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Old 09-15-2021, 10:37 PM   #1
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Solar newb: choosing MPPT?

Hey all, in the process of finalizing my solar setup. Right now I'm focused on the panels/MPPT/battery bank as it seems to be the biggest chunk of the whole setup.

I'm leaning towards 8 x 250 watt - 10 amp - 24v panels =2000watts

and 4 to 6 of the 100ah "Ampere Time" batteries in a 24v bank

Can anyone recommend an MPPT that would fit nicely? I've read everything i can find but I just get more lost as I go, I'm leaning towards the Victron 150/70 based on the math I have so far but it doesn't seem right. 2000 might also be "too much" solar but its really cheap for what I found

I'm told that to find amps for MPPT you divide 2000watts by 28.8 (for 24v) and I get 69 amps. I really can't find anything about whether the panels in series or parallel would help the situation.

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Old 09-15-2021, 10:52 PM   #2
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Is your system (battery) voltage going to be 24V nominal?

If 12V then you'll need two of those Victron.

Higher voltage panels will get you better conversion efficiency

and 1 SC per panel really helps optimize for partial shading.

So e.g. 75/15 model paired with 240-280W panels between 40 and 70Voc
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Old 09-15-2021, 10:59 PM   #3
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and 1 SC per panel really helps optimize for partial shading.

So e.g. 75/15 model paired with 240-280W panels between 40 and 70Voc
I think the battery bank will be in 24v

do you mean 1 MPPT per panel? like, 8 MPPTs?

the seller says the panels are rated at 24v. (VOC): 37.3 V - (VMP): 30.1 V
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Old 09-15-2021, 11:08 PM   #4
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While johnís right about 1 charge controller per panel helping a bit with shading youíll do just fine if you hook all the panels up in parallel.

Series connected panels will degrade the whole array if one gets shaded but parallel connected panels donít suffer the same fate. Youíll need a trunk wire that can handle 80 amps, though, (or 2 40a cables) so the cabling can be more expensive.

Victron makes great stuff, very hard to argue with that selection. You can find cheaper units but I lean towards victron myself.
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Old 09-15-2021, 11:13 PM   #5
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I, personally, don't see much benefit to having a 24V system in a 12V bus. In the off-grid systems I designed and installed, we always went with 56V systems to keep the current lower...which saved money on wire size. But in a bus, where your wire runs are relatively short, I don't recommend a higher voltage than 12V. Having a 12V system means you don't need a converter to run your numerous 12V appliances. And in a pinch, you can get a 12V battery charger at any Walmart or auto parts store if you've got shore power and your expensive inverter/charger combo takes a dump. And any truck stop has 12V inverters you could use in a pinch. Just a thought, if you're not locked in yet! I like simplicity and redundancy...
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Old 09-15-2021, 11:15 PM   #6
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While johnís right about 1 charge controller per panel helping a bit with shading youíll do just fine if you hook all the panels up in parallel.

Series connected panels will degrade the whole array if one gets shaded but parallel connected panels donít suffer the same fate. Youíll need a trunk wire that can handle 80 amps, though, (or 2 40a cables) so the cabling can be more expensive.

Victron makes great stuff, very hard to argue with that selection. You can find cheaper units but I lean towards victron myself.
parallel it is! I heard that series charges quicker but safety is the name of the game for me. Thanks for helping me decide.
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Old 09-15-2021, 11:20 PM   #7
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I, personally, don't see much benefit to having a 24V system in a 12V bus. In the off-grid systems I designed and installed, we always went with 56V systems to keep the current lower...which saved money on wire size. But in a bus, where your wire runs are relatively short, I don't recommend a higher voltage than 12V. Having a 12V system means you don't need a converter to run your numerous 12V appliances. And in a pinch, you can get a 12V battery charger at any Walmart or auto parts store if you've got shore power and your expensive inverter/charger combo takes a dump. And any truck stop has 12V inverters you could use in a pinch. Just a thought, if you're not locked in yet! I like simplicity and redundancy...
well i was thinking initially 12v for simplicities sake, but in my limited understanding.. 2000watts of solar in 12v (2000/14.4) would be 138 amps unless I'm wrong. For victrons thats like $1000 for the mppt. Of course I have to get a dc/dc converter but thats not too pricey.
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Old 09-15-2021, 11:25 PM   #8
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well i was thinking initially 12v for simplicities sake, but in my limited understanding.. 2000watts of solar in 12v (2000/14.4) would be 138 amps unless I'm wrong. For victrons thats like $1000 for the mppt. Of course I have to get a dc/dc converter but thats not too pricey.
What would it cost to have two smaller charge controllers instead of one huge one? That would be my preferred route, in keeping with my desire to have redundancy and easy-to-replace components.

My experience was split about 40% in the US and about 60% doing remote village electrification projects. Some of these places required a 4 hour trip down a dirt road, followed by a multi-hour hike. Parts needed to be robust and it was always best to have some redundancy. I try to carry those lessons to my bus systems.
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Old 09-15-2021, 11:27 PM   #9
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soooo what do you guys think? if my math is correct then 8 panels (parallel) 2000watts.. 24v bank....69 amps

Should I go victron 150/70? Or do I need to go bigger?
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Old 09-15-2021, 11:32 PM   #10
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What would it cost to have two smaller charge controllers instead of one huge one? That would be my preferred route, in keeping with my desire to have redundancy and easy-to-replace components.

My experience was split about 40% in the US and about 60% doing remote village electrification projects. Some of these places required a 4 hour trip down a dirt road, followed by a multi-hour hike. Parts needed to be robust and it was always best to have some redundancy. I try to carry those lessons to my bus systems.
well all i can find to split the difference in half (i think thats what you mean) is the victron 100/50, which is only slightly more expensive.

is that right though? you would just split it down the middle? because it does give a little extra head room. with one its 70 out of 70, with two its 70 out of 100
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Old 09-16-2021, 09:57 AM   #11
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Quote:
Originally Posted by IC08 View Post
soooo what do you guys think? if my math is correct then 8 panels (parallel) 2000watts.. 24v bank....69 amps

Should I go victron 150/70? Or do I need to go bigger?

Charge controllers are one of the main reasons to go 24, and honestly I wouldnít consider 12v at this point unless the system is small. There are very few things that you cant find in 24v and the wiring advantage for every device is significant when added up.

With flat panels youíll never reach peak current so I think youíll be fine with 70a controller.
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Old 09-16-2021, 10:15 AM   #12
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I appreciate this thread IC08. I didn't mount my solar on my bus this year but I'm planning to put together a test system on my three season patio over the winter so your questions are directly relevant to me, I've also been puzzling over the charge controllers and voltage for the system (12v vs. 24v).

I think I'll make my decision about refrigeration and buy so I'll have a major appliance that will be on the bus to put a load on the system.
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Old 09-16-2021, 10:30 AM   #13
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With flat panels youíll never reach peak current so I think youíll be fine with 70a controller.
You're really helping me put this together! I hate to ask, but what if I did somehow reach peak current? I read once the MPPT just stops the extra current from coming through so it doesn't cause any damage.. is that true in your experience?
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Old 09-16-2021, 10:38 AM   #14
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I appreciate this thread IC08. I didn't mount my solar on my bus this year but I'm planning to put together a test system on my three season patio over the winter so your questions are directly relevant to me, I've also been puzzling over the charge controllers and voltage for the system (12v vs. 24v).

I think I'll make my decision about refrigeration and buy so I'll have a major appliance that will be on the bus to put a load on the system.
Let us know how it goes, as far as I can tell my main benefit from going 24v is so I can have a higher wattage solar panel system at lower amperage. I went with the recpro 12v fridge so I'll have to convert it back down but now that I hear there are 24v fridges I think I could've gone that route and it would've been a bit easier.

They say 24v is a little more efficient, but I think I'll lose that efficiency through my dc/dc converter anyways. I can't find that many 24v components like inverters except the (very expensive) victron multiplus so there's that to consider. Its hard to tell what would be better, it would be cool to see what you end up with
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Old 09-16-2021, 11:11 AM   #15
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... as far as I can tell my main benefit from going 24v is so I can have a higher wattage solar panel system at lower amperage.
Keep in mind that the input voltage to the charge controller, from the panel array, is independent of the battery/system voltage. Having a 24V system, versus a 12V system, has no impact on the input from the solar panels...since those could be anywhere from 30-60+ volts depending upon your configuration.

And, my experience with every charge controller I've used is that they will limit the current and not just burn up...so, I agree with rffffffffffffffffff (how many fs are correct?). If you design for 80% of the time, you're probably just fine.
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Old 09-16-2021, 11:31 AM   #16
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There are a LOT of opinions out there on solar, and IMO a lot of people overthink it (Victron everything is really expensive for a system that isn't life-critical), or underthink it ("I'll just get as many panels as I can fit and a couple batteries").


I haven't finished my build and I'm not an established installer or anything, but I think it's important to consider your budget, then see what balance of panels:equipment:batteries you can achieve. If you have 8 panels but don't have enough batteries to store all that energy, you're not really any better off. Panels are cheap, batteries...not so much. I haven't reached the point where I'm ready to install panels, but I bought a controller that handles 30 amps, and two 200 AH AGM batteries to get started; I'll charge via shore or alternator for the time being. Once I get a feel for how that performs, I can make a better decision on how to move forward.
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Old 09-16-2021, 12:33 PM   #17
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Keep in mind that the input voltage to the charge controller, from the panel array, is independent of the battery/system voltage. Having a 24V system, versus a 12V system, has no impact on the input from the solar panels...since those could be anywhere from 30-60+ volts depending upon your configuration.

And, my experience with every charge controller I've used is that they will limit the current and not just burn up...so, I agree with rffffffffffffffffff (how many fs are correct?). If you design for 80% of the time, you're probably just fine.
I am just lost then, from what I hear you take total wattage of panels divided by battery bank voltage to find amperage and therefore mppt size. I hate to be so helpless as I know everyone will just say do more research but the more I look the more it seems to be different for everybody. Can anyone recommend a good place to find the math and all that for sizing? The best I've found is
https://www.mobile-solarpower.com but he focuses more on "buy this and then that" and isn't very specific on the actual science of it all (unless you buy his book I guess)

And thanks for that about the charge controller, another question answered!
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Old 09-16-2021, 12:39 PM   #18
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Originally Posted by dbsoundman View Post
There are a LOT of opinions out there on solar, and IMO a lot of people overthink it (Victron everything is really expensive for a system that isn't life-critical), or underthink it ("I'll just get as many panels as I can fit and a couple batteries").


I haven't finished my build and I'm not an established installer or anything, but I think it's important to consider your budget, then see what balance of panels:equipment:batteries you can achieve. If you have 8 panels but don't have enough batteries to store all that energy, you're not really any better off. Panels are cheap, batteries...not so much. I haven't reached the point where I'm ready to install panels, but I bought a controller that handles 30 amps, and two 200 AH AGM batteries to get started; I'll charge via shore or alternator for the time being. Once I get a feel for how that performs, I can make a better decision on how to move forward.
well from what I can find, 100 watts panels charges approximately 30ah a day. So I figured 1500-2000 watts would be 450-600 ah per day. My thoughts were that in winter and cloudy days if I was close to my total ah (400-600ah) that I could be sure to be able to charge fully no matter what.

Learning as you go and testing is what I wanted to do, but I've heard that you can't add batteries later on because they will be "on different cycles" or something. Its putting a lot of pressure on me to try to get it right the first time, but I really don't know whats true anymore lol
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Old 09-16-2021, 01:37 PM   #19
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Hey if anyone is still out there I think I finally have it down.. maybe.

My main issue now is simply this: Does the size of my panels affect the inverter/charger?

like, do I need a certain amount on my MPPT to "fit" my inverter charger?

If I'm not properly explaining this I'm sorry. I found a nice inverter/charger that seems to be about what I need but it mentions "Max pv input and max pv charging" If you check the link just look at the 6th or so picture it has a diagram

https://www.amazon.com/gp/product/B0...LFG1JAJEM&th=1

Thanks to everyone who's helping I'm so close to understanding this whole thing
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Old 09-16-2021, 03:53 PM   #20
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As a HAM radio operator, test administrator, and instructor, I find that one of the most challenging things for people to understand is the relationship between AMPS, VOLTS, RESISTANCE, CURRENT, WATTS, POWER, OHMS, etc.
SO
AMPS & CURRENT are the same and are represented by "I"VOLTS are the electromotive force and represented by "E"

RESISTANCE & OHMS are the same and represented by "R"
WATTS & POWER are the same and represented by "W"


The FORMULAS needed are E = I * R & P = I * E
Here in the 21st century, one need not break out the pencil and paper nor even a calculator as there are APPS for your smart phone that will do all the calculations for both of those formulae with your simply entering ANY TWO values.
The APP I use is titled "Ohm's Law", it is free for the Apple line and probably available for that other line from the company whose name I won't say.


The advantage of higher voltage systems is that ability to reduce wire size which has several advantages. Less copper equals less cost and less weight as well as smaller diameter wires which are more easily worked with in tight spaces and require smaller holes. Higher voltages reduce line losses by reducing the amperage required thus reducing heat production which is lost power.
There is a balancing act to perform. You probably want some redundancy in your system. You probably want to be able to find repair parts easily. And we all want to keep costs down.
Remember that we're usually only talking about the system from the PV panels to the battery bank with 12VDC the generally used voltage for DC lines.

Designing two (or more) parallel panel arrays protects against the potential of damage to a series array or failure of a controller taking out all of your PV production. If you're building a weekender or short trip rig, it's probably not a big deal. If you're building a full time live aboard, it is a much higher concern.



Finally, without knowing the voltage talking about AMPS (and vice versa) is meaningless unless talking about a specific system with a known (to all) voltage or amp rating.
100 amps @ 12VDC = 1.2kW
100 amps @ 24VDC = 2.4kW
100 amps @ 48VDC = 4.8kW
200 amps @ 24VDC = 4.8kW
400 amps @ 12VDC = 4.8kW



And finally (I lied above)... in calculating electrical needs, all draws need to be converted to WATTS. Be sure to include inefficiencies in all devices (inverters, converters, etc.) as well as all parasitic (the power used if plugged in even when not in use) draws in things such as appliances, USB ports, clocks, etc.
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