# Solar Panel Question



## advancedtel (May 7, 2008)

I was hoping some of you folks had experience with solar panels and could help me. I do not know much about solar panels at all so let me explain what I want to do. In the fall I leave my camper at a campground for about 3 months and we just go there on the weekends from time to time. The campground has full hookups and I do not do any dry camping so I only have the one battery and thats all I ever need. My problem is my refrigerator. If I go to my camper each weekend everything is fine. If it is two weeks before I get back my battery has run down and my refrigerator is not working (thus all my food has to be thrown out). I am looking for a solar panel to keep the battery charged so that my fridge will have the power it needs to run off the propane tanks while I am away. (I have to disconnect from the campground power because they store the camper)

My question is, will a 15W solar panel provide enough juice to do this? If I hook up the solar panel before leaving the camper, when I come back in two or three weeks will the fridge still be cold (providing it doesn't run out of propane of course)? I just don't know how they work and I guess how much the camper needs. Again i don't dry camp so I'm not looking to recharge my batteries for camping, just maintaining the fridge while away.


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## Insomniak (Jul 7, 2006)

advancedtel said:


> I was hoping some of you folks had experience with solar panels and could help me. I do not know much about solar panels at all so let me explain what I want to do. In the fall I leave my camper at a campground for about 3 months and we just go there on the weekends from time to time. The campground has full hookups and I do not do any dry camping so I only have the one battery and thats all I ever need. My problem is my refrigerator. If I go to my camper each weekend everything is fine. If it is two weeks before I get back my battery has run down and my refrigerator is not working (thus all my food has to be thrown out). I am looking for a solar panel to keep the battery charged so that my fridge will have the power it needs to run off the propane tanks while I am away. (I have to disconnect from the campground power because they store the camper)
> 
> My question is, will a 15W solar panel provide enough juice to do this? If I hook up the solar panel before leaving the camper, when I come back in two or three weeks will the fridge still be cold (providing it doesn't run out of propane of course)? I just don't know how they work and I guess how much the camper needs. Again i don't dry camp so I'm not looking to recharge my batteries for camping, just maintaining the fridge while away.


A small solar panel should do the job just fine. I had been using the 15 watt panel with 7 amp charge controller kit from Camping World, but the wind blew it off of the trailer roof and smashed it. I bought a new 20 watt panel on Ebay, and with my existing charge controller, it keeps my two batteries charged up. Just make sure you secure the panel in some way so it doesn't blow off or grow legs and walk away.


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## Dan Borer (Feb 6, 2009)

I think the 15 watt panel will be marginal for what you want to do. You say your single 12 volt battery is dead in two weeks. Let's assume that battery is in the 85 to 115 amp rating as is common for an RV battery. Your trailer has several 12 volt components running whenever your battery is connected in addition to the fridge, such as the stereo, CO detector and propane detector. Plus your battery has the issue of self discharge just from sitting. If your battery is dead in 2 weeks you will need to provide enough charge capacity to at least equal or exceed the drain on the battery. Your 15 watt panel has an output of about 0.9 amps per hour. Let's assume that you get about 8 hours of direct sun on that panel each day. You'll have a little over 7 amps going into your battery in a day, 49 amps in a week and 98 amps in two weeks. Figure in the lost efficiency in charging a battery and you may extend the time before the battery is dead by a couple of weeks. I think a better option would be a 30 watt panel or larger.

Dan



advancedtel said:


> My question is, will a 15W solar panel provide enough juice to do this? If I hook up the solar panel before leaving the camper, when I come back in two or three weeks will the fridge still be cold (providing it doesn't run out of propane of course)? I just don't know how they work and I guess how much the camper needs. Again i don't dry camp so I'm not looking to recharge my batteries for camping, just maintaining the fridge while away.


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## advancedtel (May 7, 2008)

Dan Borer said:


> My question is, will a 15W solar panel provide enough juice to do this? If I hook up the solar panel before leaving the camper, when I come back in two or three weeks will the fridge still be cold (providing it doesn't run out of propane of course)? I just don't know how they work and I guess how much the camper needs. Again i don't dry camp so I'm not looking to recharge my batteries for camping, just maintaining the fridge while away.


[/quote]

Is there a way to turn any of those off so the drain on the battery is not so great? Also, the existing battery gets a week with nothing. So a 15W solar panel would only get about two weeks additional? Am I understanding that correctly? (I know its not exact - just trying to get an understanding of what to expect)


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## Gilligan (Aug 25, 2006)

The biggest single drain on the battery, when everything is turned off, is the propane detector, which I have measured at about a 150mA draw. That would be 0.15 Amp, which calculates to 3.6 Amp/Hours per day. Unfortunately, the propane detector is hard-wired to the 12V, so the only option there would be to install a switch in series with the 12V supply, or possibly you could pull the fuse that supplies that circuit, assuming it is not on the same circuit as the fridge. The smoke and CO detectors run off of their own batteries, so they do not draw on the 12V. I have never measured the 12V draw for the fridge, but it maybe similar to the propane detector. And one thing to look for on the fridge is to make sure the anti-condensation switch is OFF. If it is ON, 12V will power some small heater strips around the door of the fridge to prevent condensation, and it will kill the battery fairly quickly.
Hope that helps.

Gilligan


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## advancedtel (May 7, 2008)

Gilligan said:


> The biggest single drain on the battery, when everything is turned off, is the propane detector, which I have measured at about a 150mA draw. That would be 0.15 Amp, which calculates to 3.6 Amp/Hours per day. Unfortunately, the propane detector is hard-wired to the 12V, so the only option there would be to install a switch in series with the 12V supply, or possibly you could pull the fuse that supplies that circuit, assuming it is not on the same circuit as the fridge. The smoke and CO detectors run off of their own batteries, so they do not draw on the 12V. I have never measured the 12V draw for the fridge, but it maybe similar to the propane detector. And one thing to look for on the fridge is to make sure the anti-condensation switch is OFF. If it is ON, 12V will power some small heater strips around the door of the fridge to prevent condensation, and it will kill the battery fairly quickly.
> Hope that helps.
> 
> Gilligan


I have never seen an anti-condensation switch. Where would it be located? Could it be mine doesn't have one? Its a 2008 model. Thanks for the input - it does help.


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## Gilligan (Aug 25, 2006)

advancedtel said:


> The biggest single drain on the battery, when everything is turned off, is the propane detector, which I have measured at about a 150mA draw. That would be 0.15 Amp, which calculates to 3.6 Amp/Hours per day. Unfortunately, the propane detector is hard-wired to the 12V, so the only option there would be to install a switch in series with the 12V supply, or possibly you could pull the fuse that supplies that circuit, assuming it is not on the same circuit as the fridge. The smoke and CO detectors run off of their own batteries, so they do not draw on the 12V. I have never measured the 12V draw for the fridge, but it maybe similar to the propane detector. And one thing to look for on the fridge is to make sure the anti-condensation switch is OFF. If it is ON, 12V will power some small heater strips around the door of the fridge to prevent condensation, and it will kill the battery fairly quickly.
> Hope that helps.
> 
> Gilligan


I have never seen an anti-condensation switch. Where would it be located? Could it be mine doesn't have one? Its a 2008 model. Thanks for the input - it does help.
[/quote]

It is located on the bottom of the control panel. You will have to open the freezer door to see it. In the Dometic manual, they refer to it as the "Climate Control" switch.

Gilligan


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## jake's outback (Jul 24, 2010)

Hey I learned something today! Thanks Gilligan! Not sure if it applies to the 2011 one I have but nice. I would have pulled all the fuses that powered 12V stuff other than the fridge!


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## KTMRacer (Jun 28, 2010)

you mention the campground has full hookups. If so, why don't you just leave it hooked up and run on electric??

If not, the draw on the fridge is close to 1/2 amp typically. That's what my dometic draws. so that is 12AH/day, or about 150watt/hrs day. A 15 W solar panel is really not 15W. it is 15W at 20V, or about .8A, or 10 watts into a 12V battery. (specsmanship) So if you could get 8 hrs of sunshine you might gt 80Watt/hrs day or about 1/2 of of what you need. I'd say something in the 45W panel is closer to what you need. that would be a real 30W into 12 V and in 8hrs IF you can keep it in direct sunlight for 8 hours and there is minimal cloud cover, 240W, probably more than good enough. a 30W panel may be marignal.

The spec's on solar panels are decieving. The only way to get close to say 80W out of a 80W panel is to use what is known as a MPPT controller, that is a dc-dc converer that converts the 20V from the panel to 12V at maximum efficiency. They cost in the $200 range. The common controller with these panels is a PWM controller that doesn't do a dc-dc conversion, it basically only lets the battery see 12V even though the panel is outputting 20V and the efficiency goes way down.

As an example I have two 80W panels they are rated at 80W and 4.5a @ 18V. With a PWM controller all you can get is the 4.5A so at @ 13V=60 watts at best. With a MPPT controller I get slightly less than 6A @ 13V =75W. both these conditions are with bright sun and good aiming. If they aren't tracking the sun the average power over 8 hours is probably closer to 80% of the maximum.


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## Insomniak (Jul 7, 2006)

Guess I missed the part about leaving the fridge on all the time while in storage. Unless you can pull fuses on some of the gadgets you don't need (like the propane detector), you'll need something more than a 20 watt panel. There are tons of 50 watt panels on Ebay and one of those could give you 15-20 amp-hours per day under optimal conditions. That should be more than enough to keep your fridge going for a couple of weeks.


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## advancedtel (May 7, 2008)

Thanks everyone - this was a big help! KTM - I cant leave the camper hooked up as they move it to a storage area if someone wants to rent the site while I am gone.


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## raynardo (Jun 8, 2007)

Most of the replies regarding solar power here are theoretical. Here's the voice of experience:

I actually have an 80w solar panel on the roof of my Outback. Entire system made by Samlex - it's a kit (Google Samlex and you'll get a good education). My system keeps my 2 - 6v Costco batteries fully charged every day.

I use it with an 800w inverter to allow me to watch my DirecTV HD satellite TV system whenever I want, since the next day the system is fully recharged again.

I started with a $100 15w panel and then two. Even combined they were lame. Again: you get what you pay for. When you buy the best, you get the best.


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