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Old 06-26-2018, 07:48 PM   #1
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Quick Solar Sizing Question

So I've determined I need about 380 Amp-hr battery bank capacity for my system. Based on that I've decided to go with 2 6V Trojan T-105 225 Amp-hr batteries in series (making a 12V, 450 Amp-hr system).

Unless I am confused... which is likely because this is all new to me... I think a 400 W solar system will work for this 450 Amp-hr battery bank.

If this works, I've been looking at this 400W Renology kit: https://www.amazon.com/dp/B06VYHTYK9/ref=dp_cerb_3

Question 1: Would a 400 W solar system be sufficient for my 450 Amp-hr battery bank?

Question 2: Would you recommend the above 400 W Renology kit?

Thanks!
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Old 06-26-2018, 08:01 PM   #2
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Two t-105’s will give you 225ah at 12v, so you will need 4 batteries.

Assuming you are using 200ah of power daily, this is what you must replace daily.

A 100 watt panel gives about 4.5 amps per good hour of sun, in the south you get 4 to 5 good hours of sun daily. So a 100 watt panes gives about 20 amps per day.
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Old 06-26-2018, 08:07 PM   #3
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-bubb

Oh shoot... so the Amp-hrs of the two batteries doesn't add together?
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Old 06-26-2018, 11:45 PM   #4
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You'll need more than 400 watts of PV to effectively charge 450aH of batteries! If you use too little PV you'll risk deficit-charging the batteries, and it will be hard to get any useful charge into them in the winter when the sun is lower and daylight hours are fewer. I have 2 kW of panels to charge my (eventual) 900 aH battery banks, and that gives me about a 13% charge rate; 400 W to charge half of that is about a 5% rate, the bare minimum recommended for FLA batteries (and that's assuming they're on a house roof that's ideally oriented for optimum insolation).

Double the PV, then you'll be doing well.

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Old 06-27-2018, 05:15 AM   #5
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keep in mind also that you should only discharge those batteries to 50% so the ah available is half of the battery bank rating.
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Old 06-27-2018, 10:42 AM   #6
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-bubb

Oh shoot... so the Amp-hrs of the two batteries doesn't add together?
For the record, the voltage of 2 batteries add when you hook them up in series. 2 6V 225Ah batteries in series = 1 12V 225Ah battery. The amps (and by extension the amp-hours) add up when you hook them up in parallel. So 2x 6V 225Ah bateries in parallel = 1 6V 450Ah battery. (both of these statements apply in an ideal world, not the one we live in. Batteries are slippery little devils.)

Are we cool on the definitions of the words "series" and "parallel"?
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Old 06-27-2018, 11:51 AM   #7
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I see 100w panels in the $100=$150 range. Then I saw these for $35. Could these be to good to be true? What in my little knowledge of solar am I missing here?
https://www.ebay.com/itm/12V-200W-So...0AAOSwucVbMm9P
I found this diagram to explain the system to the DIY Dummy, makes it simple to understand.
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Old 06-27-2018, 12:55 PM   #8
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I see 100w panels in the $100=$150 range. Then I saw these for $35. Could these be to good to be true? What in my little knowledge of solar am I missing here?
https://www.ebay.com/itm/12V-200W-So...0AAOSwucVbMm9P
I found this diagram to explain the system to the DIY Dummy, makes it simple to understand.

I dunno man, the seller mostly sells bongs and paint-by-number kits from what I can tell. 200w panels for 35 bucks seems a little too good to be true.
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Old 06-27-2018, 01:41 PM   #9
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Hmmm...any solar powered bongs?
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Old 06-27-2018, 01:41 PM   #10
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I dunno man, the seller mostly sells bongs and paint-by-number kits from what I can tell. 200w panels for 35 bucks seems a little too good to be true.
I didn't see any bongs, but panels, inverters and tattoo ink. All totally related right?
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Old 06-27-2018, 01:44 PM   #11
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I didn't see any bongs, but panels, inverters and tattoo ink. All totally related right?
I didn't look at current listings, just at his feedback. Seems like wholesale tchotchkes from China. I can't think of a punchline, but it sounds like a joke.

I once bought solar panels from a Hong Kong Bong salesman.....
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Old 06-27-2018, 02:01 PM   #12
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I didn't look at current listings, just at his feedback. Seems like wholesale tchotchkes from China. I can't think of a punchline, but it sounds like a joke.

I once bought solar panels from a Hong Kong Bong salesman.....
I was high at the time, what was I thinking.
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Old 06-27-2018, 03:39 PM   #13
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I was gonna build a kick ass solar power system, but then I got high.
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Old 06-27-2018, 03:43 PM   #14
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I was gonna build a kick ass solar power system, but then I got high.
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Old 06-27-2018, 09:01 PM   #15
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Ok I guess it turns out I was very confused after all haha... Thanks for all the help guys

Upon some reconsideration I've decided to just build a system around 400 Watts of panels because thats all I've got the roof room and budget for at the moment anyway. I will adjust my consumption accordingly.

The following is my revised plan of attack: (please correct me if I am still confused)

So if each 100 W panel can provide 20 amphrs daily, at 400 W total I could replenish 80 amphrs daily. Which means I can consume 80 amphrs daily.

If I plan to keep my battery bank charged at at least 65% charge even after using my allotted 80 amphrs of consumption, then I would need a battery bank capacity of around 225 amphrs.

If I hook the 2 T-105 6v trojans in series (12v total, 225 amphrs total) it should pair nicely with 400 Watts of panels.

Am I missing anything here?
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Old 06-28-2018, 08:18 AM   #16
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Well... maybe.

With solar, there are no guarantees due to cloud cover, shade, mounting, location (insolation - see this), etc. The rated panel output is under ideal conditions. It is unlikely that you will achieve that rated output. With flat mounted panels in the southwest, my panels produce about 80% of rated output during the summer, 60% in the winter. Number of charging hours in the day varies widely.

However; none of that matters too much if you only have room for X panels. The best you can do is fill all available space.

The big, high voltage "residential" panels are the most efficient. The negative is their large physical size. I think the high voltage is a benefit, some do not. It does require an MPPT charge controller but allows the use of fairly small gauge/size cable.

$1/watt is a reasonable price to pay for panels these days. Some find cheaper, some not.

No matter how much solar you have, you will be happy you have it. A generator (yuck, not my ideal) can be used to bulk charge the batteries to the point at which they begin charging much more slowly (about 80%) and then let the solar finish the job over the course of the day. Or some scenario like that - whatever works in your situation.

Already noted but important to remember, a lead-acid battery bank should not be discharged below 50%. A good battery monitor is important if you want to take care of your battery investment. Google Trimetric for a good one, Victron has a reasonable unit but cannot be tuned.
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Old 06-28-2018, 04:50 PM   #17
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JDonthego,

How many amphr battery bank would you match to a 400 W panel system?
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Old 06-28-2018, 05:30 PM   #18
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How many amphr battery bank would you match to a 400 W panel system?
You can certainly spend time on such matters and doing so will likely achieve peak performance/battery life. However; you are limited on roof space so you can do as well as you can do and be happy with the results.

The theory is that you size the battery bank to meet your needs and then build a solar array to match the battery. That is often not possible with an RV/bus due to limited roof space for panels as well as the ability to carry a large battery bank. So, you do the best you can.

Figure out what your energy budget is - how much power you need on an average day - and shoot to store enough power to get by for a couple days. More is better, of course, but be realistic. That locks in your battery bank. If you can only install 400 watts of solar, do it. Then, plan to use a generator to fill in the gap. No, not ideal but you have constraints.

The math, based on lead-acid battery bank size/capacity is (AH * .05) and (AH * .13). The 5% and 13% is the ideal charging current for a battery bank.

In reality, your 400 watts of panels is going to produce a maximum of something like 20 amps (at charging voltage of 14.5). There are other losses that could be considered but that is a nice round number. In theory, that is enough to support a 155 Ah battery (at the 13% maximum - better) all the way to a 400 Ah battery (at the 5% minimum - worse).

All that said, it doesn't matter. You can fit no more than 400 watts on your roof and, after learning to live on battery, you are going to use whatever power you use. 400 watts of solar won't be enough (unless you are really frugal with power) and so you will run your generator. It is better to run the generator during bulk charging (when it is below 80% SOC) and then let solar top it off over the course of the many hours of daylight.
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Old 06-28-2018, 08:59 PM   #19
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JDonthego,

If I understood you correctly you are saying 400 W could support between 155 amphr to 400 amphr battery bank, correct?

So using the 2 T-105 6v trojans in series at 225 amphrs is a reasonable battery bank size for a 400 W system?

Sry if this all seems a bit repetitive.
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Old 06-29-2018, 09:53 AM   #20
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If I understood you correctly you are saying 400 W could support between 155 amphr to 400 amphr battery bank, correct?
I am saying that the rule of thumb is solar charging output (amps) that is a minimum of 5% of battery bank capacity to a maximum of 13% battery bank capacity (lead-acid batteries). As I understand it, closer to 13% is the 'ideal'. In my opinion, the 'rule of thumb' part is important (something to shoot for but not mandatory).

I am also saying that I think it is good to know and shoot for these sorts of 'ideal' but it is often not possible in our environment and getting wrapped up on such things might be counter-productive.
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