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How much solar do I need?
As an retired engineer I would like to add that you actually look and see how many watts each of your items are rated. This will be your watts/ hour you may need. 

Like mentioned in above post your batteries should not be discharged below the 50% level.

 If you have 100 amp/ hour batteries, that's 1200 watts . Taking half of it = 600 watt/hr available at full charge. 

One other item to remember is a solar panel is not  100 % efficient. Meaning you will not get 100 watts out of a 100 watt panel. The standard rule of thumb is 80%. 

Other than the physical requirements of being roof mounted as mentioned in other posts ,  hope this info will help.
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RVTravel (10-10-2017)
(10-09-2017, 07:47 PM)musicman Wrote: As an retired engineer I would like to add that you actually look and see how many watts each of your items are rated. This will be your watts/ hour you may need. 

Like mentioned in above post your batteries should not be discharged below the 50% level.

 If you have 100 amp/ hour batteries, that's 1200 watts . Taking half of it = 600 watt/hr available at full charge. 

One other item to remember is a solar panel is not  100 % efficient. Meaning you will not get 100 watts out of a 100 watt panel. The standard rule of thumb is 80%. 

Other than the physical requirements of being roof mounted as mentioned in other posts ,  hope this info will help.

Great post.

I was just looking at some info last night trying to figure out if I can put a fridge in my van.

I have a 2.3 cubic foot fridge, and it says on the back it uses 1.3a (110v) but like 6.3 max.

I have (2) 35ah batteries in parallel, and (2) 100w panels on the roof of the van. I estimate that on a good sunny day, I can receive 8-11a of 12v power into the batteries. I checked charts for average usable sunlight for solar panels for where I live. It's on average, about 5 hours per day for full charge. I guess that's how I arrived at some of the info below.

So I start thinking... can I run this fridge? How much power is left over. I may update the post later with my crazy math, but what it seemed to boil down to was this -

I have about 660 watts available that come from the solar each 24 hour period. Since it's not sunny 24 hours a day, that was the best way I could account for solar generated watts as a "unit". I end up left with 240 additional watts to "play with" over a 24 hour period, once the battery is fully charged (from dead). 

I arrived at these numbers thinking that if I ran whatever I have, it would take 660 watts of usage to end up at a point where I should not use the battery anymore - 50% used, or 35ah.

I think it's way better to think of everything in watts and watt hours than amps and volts.

I'm thinking I'm undersized on my batteries now, and maybe I should have 150ah or 200ah, but I don't know how folks are running standard 110 fridges off of 200w of solar. Seems like you have almost nothing left over, and really the only thing upsizing the battery does, is to give you a buffer for cloudy days.

Otherwise, you really do have to add more panels. Or spend 500 or so on a really nice 12v fridge/freezer with really low amp draw... like a dometic unit.

Is this right, or have I lost my mind? Smile
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> I think it's way better to think of everything in watts and watt hours than amps and volts.

Probably (and definitely when comparing 12v apples to 24v oranges).  But for whatever reason Amps and Ah are the most commonly used.


> I'm thinking I'm undersized on my batteries now, and maybe I should have 150ah or 200ah, but I don't know how folks are running standard 110 fridges off of 200w of solar.

As you suspected, most folks are running $$$ 12v compressor fridges if they are doing it on 200w of panel.

IMO it is better to be undersized on batteries rather than oversized;  at least they won't be killed by undercharging.  St. Sternwake told us in 1st Absorption, verse 42, that a 2:1 or 3:1 ratio between panel wattage and battery Ah is best for regularly cycled lead-acid.   You are sitting right at 3:1 so he is smiling down on you right now.  :-)

To help a small bank make it through the night it may be necessary to move all time-shiftable loads to periods when there is excess power.  That way they wouldn't compete with the fridge for precious power at night.
frater/jason @ Quartzsite
blog | Promaster van | offtopic answers
"I would unite with anybody to do right; and with nobody to do wrong" F. Douglass
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Ok we need lots of help Gary and I want to do solar want to get 33 quart fridge and won't know what to do with solar kit when we get them or what kind of battery to get or inverter or what else this is so over our heads.
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Laurie, you may want to start a new thread. 

Here is an intro to solar I'm working on.  It's rough but might help you understand what questions to ask.
frater/jason @ Quartzsite
blog | Promaster van | offtopic answers
"I would unite with anybody to do right; and with nobody to do wrong" F. Douglass
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free2enjoy (12-12-2017)
you can break it down to 3 basic steps,

1. figure out your daily power usage.
2. size your battery to your daily power usage.
3. size your solar to your batteries.

that's it, it's easy. well not that easy but as you go through the steps you will gain knowledge and understanding. we will help. try to avoid an inverter most things can run 12v. also remember it's always easier and cheaper to conserve energy then it is to make it. highdesertranger
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frater secessus (11-15-2017)
I personally tell people to fill the roof with as many solar panels as they can (because on a vehicle, it is the largest constraint), then build a battery that can charge at a safe rate with the solar panels that you put on the roof, then buy the largest solar charge controller they can. Then buy the largest inverter they can afford.

Try to build the biggest system you can. Solar panels are cheap. If you need to scale your system in the future, be sure to have a larger than needed solar charge controller so you can continue to make your system bigger if you need to.
Off-grid for 8 years in 23 foot ford jamboree rv, toyota new horizons rv, 1989 vanagon (rebuilt from the ground up) and a 2010 toyota sienna

Check out my solar how-to site:
http://www.mobile-solarpower.com/
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CapnGimp (12-04-2017)


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