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Ask Dave: How long will 100 W solar panel and two batteries run residential fridge?

Dear Dave, 
When I purchased my RV from Camping World they never told me anything about the solar panel on the roof and the inverter. I know now that the residential refrigerator runs off the two batteries that are charged from the solar panel or shore power, but the inverter is in the off position and has to be turned on manually, I assume. I would think that if we lost shore power it would automatically switch on the inverter. I’m trying to understand the solar system and the RV, and if I should upgrade the 100 W panel to a bigger panel. I heard that there’s a 300 W or 350 W panel available now. —Joseph, Cedar Creek 38ftk 43’

Dear Joseph,
I cannot find the model you have listed. However, it seems that Forest River uses a 1000W inverter in most of their larger 5th wheel units. Typically these units have a 12-volt cable coming from the battery to the inverter and an automatic transfer relay that determines if you are plugged into a 120-volt source such as the campground pedestal or generator. When 120-volt power is present, the transfer switch goes into pass-through mode and allows the 120-volt power to pass through to the appliances that the manufacturer has determined need AC power while boondocking. Typically this is a residential refrigerator and a few outlets such as the TV. Some manufacturers are now running the line to the bedroom for outlets for CPAP machines.

When there is no 120-volt power being supplied to the inverter, the relay switches and draws 12-volt DC power from your batteries and inverts it to 120-volt AC power to supply those chosen outlets. And, yes, it does have to be turned on. You would want to always leave the inverter on so it can switch, as you indicated, except during storage.

Size of solar panels

Let’s take a look at your other question about the size of your solar panels. You state there is a 100W panel on the roof and two house batteries. The first question is, what size batteries and what condition are they in? If they are lead acid or AGM batteries, you will only be able to draw 50% of the listed amp hours. So if you have two 100 AH batteries connected parallel, which is positive to positive and negative to negative, you will have 200 AH but can only use 100 AH. If you have two 6-volt batteries connected in series, which is positive to negative, it will give you a 12-volt bank but does not double the amp hours. So you would only have 100 AH and can only use 50%.

I know it’s confusing and nobody told you that there would be math required. Well, we are not done yet! If you have two lithium-ion batteries, then you would have 200 AH and can use almost all of the available amp hours.

How long will you run unit with just batteries?

The answer to the original question depends on several factors, starting with how long do you estimate needing to run the unit with just the batteries? Keep in mind when the power goes off or you are dry camping, you will be using other 12-volt components, so you need to calculate what will be used and for how long. This is not a perfect science as it is difficult to guess what might be running and for how long. Go Power! has a good calculator sheet that will at least help to identify these components and calculate what size solar panel system as well as battery power you need. Download a copy here.

If you have two 12-volt lead acid batteries, you will not be able to run the refrigerator very long, and the 100-watt solar panels will not be enough to keep up for very long. If it’s just overnight, it might be fine, depending on the condition of your batteries due to sulfation.

According to the Zamp Solar website, they do offer a Forest River 100-watt kit, which seems to be what you have on your rig. You can add a second 100-watt panel or go bigger. However, I would also suggest looking at adding a portable model. This would allow you to park the rig in the shade. That helps keep the rig cooler during hot temperatures, and you can put the panel out in the sun to provide a charge. Both Zamp and Go Power! have the permanently mounted panels as well as portables. They are both owned now by Dometic.


 You might also enjoy this from Dave 

Why aren’t my solar panels charging the house batteries?

Dear Dave,
I have solar panels to help charge the house batteries while boondocking, but after a couple of days the house batteries are dying. I turn on the engine to recharge the house batteries, but often it won’t charge them, even if I let the engine run for 15 minutes or more. Sometimes it charges as soon as I start the engine. What’s going on? —Brian, 2017 Leisure Travel Van (Unity)

Read Dave’s answer.


Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here

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Mike
1 month ago

Hope this helps…I have a large residential refrigerator that pulls 4 amps when running with 8 lead acid, 7 year old batteries, without solar and I can run a weekend on the batteries.

Admin
Diane McGovern
1 month ago
Reply to  Mike

Sorry, Mike. I have no idea why our filter held your comment for moderation. 😯 Have a great day! 😀 –Diane at RVtravel.com

Mike
1 month ago
Reply to  Diane McGovern

No problem. Thanks, you too.

Admin
Diane McGovern
1 month ago
Reply to  Mike

Thanks, Mike. I’m getting the hang of my new computer (after 10 years on my previous one and being a “computer idiot,” which is much worse than being computer illiterate), and our snow has finally melted. It’s going to be a good day. 😀 –Diane at RVtravel.com

Left Coast Geek
1 month ago

I have a Norcold N2175 12VDC compressor fridge, and 406AH of Lithium…. without any sun, I can run the whole trailer for at least a week on pure battery, and the 360W solar panel will fully recharge it from nearly dead in 2-3 days of full sun. As long as I’m parked in the sun, and get at least 30% sunshine average each day, it will run forever.

this fridge is MUCH more efficient than the typical 120VAC residential fridge. its 6.3 cubic feet, has a separate freezer drawer.

Snoopy
1 month ago

I have a 2017 F.River & have a 16 cu ft residential refer. It draws 6 amps when on shore power & when on inverter I’ve seen it draw 84 amps from my batteries, but only for short period. I’m thinking with Joes set up, it won’t even cool his refer down! He’s going to spend a lot of money to make his fifth wheel boondockable! Currently I have 4 AGM L16 6V 390Amp batteries & 480 watts of go power solar! At best I can run 24 hrs. & not have to worry. With 390 amps of available battery, I only need to run my gen for a couple hours & the solar kicking in its share. Wish I knew about Lithium-ion batteries when I done my system, tho 10K minimum is a huge bite out of the grocery bill!
Snoopy

Bob p
1 month ago

In the first letter I’ll bet camping world didn’t have anyone capable of telling him anything about the solar charging system. Solar energy is not new, but the sudden arrival in the RV industry is, and unless a technician takes it upon themselves to learn about it, it’s not going to be part of the training program for several years. The older tech’s that are running the shops don’t see a need for it so they’re not going to their super to push for training.

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