Question About My Setup

LeoLopez

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I have not installed anything and my questions come due to my limited knowledge of solar power. My family and I are converting a school bus into our home and I am coming close on the electrical setup. We have 4 405W Solar Panels. I have two MPPT 100/50 solar charge controllers from victron because the one solar charge controller from victron big enough for the solar panels was a bit expensive and this other route was at the time a couple hundred cheaper. I am planning on connecting 2 panels per controller and my questions is, do I have to have my solar be a 24v system or can my solar charge controllers sustain me at 12v if I do 2 panels per controller?

Please help,
TIA for any and all guidance!
 

MichaelK

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Will this bus still be mobile, with a running engine? For anything with wheels, sticking with 12V is a compelling reason. Otherwise, I'd be starting at 24V.

Will the panels be laying flat on the roof? If yes, assume they'll never be at an optimal angle towards the sun, and you'll need to downgrade output to 75%. That is, don't expect to get more than 300W out of those 405W panels.

To work the math, that works out to be { (405W X 2 panels)/13V charging } X 0.75 = 46.7A. So, one 50A controller should be able to handle one string of your panels.

With 47A + 47A of charging current, what battery will you be charging? Lead-acid likes 1/8C, so 94A X 8 =750Ah. That's a BIG lead-acid battery. If you made two parallel 12V strings of Trojan L-16 batteries, that would be about the right size. If going the 24V route instead, then four L-16s wired in series for 24V.

For Li, going with charging at 1/4C that would be 94A X 4 = 376Ah. Or, 188Ah at 24V.
 

LeoLopez

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Yes mobile. Yes flat on the roof. For batteries, I am going with 4 200ah LiFePo. So you think that with them flat on the roof, I could get away with 12v?
 

rmaddy

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FYI - you could use one 150/70 charge controller with your 4 panels wired in 2S2P if used with a 24V battery setup.

With a pair of 100/50 charge controllers you need to put each pair of panels in parallel otherwise the Voc will be too high. Two of those panels have the potential to result in over 70A of output current to the battery under ideal conditions with a 12V system. Since the charge controller only supports 50A, the extra amps will essentially go to waste. But as pointed out you won't likely get that full potential with flat mounted panels. But it can happen. I have flat mounted panels on a trailer and there have been a few short occasions where I registered the full power of the panels. With a 24V system you can take full advantage of the panels without anything going to waste.

One other consideration is your inverter. What size inverter are you planning? If it's 3000W then that would be another good reason to go with 24V.
 

LeoLopez

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Thanks so much! All of this is super helpful! I have not yet bought the inverter so I will plan on the 3000w and I'll also purchase a 24v to 12v converter! I was just worried I would fry my chargers with those panels!
 

MichaelK

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Thanks so much! All of this is super helpful! I have not yet bought the inverter so I will plan on the 3000w and I'll also purchase a 24v to 12v converter! I was just worried I would fry my chargers with those panels!
I would NOT recommend you buy a 3000W 12V inverter. I firmly believe that 3000W at 12V is just a marketing scam. In the real-world, you will never actually see an honest 3000W out of a 12V inverter. If you really need 3000W you should be going at least 24V.
 

LeoLopez

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I would NOT recommend you buy a 3000W 12V inverter. I firmly believe that 3000W at 12V is just a marketing scam. In the real-world, you will never actually see an honest 3000W out of a 12V inverter. If you really need 3000W you should be going at least 24V.
I'm confused, once again I'm sorry because I am a noob, but what would I do for my 12v lights and other stuff?
 

MichaelK

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I'm confused, once again I'm sorry because I am a noob, but what would I do for my 12v lights and other stuff?
I didn't say you can't buy a 12V inverter, I said to not buy a 3000W 12V inverter. Look at the fine print. All of these so called 3000W inverters are NOT UL listed. That's because at UL you must demonstrate your equipment can operate safely at specification. To put out a honest 3000W, you need to pull 250amps out of your battery. In the real-world that is not a very realistic number.

Buy a 1000W 12V inverter that's UL listed and you won't go wrong. Just don't expect to run your toaster oven with it. Keep in mind that it costs money to get UL listed. So, you'll pay extra for the UL rated product. Remember, anyone that's marketing something that's not UL listed is cutting corners to make the cheapest product possible. Is that really what you want to buy?

If you want to run really big loads, then go with at least 24V. I like MS's idea of a 24V system with 12V converters. I know they run only 15-20$ on Ebay. That might be the best of both worlds for you. Whatever you design, it's a good idea to limit your amp flow to less than 100A.

I always design things with the 2X rule. That is, don't run your equipment at more than 50% of potential capacity. So, if you need to run a 2000W load, get at least a 4000W inverter. Adhering to this rule has worked well for me over the years. 3X is even more reassuring.
 
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rmaddy

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Thanks so much! All of this is super helpful! I have not yet bought the inverter so I will plan on the 3000w and I'll also purchase a 24v to 12v converter! I was just worried I would fry my chargers with those panels!
This post implies you have chosen to go with a 24V battery bank. But it sort of also implies you've decided on a 3000W inverter just for the sake of getting a 3000W inverter. Have you done a full energy audit of your needs? Figure out your actual DC needs and your actual AC needs. Your AC needs will determine what size inverter you need. Figure out what all of your DC needs are. Then based on the overall needs you can decide whether 12V or 24V makes sense. Then you can decide how much solar you really need and how much battery you really need.

You really shouldn't buy any solar/electrical components until you have a clear understanding of your needs and a clearly defined and designed system. This stuff is too expensive and complicated to start buying without a concrete plan.
 

LeoLopez

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I have calculated my electrical needs and have determined that the batteries should be more than enough. My question was regarding my solar panels and controllers and make sure that it is going to hold up. For the inverter I will need to do some math to know what exactly I need for AC and DC but if I go through the 24v system I will need a 24v to 12v inverter period. I was not stating what size yet. Sorry if I confused anyone. Once again I am starting out and not really hooking anything up yet. I do appreciate the look out though! Thanks!
 

rmaddy

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If you go with a 24V battery bank then you need a 24V inverter no matter how many watts it is and if you have any 12V DC loads then you will need a 24V->12V DC converter for the 12V loads. I used as many 24V components as I could with my 24V system but some things are only available in 12V.
 

wiseacre

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4 405W Solar Panels. I have two MPPT 100/50 solar charge controllers
What is the VoC of the panels?
Some of the larger panels have a higher voltage (saw one today at 50v)
considering a panel's voltage goes up as the temp goes down and you may find even 2 panels in series too close to the SCC's PV input voltage for comfort. post the panel's specs for confirmation

I'd go 24v system. As already mentioned drawing a high wattage to the inverter would mean pulling very high amps from the battery.
3000w is a hefty load

I use a 3500w inverter on a 24v system. The inverter is generally twice the size I need. I'd rather limit the inverter's use to a moderate level insted of continually pushing a smaller unit. Then when I want or need to use a high wattage it is available.

Not recommending but I've been using this for over a year (about 12 hours a day) without so much as a hiccup:
https://www.eco-worthy.com/collecti...d-pure-sine-wave-inverter-12v-24v-48v-to-110v

The only problem with my inverter is - NOT ENOUGH BATTERY for 24 hour constant use
 

Short_Shot

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I didn't say you can't buy a 12V inverter, I said to not buy a 3000W 12V inverter. Look at the fine print. All of these so called 3000W inverters are NOT UL listed. That's because at UL you must demonstrate your equipment can operate safely at specification. To put out a honest 3000W, you need to pull 250amps out of your battery. In the real-world that is not a very realistic number.

Buy a 1000W 12V inverter that's UL listed and you won't go wrong. Just don't expect to run your toaster oven with it. Keep in mind that it costs money to get UL listed. So, you'll pay extra for the UL rated product. Remember, anyone that's marketing something that's not UL listed is cutting corners to make the cheapest product possible. Is that really what you want to buy?

If you want to run really big loads, then go with at least 24V. I like MS's idea of a 24V system with 12V converters. I know they run only 15-20$ on Ebay. That might be the best of both worlds for you. Whatever you design, it's a good idea to limit your amp flow to less than 100A.

I always design things with the 2X rule. That is, don't run your equipment at more than 50% of potential capacity. So, if you need to run a 2000W load, get at least a 4000W inverter. Adhering to this rule has worked well for me over the years. 3X is even more reassuring.
On the whole I agree that you typically don't see 3kw out of most inverters.

However even if you do pull 250a, that's not in itself a problem. 4/0 cable has no problem with 250 amps over shortish runs.

Designing to a 100% safety factor is absurd though. If you buy good quality equipment it will have no issue running at its rated output indefinitely.
 

MichaelK

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Designing to a 100% safety factor is absurd though. If you buy good quality equipment it will have no issue running at its rated output indefinitely.
Sorry, but I'm going to firmly disagree with you on this point. As I indicated in my post above, if it isn't UL listed, then it just ain't quality equipment.

Show me a 12V 3000W inverter that is UL listed, and I'll seriously consider it.
 

Short_Shot

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Sorry, but I'm going to firmly disagree with you on this point. As I indicated in my post above, if it isn't UL listed, then it just ain't quality equipment.

Show me a 12V 3000W inverter that is UL listed, and I'll seriously consider it.
The fact that you think UL is the only standard on the planet worth having is comical.

That said, here you go.


But that gets into your complaint about it not "really" being 3kw output. If you really actually need 3kw output then....

The Renogy 3kw inverter however actually will output 3000w and is also UL listed.

At least, some sites seem to indicate as such.
Some others just state UL conformity.

However if you think a renogy inverter isn't a quality product regardless then this conversation isn't worth continuing.

 

MichaelK

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The fact that you think UL is the only standard on the planet worth having is comical.


However if you think a renogy inverter isn't a quality product regardless then this conversation isn't worth continuing.
I never said that UL was the only standard worth considering. There is also the European CE standard. I'd consider that just as good or maybe better?

I have bought Renogy before, and yes I know what Renogy's quality has become, so I guess the answer is bye.
 

Short_Shot

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I never said that UL was the only standard worth considering. There is also the European CE standard. I'd consider that just as good or maybe better?

I have bought Renogy before, and yes I know what Renogy's quality has become, so I guess the answer is bye.
So then the point stands that there are several options available with meet or exceed your requirements regarding meeting various standards.

Your previous statement implied that there weren't.
 

LeoLopez

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What is the VoC of the panels?
Some of the larger panels have a higher voltage (saw one today at 50v)
considering a panel's voltage goes up as the temp goes down and you may find even 2 panels in series too close to the SCC's PV input voltage for comfort. post the panel's specs for confirmation

I'd go 24v system. As already mentioned drawing a high wattage to the inverter would mean pulling very high amps from the battery.
3000w is a hefty load

I use a 3500w inverter on a 24v system. The inverter is generally twice the size I need. I'd rather limit the inverter's use to a moderate level insted of continually pushing a smaller unit. Then when I want or need to use a high wattage it is available.

Not recommending but I've been using this for over a year (about 12 hours a day) without so much as a hiccup:
https://www.eco-worthy.com/collecti...d-pure-sine-wave-inverter-12v-24v-48v-to-110v

The only problem with my inverter is - NOT ENOUGH BATTERY for 24 hour constant use
So now that you mention VoC, my panels do say 49.86V. Would two of them not work to a 100/50a Victron Solar Charge Controller?
 

MichaelK

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So now that you mention VoC, my panels do say 49.86V. Would two of them not work to a 100/50a Victron Solar Charge Controller?
That's because the 49.86Voc is determined at a controlled temperature of 25C. Voltage goes up when the temperature goes down. What are the winter lows at your location? On paper, those two panels in series would be 99.7V at 25C, but the voltage would go up to 112V on a frosty morning. You need a string calculator to determine it exactly.

It looks like what I wrote above about using 2S2P would be incorrect for your particular panel. Two in series would go over the 100V limit in cold weather. Looks like the only way to wire them would be in parallel, two panels per controller. The Vmp is ~40-41V? That would be ~ 10A + 10A at 40-41V. Once transformed by the MPPT controller, your output for a 24V system would be 810W/25Vcharging = 32.5A. So, one 50A controller could work with two panels, at 24V. If instead you decided to go back to 12V, the numbers would be 810W/13Vcharging = 62A, more than the controller can handle.
 
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