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Old 12-23-2021, 01:26 PM   #1
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300W of Solar should replace 100AH of usage daily?

So I am new to this and suck at math. But here's what I think I need.
I would appreciate you correcting or confirming my research.

I have a 200AH AGM battery. I have never been below 12.3 V with overnight usage, but want to use the 50% (12.0ish V's) or 100AH's of usage as my benchmark for completing my solar build.

I understand the variables of conditions and overall efficiency of advertised claims, but lets use "optimal" to keep it simple.

Renogy claims 1200WH of daily output depending on 4 hours of sunlight availability. (3 x 100W panels @ 400WH ea.)

1200WH divided by 12.7V = 94.48AH.

I will be using an MPPT controller. I also have 100w of portable solar to supplement, if necessary.

So "in theory" if I left the solar charging all day even in conditions that are a little less than optimal, this much solar should have no trouble completely recharging my battery?

Thank you in advance for any help!
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Old 12-23-2021, 08:25 PM   #2
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I'm having a little trouble following your math. Not because of your description. It's because I can't jump back and forth between Amp hours and watt hours. So let me explain it my way.

You have a 200Ah battery. Using the "don't use more than 50%" means you have 100 usable Amp hours. So you're trying to make sure you can put 100Ah back into the battery even on less than an optimal day.

I've installed renogy solar panels on my last three RVs. Generally speaking you'll get 5A per hour for 4-5 hours a day. And that's on a GOOD day. On overcast or rainy days you'll get a lot less.

So 3x 100 watt panels = 15 amps per hour, times 4 hours = 60Ah... not the 100Ah that you need.

4 panels would give you 80Ah. Remember, these are good sunny days.

Now I have to say, you may get 5 hours of good light, plus you do get less than perfect sun on both sides of the best 4-5 hours, and that helps.

But I recently used about 107Ah two nights in a row. I have 320 watts of solar (two 160 watt renogy panels). For both days it was very overcast and light rain. The solar could not put a dent in recharging the batteries. I needed to plug in to a shore power to recharge. It proved to me I don't have enough solar for crappy days. On sunny days I'm golden. On crappy days I'm not.

So, you need to figure out how many amp hours you'll use (how many you have to replace into the battery) and how many panels you "need" to get your recharge.

Hope that helps. Let me know if you have any questions about what I said.
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Old 12-24-2021, 10:03 AM   #3
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I'm having a little trouble following your math. Not because of your description. It's because I can't jump back and forth between Amp hours and watt hours. So let me explain it my way.

You have a 200Ah battery. Using the "don't use more than 50%" means you have 100 usable Amp hours. So you're trying to make sure you can put 100Ah back into the battery even on less than an optimal day.

I've installed renogy solar panels on my last three RVs. Generally speaking you'll get 5A per hour for 4-5 hours a day. And that's on a GOOD day. On overcast or rainy days you'll get a lot less.

So 3x 100 watt panels = 15 amps per hour, times 4 hours = 60Ah... not the 100Ah that you need.

4 panels would give you 80Ah. Remember, these are good sunny days.

Now I have to say, you may get 5 hours of good light, plus you do get less than perfect sun on both sides of the best 4-5 hours, and that helps.

But I recently used about 107Ah two nights in a row. I have 320 watts of solar (two 160 watt renogy panels). For both days it was very overcast and light rain. The solar could not put a dent in recharging the batteries. I needed to plug in to a shore power to recharge. It proved to me I don't have enough solar for crappy days. On sunny days I'm golden. On crappy days I'm not.

So, you need to figure out how many amp hours you'll use (how many you have to replace into the battery) and how many panels you "need" to get your recharge.

Hope that helps. Let me know if you have any questions about what I said.

I'm new to solar also, so what would happen if you upped to 2 batteries. The solar wattage would not be able to get the 2 batteries topped off or close? Thanks
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Old 12-24-2021, 10:17 AM   #4
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I definitely like your way better, LOL. basically 5 A per panel is much easier for me to get my mind around. Your real world input is exactly what I was looking for. I am overthinking this. Your solar equipment and usage are pretty similar to what I am going to end up with. I suppose the 300 W I am installing will be adequate for at least reducing my generator run time under high usage. Im going to upgrade my converter to help as well. I guess in a nutshell 400 to 600w would be better to recharge 100 amp hours of usage but not practical for my application. Thanks for simplifying this, Mike!
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Old 12-24-2021, 10:23 AM   #5
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Wls,
The 200 AH battery I have is basically equivalent to two standard size travel trailer batteries. Here in lies the problem. I didn’t want to worry about how much I’m using. Having solved this problem, now the problem is charging it back up daily. Trying to avoid a downward spiral of using more than I can put back in. One or two nights of camping, this wouldn’t be a problem. Extended dry camping is.
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Old 12-24-2021, 11:10 AM   #6
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All bets are off when the sun don't shine! I would only build enough solar to top off batteries under good solar charging conditions. Use the generator as necessary on bad solar charging days.
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Old 12-24-2021, 11:21 AM   #7
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I have 600 watts flat mounted on the roof, feeding two Trojan T105 deep cycle batteries through a Bogart Trimetric controller/ monitor.

I have lived in the Quartzsite desert with unobstructed sun in January and February -- with pretty much unrestricted 12 volt and inverted 120 volt use ( TV, DVD player, microwave, etc ) -- without ever having to fire up the generator.

Put me in a shady area, and things would change. But give me longer days with a higher sun and that would take me the other way.

Point is, there are a number of variables, including use patterns, length of sun exposure, tilted/non tilted panels, etc. Learn from others, experiment, live and learn what works best for you.

Good luck

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Old 12-24-2021, 12:55 PM   #8
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https://pvwatts.nrel.gov/pvwatts.php
Location, location, location
and a little tilt helps.
For a 100w panel in Quartzsite, AZ the annual average is 38 amps per day but seasonal is 19.5amps winter and 53amps summer
For the same system in Portland, OR the annual average is 23 amps per day and seasonal is 5.5 in winter and 44.5 in summer.
Tilting and pointing gives about a 13% increase.
If you are going to be boondocking in the summer the basic ratio of 200 watts for every 100 amps of battery works.
In a shitty solar location such as Portland in winter you might want to buy a generator.
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Old 12-24-2021, 12:56 PM   #9
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Cool location calculator.
How did you get the amp hours per day figure? I inputted my location, 10 degree tilt facing West, and 0.1kw for my rooftop 100w panel, and not sure how to interpret the table of results. The panel direction is just when parked in driveway.
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Old 12-24-2021, 12:58 PM   #10
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This guy at Quartzsite a few years ago has it nailed!!!
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Old 12-24-2021, 01:24 PM   #11
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Just a couple more thoughts to throw in the discussion,

If one has a lead/acid battery bank there is a lot of solar power that doesn't directly get stored due to the charging inefficiencies of Lead/Acid chemistry. It's generally accepted that they are only 85% efficient in utilizing charging current.

On the other hand LiFePo4 batteries are more like 99% efficient when charging. Out of every 100 watts received, 99 watts are stored.

Even more important is the nature of a Solar Charging Day. Little energy in the early hours, building to it's peak at mid-day when Sun is at it's zenith, and then waning as the day approaches sunset. Even if one were to "aim" the panels throughout the day the sunshine has to fight it's way through the thicker atmosphere early and late in the day, with efficiency highest around noon.

With this in mind LiFePo4 batteries will absorb far more power during an average solar day than Lead/Acid thus a smaller solar array can replace the same energy.

Yes, cost is a factor but considering the lighter weight, greater charging efficiency, ability to use more of the stored energy, the reduced cost of a solar array can offset some of that cost.

Based on my needs for 12v power, when camping in places with good sun exposure, I can replenish my two 100ah Battleborn batteries with a 200w portable solar system usually by 2 PM which are around 50% discharged on a heavy use day.

I don't really think there's any "Rule of Thumb" for solar array size to battery bank size as I doubt there are two campers that have the same power use/requirements.

Best approach would be to start with a Solar Controller that can handle the largest possible array one could put on their RV and then start with a couple of panels. See how it works and if it comes up short, add a panel or two.

Just remember, LiFePo4 batteries will usually not require as much solar to recharge as Lead/Acid.
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Old 12-24-2021, 02:07 PM   #12
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This guy at Quartzsite a few years ago has it nailed!!!
Is it possible the trailer is also for carrying ectra batteries?
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Old 12-24-2021, 04:04 PM   #13
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I definitely like your way better, LOL. basically 5 A per panel is much easier for me to get my mind around. Your real world input is exactly what I was looking for. I am overthinking this. Your solar equipment and usage are pretty similar to what I am going to end up with. I suppose the 300 W I am installing will be adequate for at least reducing my generator run time under high usage. Im going to upgrade my converter to help as well. I guess in a nutshell 400 to 600w would be better to recharge 100 amp hours of usage but not practical for my application. Thanks for simplifying this, Mike!

You're welcome. If you do upgrade your converter I highly recommend getting one that includes a lithium battery charging profile for future use (in addition to the lead acid profile you presently need).

I switched to two 100Ah lithium batteries. That gives me the equivalent of 400Ah of lead acid because I can drain to 0 state of charge instead of only 50% state of charge. Of course I don't drain to 0, but it's nice having 200 usable amp hours.

Regarding tilting the solar panels. The benefit is real but the amount of benefit varies by the geographic location. I was in Quartzsite AZ one January with 3 100-watt renogy panels facing south. I never saw the sun stay so low in the sky. I thought to myself, this must be what it's like in Alaska. LOL. But I digress. I was getting only 10 amps at noon. Then I tilted all three panels. That upped it to 15 amps. That's a 50% increase. Not a lot of prime sun hours there in January, but the extra amps helped.
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Old 12-24-2021, 04:37 PM   #14
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Mike has it right on tilting.

But at 75 years of age, I resist going on the roof any more than absolutely necessary. So I went from 300 watts one year at Q (using the generator every now and then), to 600 the next (using it not at all).

Lots of folks there have panels on the ground that get the low sun very well. Myself, I keep trying to figure out how lithium might fit into my plans. As old as I am, the payback has to be operational, not financial.

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Old 12-24-2021, 04:39 PM   #15
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Hey all,
my months of scouring the Internet are nothing compared to your responses to this thread.
Thank you very much for all of the info, please keep it coming. So much to learn.
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Old 12-24-2021, 05:58 PM   #16
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By the way, RichP is right. Tilting is for young people. If at all possible, put more/bigger solar panels on the rooftop so you don't have to climb up there.

Also, I think lithium is an "ease of use" analysis, not so much a economic analysis. Now with batteries at sub-$500, the decision may be a little easier than when I did it.
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Old 12-24-2021, 07:12 PM   #17
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Mikec557

I switched to two 100Ah lithium batteries. That gives me the equivalent of 400Ah of lead acid because I can drain to 0 state of charge instead of only 50% state of charge. Of course I don't drain to 0, but it's nice having 200 usable amp hours.


I currently have 4 6 vt batteries, very similar to T-105, so would 2 100ah lithium batteries be more than the total AH of my 4 batteries. Thanks
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Old 12-24-2021, 07:18 PM   #18
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Mikec557

I switched to two 100Ah lithium batteries. That gives me the equivalent of 400Ah of lead acid because I can drain to 0 state of charge instead of only 50% state of charge. Of course I don't drain to 0, but it's nice having 200 usable amp hours.


I currently have 4 6 vt batteries, very similar to T-105, so would 2 100ah lithium batteries be more than the total AH of my 4 batteries. Thanks
4 t-105 batteries would give you 450 amp hours. Best practice is 50% discharge, so you have 225 amp hours available, more than what the lithiums would give you. Plus, those batteries are made for deep discharge, you can even go down to 20% SOC, which would give you 360 amp hours. But you're hauling almost 300 lbs in batteries. Certainly you have a large RV to need that much battery, and weight would probably not be a big deal in a large RV.
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Old 12-24-2021, 08:42 PM   #19
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4 t-105 batteries would give you 450 amp hours. Best practice is 50% discharge, so you have 225 amp hours available, more than what the lithiums would give you. Plus, those batteries are made for deep discharge, you can even go down to 20% SOC, which would give you 360 amp hours. But you're hauling almost 300 lbs in batteries. Certainly you have a large RV to need that much battery, and weight would probably not be a big deal in a large RV.
I have the res refrigerator also
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Old 12-24-2021, 08:42 PM   #20
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Great info! So I’m starting a shopping list based on your input.
I already have a 100 W Renogy panel on the roof. I also have 100 W of portable Harbor freight with their cheap arse controller. Also, a 4200/3500 generator on the bumper.
I understand lithium is the way to go but I can’t see that much investment at this time. I’m going to stick with the Renogy 200ah AGM battery (which was $320 at the time) until it peters out. (Yeah, I know it’s heavy)
The harbor freight portable stuff I will retain to supplement if I find a shady campsite that is too nice to pass up. Most of my camping will be Eastern Sierra with limited sun early and late due to being in the shadows of 14,000 foot peaks. I am learning that doing stuff on the cheap comes back to bite you eventually.

I’ll add a 40 amp MPPT charger, three more panels and swap my WFCO for a nice Progressive Dynamics (Li capable) controller. I suppose I might be able to fit one or two more panels eventually, but real estate is definitely limited. All of this stuff is sitting in my Amazon cart right now, but Santa Claus stole my wallet this year so I may wait a couple of weeks..

I agree with the older guys (as I am getting too old to hang out on the roof myself) and won’t be tilting or using the portable much. I want this to be as low maintenance as possible as I spend most of the day away from the trailer. Watch a little TV, charge electronics, eat, sleep, shower and crap, basically.

I am told the 20’ of 8 ga from the controller to the battery should be adequate.

Merry Christmas!
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