". Your calculations seem to be fine, just the units are wrong.
TOTAL BANK CAPACITY @ 80% battery efficiency = 74.83wh x 1.8 = 93.54ah
Similarly, this is really 74.83Ah / 0.8 = 93.54Ah. The equation is just incorrect arithmetic and might confuse people trying to learn from what you're doing. Again, it appears you are thinking it through correctly, just want it to be clear to other people.

I like to think about PV energy collection in terms of hours of full-wattage sunlight per day. For example on a nice sunny February day, a friend collected 30kWh in a day at middle-of-USA latitude using his 8.1kW Tesla solar system. This is equivalent to 3.7 hours of sun. You can actually look up your average solar "insolation" as it is called from NREL: if you're in Texas it might be 5.5-6hrs of sun on average. This makes estimates easier because you can say 1.28kW PV capacity * 5 hours/day = 6.4kWh/day, so it would take you only 2.24kWh / 6.4kWh/day = about 35% of a day to fill up your battery, assuming no load, or a bit longer (1280/(1280 - 369) = 41% longer) if there is your peak 369W load also using solar power while it is sunny. So that's half a day. (Someone please check the math). To me this says your battery capacity is too small.

There can also be cloudy days and you probably don't want your fridge to warm up if you have a cloudy weekend, so in addition to sizing for 80% usable battery capacity, you should assume you need to run for more than a day (maybe a couple days) on average to keep the lights on using the battery.

Relatedly, perhaps you can use a duty-cycled chest freezer to make a refrigerator, since this is just a weekend home, and dramatically cut your energy usage, reducing the cost of everything else. Those have thicker insulation than refrigerators and they don't lose as much coldness when you open them because the air doesn't all spill out like a front-opening refrigerator does.

Just some ideas. Let me know what you think.
 
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If you wire 4 panels in series, you will need a charge controller that can handle 162.8V and 10.1 amps
If you wire 4 panels in parallel, you will need a charge controller that can handle 40.7V and 40.4 amps
If you wire 4 panels with 2 in series and 2 in parallel, you will need a charge controller than can handle 81.4V and 20.2 amps
This is not really true so be careful using these numbers blindly.

Yes, it would be 10.1A PV current in series. But MPPTs, for example Victron MPPTs, are specified as PV current AND battery current, and since the battery is always at a lower voltage than the PV array when energy is being collected, in practice this means an MPPT 100|20 can only support 20A of battery charge current or 20A of PV current max (for each one). The battery current is *always* higher since its voltage is always lower when the PV system is charging, for that MPPT.

Your actual peak battery charge current would be PV watts / Battery volts. For example, if 1.28kW PV and 24V battery, following magic8192's estimate would say you need a 10.1A MPPT but really you need 1.28kW / 24V = 53.3A MPPT (nominally), and therefore a bigger MPPT. (Edited a couple times for clarity).
 
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You are correct, that would be amperage/voltage on the solar side of the MPPT, not out of the MPPT.
 
Should I prefer either series over parallel or vice versa? Here are some options below I think might work based on your feedback. If it's all the same I'd likely go with the OOYCOYOO in parallel for no particular reason.
For 4x of the JKM315M-60HBL panel

Voc = 40.7V * (1 + 0.28%/degC * deltaC) = 40.7V + 114mV/degC * deltaC = so if 25C nominal and you want to operate down to -20C, that would be 45C of change in temperature so 5.13V additional open-circuit volts, giving 45.83Voc per panel. Sorry about your recent freeze, by the way, I hope Texas is recovering now.

Isc = 10.1A

Ibat = 1.28kW / 24V = 53.3A (or less).

Based on this I think you could do 4 panels in 2 parallel strings of 2 series panels (2s2p) with a Victron MPPT 100|50. As a matter of fact, that's the exact part I am using in my solar/battery project (but only with 0.84kW of PV), and I happen to know that the SmartSolar one with bluetooth costs $323 right now.
 
Thanks guys, this is exactly the help I was looking for! Ok, so I'll double my battery bank and go with the 100/50 Victron. I updated my plan below to capture corrections but likely missed corrections to the array.....it was above my head. I'll start assembling this in my garage before I take it out to the ranch and report back with any issues. Thanks again.

Load
* Fridge - 353w @ 4hrs = 1412w (assuming the compressor runs < 3.5hrs per day)
* Router - 7w @ 24hrs = 168w
* Wireless Security Cam - 9w @ 24hrs = 216w
* TOTAL LOAD = 1,796wh
* PEAK LOAD = 369w

Battery Bank (24v)

* capacity requirement is 1,796wh x 24v = 74.83ah
* TOTAL BANK CAPACITY @ 80% battery efficiency = 74.83ah / 0.8 = 93.54ah
* Using 4x Interstate deep cycle 12v 110ah batteries in two series strings combined in parallel for 24v 210ah. <--$1000

Array
* Using 4x Jinko 315w 120 half-cell panels
* Open Circuit Voltage = 40.7v
* Total Series Voltage = 40.7v x 4 = 162.8v
* Configured in 2 series strings combined in parallel.
* Peak Watts = 1,260w

Charge Controller

* Victron MPPT 100v/50a <--$325

Inverter
* Giandel 24v 1200w Pure Sine Inverter (allows for future expansion) <--$240

Breaker Panel
* Advice on a panel to supplement my main (generator) panel?
 
Sorry about your recent freeze, by the way, I hope Texas is recovering now.
Thanks...it was pretty bad. Seems silly that cold weather would knock us down like that but it was unprecedented and we were clearly unprepared. We seem to be recovering fine but I'm really missing 2018/2019 right now!
 
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