Did I buy the wrong inverter?

[djdalfaro]

Hello, new member here. I am in the early stages of planning and acquiring components to add a solar system to my school bus RV conversion. Unfortunately I found this forum and the information herein after purchasing a few components at a deep discount. First I purchased 4 245w Bosch Mono PV panels. Voc = 37.27, Isc = 8.61. Then I purchased a Xantrex Freedom 458 MSW 12 volt inverter (2500 watt). My plan is to run the majority of my RV appliances on 12 VDC, with only a few components/outlets on 120 VAC. Now that I have read the stickied posts I'm learning a lot of things about mistakes I have already made in my planning.

My initial plan was
(4) 245w PV panels wired in 4p *already purchased*
(1) Midnite Solar Classic 150.
12v battery bank
(1) Xantrex 458 MSW 12v inverter (2500w) *already purchased*

Are there any concerns with wiring all 4 panels in parallel? It has been my understanding that parallel wiring helps to prevent losses due to partial panel shading. As always with electricity I know I would have to ensure adequate wire size. I think to the CC it would have to handle ~40A (8.61A * 4 = ~35A).

For the MPPT CC, I was planning on purchasing a Midnite Solar Classic 150 when I get closer to the install date, as it is the only CC I've found that can handle the amperage necessary to run a 12V system with my panel wattage. 980w/12v = 81A necessary. Midnite Solar Classic 150 can handle 96A.

Concerning the battery bank, I have not purchased anything and so can buy whatever is necessary for a safe system. As I understand if I use a 12V system I should have at least 720ah of FLA or 360ah of AGM. I was planning on AGM as I could mount them inside without danger of offgassing and they are maintenance free. If the FLA is a much better buy I can build underbody battery storage.

I also read that a big problem is having an 12v inverter over 1000 watts. Before I found this forum, I purchased a used Xantrex Freedom 458 (2500 watt) fairly cheap. I'm hoping that there is a way to use it without creating a hazard to my family and I. Is there some way to ensure that I don't exceed the recommended 1000 watts and make it safe for use? If there is not I dont have any qualms about selling it as I got it pretty cheap, I had just heard good things about the company and thought it was a good deal.

Please don't pull any punches, I've got thick skin.

~Alf

[Logan5]

You are stuck in a 12 volt box.Your system should be a 24v battery not 12v, your inverter should also be 24v. If you just purchased the inverter, return it and get a 24v model.

[Sunking]

Are there any concerns with wiring all 4 panels in parallel? It has been my understanding that parallel wiring helps to prevent losses due to partial panel shading. As always with electricity I know I would have to ensure adequate wire size. I think to the CC it would have to handle ~40A (8.61A * 4 = ~35A).

You want to wire them 2S2P. Your current calculation mised plant earth and no where close.

MPPT Output Current = Panel Wattage / Nominal Battery Voltage.

or

980 watts / 12 volts = 82 amps. You are off by over 100%

For the MPPT CC, I was planning on purchasing a Midnite Solar Classic 150 when I get closer to the install date, as it is the only CC I've found that can handle the amperage necessary to run a 12V system with my panel wattage. 980w/12v = 81A necessary. Midnite Solar Classic 150 can handle 96A.

Wrong all you need is 80 amp controller, and there are several of them. Midnite Solar makes a good controller, but all you math is wrong.

Concerning the battery bank, I have not purchased anything and so can buy whatever is necessary for a safe system. As I understand if I use a 12V system I should have at least 720ah of FLA or 360ah of AGM. I was planning on AGM as I could mount them inside without danger of offgassing and they are maintenance free. If the FLA is a much better buy I can build underbody battery storage.

AGM has place in solar, but you will pay up 400% premium using them. AGM cost twice as much and last half as long as FLA. So no problem you have more money that you know what to do with. Go for it, you battery dealer will be a lot happier with you.

Is there some way to ensure that I don't exceed the recommended 1000 watts and make it safe for use?

Absolutely, buy a 1000 watt Inverter. While you are at it and you have all that money to spend, buy your kids brand new Corvettes. They promise to never go over 50 MPH or put themselves in danger.

[djdalfaro]

You want to wire them 2S2P. Your current calculation mised plant earth and no where close.
MPPT Output Current = Panel Wattage / Nominal Battery Voltage.
or
980 watts / 12 volts = 82 amps. You are off by over 100%

RE: 2S2P vs 4P, Is this just to raise the voltage in the string thus bringing down the amps per wire and voltage drop? Or is there another purpose? I was under the impression parallel was better for preventing losses. Did I get that wrong?

RE: ~40A, I was referencing the amperage that the wires would be carrying.

Wrong all you need is 80 amp controller, and there are several of them. Midnite Solar makes a good controller, but all you math is wrong.

Can you recommend some brands. Or where to go to find good brands? I admit, this is my first foray into solar, and I'm not familiar with any of the companies or products.

AGM has place in solar, but you will pay up 400% premium using them. AGM cost twice as much and last half as long as FLA. So no problem you have more money that you know what to do with. Go for it, you battery dealer will be a lot happier with you.

I live in Houston, and part of the reason I am building a bus conversion vs buying a travel trailer/camper/rv is watching Hurricane Harvey and seeing the rising storm waters. My understanding is that FLA need to be vented. Is this safe to vent in a living space? I was hoping to keep the entirety of the electrical system above the floor level of the bus if possible. If I can't vent the FLA to the living space, I would have to mount below and then subject them to flood waters. Are my concerns unfounded?

Absolutely, buy a 1000 watt Inverter. While you are at it and you have all that money to spend, buy your kids brand new Corvettes. They promise to never go over 50 MPH or put themselves in danger.

Not lots of money, although I do have a decent job, my 5 kids and wife get most of it. My spending philosophy for this project is to save when I can, and spend when I have to. Each of the PV panels was $75, and I got the inverter for $145.

Care to buy a 12V inverter? Come on... 2500watts, it's got all the power you need... Only $300. =]

[SunEagle]

RE: 2S2P vs 4P, Is this just to raise the voltage in the string thus bringing down the amps per wire and voltage drop? Or is there another purpose? I was under the impression parallel was better for preventing losses. Did I get that wrong?

RE: ~40A, I was referencing the amperage that the wires would be carrying.



Can you recommend some brands. Or where to go to find good brands? I admit, this is my first foray into solar, and I'm not familiar with any of the companies or products.



I live in Houston, and part of the reason I am building a bus conversion vs buying a travel trailer/camper/rv is watching Hurricane Harvey and seeing the rising storm waters. My understanding is that FLA need to be vented. Is this safe to vent in a living space? I was hoping to keep the entirety of the electrical system above the floor level of the bus if possible. If I can't vent the FLA to the living space, I would have to mount below and then subject them to flood waters. Are my concerns unfounded?



Not lots of money, although I do have a decent job, my 5 kids and wife get most of it. My spending philosophy for this project is to save when I can, and spend when I have to. Each of the PV panels was $75, and I got the inverter for $145.

Care to buy a 12V inverter? Come on... 2500watts, it's got all the power you need... Only $300. =]

With a quality MPPT type CC your best bet is to wire the panels as much as possible in series per the maximum DC input voltage rating.

Also an MPPT will utilize close to 100% of the panel wattage no matter how they are wired to it. A PWM type CC will harvest more charging amps if the panels are wired in parallel but you will lose about 33% of the charging amps the panels can put out so an MPPT is a better way to go.

MPPT - Watts in = watts out

PWM - Amps in = Amps out

You can keep the 2500w inverter but please understand that at 12V you can draw over 200amps so the wire between the batteries and inverter need to be big enough to handle that current and the over current protection needs to be rated less amps to protect the wire. You also run the risk of draining the battery system if the inverter is much bigger then the total panel wattage.

[jflorey2]

RE: 2S2P vs 4P, Is this just to raise the voltage in the string thus bringing down the amps per wire and voltage drop?

Basically yes.

Or is there another purpose? I was under the impression parallel was better for preventing losses.

Parallel is better for reducing losses in the controller. Controllers are more efficient when their input voltage is closer to their output voltage - and parallel wiring reduces the input voltage.

Series is better for reducing losses in the wiring. Losses in wiring are caused by current. Higher voltage at same power = lower current.

For an RV conversion you might be able to get away with parallel wiring since your runs will be short. However, you will need to add fuses and/or breakers to each string.

Can you recommend some brands. Or where to go to find good brands? I admit, this is my first foray into solar, and I'm not familiar with any of the companies or products.

Midnite and Outback both make good charge controllers.

I live in Houston, and part of the reason I am building a bus conversion vs buying a travel trailer/camper/rv is watching Hurricane Harvey and seeing the rising storm waters. My understanding is that FLA need to be vented. Is this safe to vent in a living space? I was hoping to keep the entirety of the electrical system above the floor level of the bus if possible. If I can't vent the FLA to the living space, I would have to mount below and then subject them to flood waters.

Mount them below in an easy to access location. Keep the tops as close as possible to the underside of the bus. This both helps with CoG and reduces the risk of batteries in the living space.

If you are worried about flooding, keep in mind that the 'jar' (the bottom of the battery) could care less if it sees water. So you only care about the level where the terminals are. And if the waters are that close, a few inches likely won't make a difference.

But in any case back up a second. Figure out your loads FIRST and then design the system. It is impossible to choose an inverter without knowing what it has to do.

I am going to disagree with Logan5 on the 12 vs 24V thing. If your analysis shows that you won't need more than about 1000 watts or so at any one time, 12V works just fine. And 12V is superior to 24V on most vehicles because you can charge from the vehicle alternator, and that's an important benefit for something designed for emergency use.

[djdalfaro]

With a quality MPPT type CC your best bet is to wire the panels as much as possible in series per the maximum DC input voltage rating.

I had read somewhere to stay less than 80 VDC, or there was a risk of arc flash? Is that not as big a concern?

Also an MPPT will utilize close to 100% of the panel wattage no matter how they are wired to it. A PWM type CC will harvest more charging amps if the panels are wired in parallel but you will lose about 33% of the charging amps the panels can put out so an MPPT is a better way to go.

So partial panel shading is not a concern with MPPT?

You can keep the 2500w inverter but please understand that at 12V you can draw over 200amps so the wire between the batteries and inverter need to be big enough to handle that current and the over current protection needs to be rated less amps to protect the wire. You also run the risk of draining the battery system if the inverter is much bigger then the total panel wattage.

If I were to put an 80A breaker DC breaker between the battery bank and the inverter, that should limit me to 960watts. And as long as I used AWG 4 75deg C cable or larger (rated at 85A), it should be ok? Are there any other concerns with going that route?

[SunEagle]

I had read somewhere to stay less than 80 VDC, or there was a risk of arc flash? Is that not as big a concern? It all depends on what the CC is rated for. The good ones may be able to handle around 100VDC while others may not even make it to 80VDC


So partial panel shading is not a concern with MPPT? Partial shading will affect all panels on the same "string". So while parallel wired panels have a better chance of working when one is shaded you still lose about 33% of the wattage using a PWM type CC. You can wire all of the panels in parallel to an MPPT type CC but you then need to use overcurrent devices and a combiner box once you have more than 2 Stings. Also the wire size needs to be able to handle the total amps the panels can generate which may make them much bigger and more expensive.


If I were to put an 80A breaker DC breaker between the battery bank and the inverter, that should limit me to 960watts. And as long as I used AWG 4 75deg C cable or larger (rated at 85A), it should be ok? Are there any other concerns with going that route? An 80A CB should be ok to protect that #4 wire. Just remember that while CB's trip on over current or heat some are not rated to trip for more than a couple of times and then they need to be replaced. That could get expensive with breaker replacement if your inverter should see a total inrush current draw more than 80A.

See my comments above

[Mike90250]

Flooded batteries vs Sealed AGM

Both have to be treated as venting fumes and hydrogen. Flooded does this at every recharge, AGM does it at end of life.
I would suggest flooded under the chassis, with a easily removable cover in the floor of the bus so you can service the batteries every 2 weeks (check water levels.
Or pay the expenses for AGM. You could put batteries inside in a plastic tote, or well painted plywood box.
i would consider 6v golf cart batteries, not too tall, cheap to locate and buy. 4 of them would give you 12V @ 400ah, or 24v @ 200ah

[djdalfaro]

Parallel is better for reducing losses in the controller. Controllers are more efficient when their input voltage is closer to their output voltage - and parallel wiring reduces the input voltage.
Series is better for reducing losses in the wiring. Losses in wiring are caused by current. Higher voltage at same power = lower current.
For an RV conversion you might be able to get away with parallel wiring since your runs will be short. However, you will need to add fuses and/or breakers to each string.

Forgive my ignorance, I'm still confused about the advantages/disadvantages of series or parallel wiring in a small system such as mine.
The longest run between a panel and the CC in my case would be 20ft. I originally had been lead to believe the benefit of parallel panel wiring was because it would help to eliminate loses due to partial panel shading.

SunEagle implied (if I understood correctly) that if I went MPPT it didn't matter.

SunKing stated that I should do 2S2P, but didn't elaborate why (I'll go back and reread his stickies, maybe I missed something).

You are saying (I think) that as long as my runs aren't too long, I should go 4P in an attempt to keep the difference between the panel voltage and the system voltage low, except that if I do, I will have the added expense of fuses and/or breakers for each string.

Mount them below in an easy to access location. Keep the tops as close as possible to the underside of the bus. This both helps with CoG and reduces the risk of batteries in the living space.

If you are worried about flooding, keep in mind that the 'jar' (the bottom of the battery) could care less if it sees water. So you only care about the level where the terminals are. And if the waters are that close, a few inches likely won't make a difference.

Thanks for the suggestion. I've certainly got some thinking to due to determine how high I could possibly mount the batteries and still make them accessible for maintenance.

But in any case back up a second. Figure out your loads FIRST and then design the system. It is impossible to choose an inverter without knowing what it has to do.

I have done this, adding up my proposed appliances/lighting, and I'm coming up no higher than 700W total if everything was on, I think realistically it'd be no higher than 500W, but really the system will almost entirely be DC, regardless of whether I go 12v or 24v. The AC will just be there for a few things mostly outlets. Yeah, I know I bought too much inverter. Honestly I didn't realize before reading SunKing's posts that it was a bad idea. I asked in my previous reply to SunEagle if it would be sufficient to put an 80A breaker on the inverter input, to prevent it from overdrawing on the system. Do you have any recommendations for/against this?

[djdalfaro]

SunEagle,

I had not considered using a PWM CC. From my reading it sounded pretty definitive that MPPT was the way to go and it sounds like you agree. If I were to only have 2 strings (2S2P) would I not require overcurrent protection or a combiner box?

[Sunking]

RE: 2S2P vs 4P, Is this just to raise the voltage in the string thus bringing down the amps per wire and voltage drop? Or is there another purpose? I was under the impression parallel was better for preventing losses. Did I get that wrong?

Yep you got it wrong. Low Voltage = Poor Efficiency and a lot more expensive. In 4 P means you need combiners, fuses, and a lot more and larger wire to overcome the losses of low voltage. Much more efficient and less expensive to run 900 watts at 900 volts @ 1 amp using wire the size of a hair, vs 1 volt @ 900 AMPS using wire as large are your wife. Wire 2S2Pand you avoid all that expense and loss.

So I say wire them in parallel, I own stock in copper mining and would to take all your money away from you. .

RE: ~40A, I was referencing the amperage that the wires would be carrying.

Yes I know, I own copper mining stock. Of course if it were me I would only run 16 amps on the panels to save my money so I can invest to make more money off you.

Can you recommend some brands. Or where to go to find good brands? I admit, this is my first foray into solar, and I'm not familiar with any of the companies or products.

Morningstar and Midnite Solar.

[djdalfaro]

Yep you got it wrong. Low Voltage = Poor Efficiency and a lot more expensive. In 4 P means you need combiners, fuses, and a lot more and larger wire to overcome the losses of low voltage. Much more efficient and less expensive to run 900 watts at 900 volts @ 1 amp using wire the size of a hair, vs 1 volt @ 900 AMPS using wire as large are your wife. Wire 2S2Pand you avoid all that expense and loss.

If 2S2P is better than 4P, is 4S better than 2S2P?

Morningstar and Midnite Solar.

I thought Morningstars MPPT CC topped out at 60A. Are you suggesting I go to a 24V system? I really don't need more than 1000W.

Would putting an 80A CB between the battery and inverter counteract all the possible damage done by installing a 2500W inverter?

[Sunking]

If 2S2P is better than 4P, is 4S better than 2S2P?

You want as many panels in series as possible, to make it the most efficient and lowest cost. 4p Is the most inefficient and highest cost option.

I thought Morningstars MPPT CC topped out at 60A. Are you suggesting I go to a 24V system? I really don't need more than 1000W.

Too late to go to 24 volts. My bad, you need an 80 amp controller to run 12 volt battery. 24 volts would have only required 40 amps. If it were me I would have ran 48 volt battery and a 20 amp controller at that power level. I have better things to spend money on.

As for battery, the minimum size required to handle 1000 watts @ 12 volts is 500 AH up to 800 AH. That would be 2 x 6-Volt batteries like these, wired in SERIES. Never ever wire batteries in parallel.

[djdalfaro]

You want as many panels in series as possible, to make it the most efficient and lowest cost. 4p Is the most inefficient and highest cost option.

Gotcha, so in your expert opinion the partial shading that I would undoubtably see in RV usage should not be severe enough to warrant the extra expense/hazard of running the PV in parallel.

Too late to go to 24 volts. My bad, you need an 80 amp controller to run 12 volt battery. 24 volts would have only required 40 amps. If it were me I woul dhave ran 48 volt battery and a 20 amp controller at that power level. I have better things to spend money on.

It's not too late. If 24V is the answer, I can do 24V. I can sell the 12V inverter. Of course if I go 24V I can't charge off the alternator. I don't know about going 48V. Would I have to step it down to 24V to run appliances. They make RV appliances for 24V systems, I would just need to figure out how to wire all the 12v lights/outlets. Regarding 24V, as I've stated I'm only looking to really have about a 1000W system, can you recommend a couple inverter brands that I should start looking at. I don't want to buy too big again, and should I be looking for 1000W inverters or 2000W inverters. I'm not seeing many 1000W 24V, but I know you recommend not buying more than you need. For example Xantrex's Freedom SW 24V only comes in 2000W, and 3000W.

Thank you for your expertise.