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Old 02-23-2019, 01:40 PM   #1
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How much solar?

I've read a number of solar threads and have a basic question, being new to the solar thing, My power need consists of DW's O2 machine, which draws(according to the label) .9 amps which I figure converts to about 99 watts at 110 voltage. I have 4 lead acid batteries (100 ah each) and 2000w inverter which I used a couple years ago to power her (at that time)3.4 amp O2 machine, they lasted about 8 hours on full charge. I have purchased the 100 watt Renogy panel kit and I also have Honda eu2000 generator. I figure I can run the generator with battery chargers to charge the batteries if the solar can't keep up, of course, restricted to quiet times, especially in USFS campgrounds in Alaska. Solar and batteries will be used ONLY to power her machine, nothing else, as that is top priority. Can I keep the solar attached while drawing power? If I need to run the generator and battery chargers should they be separated from the solar? When the weather warms up here, I will try to test run the system, but thought I could get a head start from folks wiser than I.
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Old 02-23-2019, 02:23 PM   #2
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The question should always be, how much battery capacity? Then the solar component is easy...... 1.5 watts per Amp hour of battery!
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Old 02-23-2019, 05:49 PM   #3
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You do not have to disconnect the solar charge controller when you are charging the batteries via another power source.
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Old 02-23-2019, 06:13 PM   #4
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As mentioned, when it comes to charging batteries, most solar chargers play well with the DC converters/chargers in our RV's.



As for how much power you can expect, as a general rule, I've found I can expect 100 watts of panel on the roof equates to about 25ah to the batteries on a sunny day with little to no shade or clouds. You'll get somewhat less than that in the winter, somewhat more in the summer due to different day lengths, and the angle of the sun. How far north or south you are etc. also come into play.


While a single hundred watt panel won't keep up with what you're using power wise, it would be a nice compliment to the generator. You can run the generator in the morning for the bulk charge, then let the solar continue on later in the charge cycle when your lead acid batteries won't accept as much current anyway.
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Old 02-23-2019, 06:18 PM   #5
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Tdansby is correct in stating about 1.5 watts per amp hour of battery as a good rule of thumb.
The type of solar panel and charge controller you buy make a significant difference in how much power you will generate in equal conditions.
A basic 100W Renogy kit may only come with Polycrystaline panels and PWM controller with each only ~70-80% efficient or ~60% total, which means on a sunny day with the panel perfectly aligned you are only producing 60 watts from your 100 watt panel. The higher end Monocrystaline and MPPT combo will more consistently be ~90-95% efficient.
As/or more important is how much of your battery capacity you can use, reliably and repeatedly. For best bang for buck, use four 6-volt golf cart batteries wired in pairs for 12 volt ea pair. This will allow you to use 50% of the total capacity (i.e.200ah for a 400ah battery bank) for over 600 cycles, while you can only use 20% of "deep cycle 12 batteries" (i.e.100am for a 400ah battery bank) repeatedly for the same number of times. More than that, you damage the batteries and shorten their life.
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Old 02-24-2019, 08:05 AM   #6
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Thanks for the input everybody, my solar panel is a monocrystalline style. Just to be clear, the battery chargers I will be using off the generator are automotive style, so what I was wondering was if the solar controller would cut back while they were running if all hooked together. In my previous boondocking I actually unhooked the batteries from each other and they had their own dedicated charger, as I could charge them faster (or so it seemed to me)
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Old 02-24-2019, 12:02 PM   #7
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I just tested my solar charger readout before replying and can say that your solar will continue to send power to your batteries IF they are not fully charged AND/OR you have anything drawing power (O2 converter, etc.) REGARDLESS of whether you are also on shore power (charger) or generator powered charger.
My batteries were fully charged before I started the test and the solar controller only read 0.7amps. I turned on a space heater and the solar controller slowly increased to max amps to 33 amps as the inverter drew the batteries down. I then started the generator, which first drew the batteries down further but after starting and generating, the solar still read ~30 amps and decreased (faster than solar alone) as the batteries were recharged by both the solar and the generator. I repeated the same test with the shore power/charger with similar results.
I can't tell you if or to what extent the shore or generator charging amps are affected, but solar will continue to provide all it can and needed.
Unfortunately, all of chargers will adjust based on battery voltage and not the true State of Charge (SOC) so will back off charging amps independent of your particular batteries capacity or ability to absorb amps.
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Old 02-26-2019, 12:12 PM   #8
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The good news is that you have a fairly predictable load.

In an ideal world, you want the batteries to finish bulk charging by 12 - 1 pm and then have them doing the "finish charging" the rest of the day.

As an example, if your night use is:

(100 watts x 8 hrs) = 800 watt-hrs

Then either with solar or the generator, you should try to push this much back into the batteries by 1 pm if possible. The reason is that it takes "time" to complete the "finishing stages" and if they haven't reached these stages by 1 pm, there really isn't enough time to complete the charging correctly.

Assuming the panels are putting out 50% of their "rating", then you have 3 - 4 hrs to make up 800 watt-hrs.

(800 watt-hrs) / (4 hrs of morning sun) = 200 watts

Assuming 50% panel output compared to the official rating:

(200 watts ) x (2) = 400 watt of panels

That would be a good start for a system to run your wife's power loads if I am understanding them correctly.
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Old 02-26-2019, 12:21 PM   #9
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Quote:
Originally Posted by Tdansby View Post
The question should always be, how much battery capacity? Then the solar component is easy...... 1.5 watts per Amp hour of battery!
I'm in "trouble" then 1020 amp hours of AGM's and only 400 watts solar.
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Old 02-26-2019, 12:22 PM   #10
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Alternatively, you could use a 75 amp x 12 volt charger with your generator for 1 - 2 hrs to bulk charge the battery pack. (also by 1 pm)

The 1 solar panel that you have might be able to complete the finishing charge at that point.

It depends somewhat on the exact batteries that you have. Some can accept bulk charging at quite high rates, while others are like trying to push electricity through a straw.

edit - To be honest, I think of a 100 watt panel as a trickle charger. Especially on an overcast day, those renogy mono panels don't make much power - maybe 10 watts.
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Old 02-26-2019, 02:22 PM   #11
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I suspect you are running more stuff than you know, for example my Dometic refrigerator when running on Propane consumes about 12 watts continuously to operate the logic board, solenoid valve, door defrost heater, etc. This results in a 288 watt hours per day of electrical draw. A 60 watt replacement LED light bulb draws about 9 watts in comparison. Lets assume 3 such LED light bulbs used for 3 hours per day each this would give us about another 80 watt hours per day of draw, maybe another 2-3 hours watching TV at 15 watts is another 30-45 watts hours per day. Then there are phone chargers, computers, tablets, etc. Add to this that many inverters are not very efficient at light loads, so that 2000 watt inverter may be drawing 50 watts just to run at 100 watts of output.


All things considered expect flat roof mounted solar panels to put out around 5 times their face value in watts per day, so a 100 watt panel will on average in the summer in mid US conditions put out about 500 watts total per day. In Alaska you will have more hours of daylight, though the sun will be lower in the sky in the summer.
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Old 02-26-2019, 02:32 PM   #12
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Quote:
Originally Posted by HaRVey 74.5 View Post
I just tested my solar charger readout before replying and can say that your solar will continue to send power to your batteries IF they are not fully charged AND/OR you have anything drawing power (O2 converter, etc.) REGARDLESS of whether you are also on shore power (charger) or generator powered charger.
My batteries were fully charged before I started the test and the solar controller only read 0.7amps. I turned on a space heater and the solar controller slowly increased to max amps to 33 amps as the inverter drew the batteries down. I then started the generator, which first drew the batteries down further but after starting and generating, the solar still read ~30 amps and decreased (faster than solar alone) as the batteries were recharged by both the solar and the generator. I repeated the same test with the shore power/charger with similar results.
I can't tell you if or to what extent the shore or generator charging amps are affected, but solar will continue to provide all it can and needed.
Unfortunately, all of chargers will adjust based on battery voltage and not the true State of Charge (SOC) so will back off charging amps independent of your particular batteries capacity or ability to absorb amps.
Just to clear something up.

Your generator probably doesn't have a charging system, many don't.

The generator powers the same charger that the shore power does.
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Old 03-05-2019, 06:13 PM   #13
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Originally Posted by Tdansby View Post
The question should always be, how much battery capacity?
This.



When off grid it's all about batteries. You have no solar at night. How much battery capacity do you need? Now determine how you will charge them.


My off grid camper has 200 AH AGM batteries so 100AH usable. It's just me, spend all my time outside. No furnace, no game machines, no TV. My god I'm camping with a mobile apartment.


I run my champion dual fuel inverter generator on propane through my Magnum 2000 watt inverter/multistage charger an hour a day. Sometimes less.



It's all about power usage. How much power do you "need" and how much battery will provide this?


Now how do you replace this used power.


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Old 03-05-2019, 06:37 PM   #14
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We can estimate...but you have the ability to go one step better.

Invest in a good Battery Monitor. A Shunt device which measures actual current flow in and out of your battery bank.

Next, go out and camp with utilities available...but zero out your counter...and unplug the rv. Take measurements over time. You will then have two things you didn’t have before...

1) An actual measure of your needs...

2j A management tool that is a very, very useful for budgeting your power usage. Not everyday is going to be perfectly sunny...you need to know how you are doing... know what times you can splurge, what times you need to conserve and when you need to go for a generator bump up to be in a good place.

It is the very first thing I did...before I purchased anything at all related to solar.

If you were just looking for a battery maintainer...it is easy to recommend a small system for that purpose. When planning for real use...it helps to have real numbers. And it is obviously even more important when medical equipment is the focus. It’s one thing to have to spend the night without tv or microwave...it’s another thing all together not to have Oxygen...

I highly recommend the Victron BMV-712...if you do a search on YouTube, there are great videos explaining what it does and the basics of installation.
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