How much Solar is Needed?

The friendliest place on the web for anyone with an RV or an interest in RVing!
If you have answers, please help by responding to the unanswered posts.

Greyghost435

New member
Joined
May 20, 2022
Posts
1
Location
B
We just bought a 2022 transcend by grand design end it comes with a 12 V fridge. There are 2 6 V batteries on it and a 165 W solar panel. I know there are many scenarios that will impact how much solar you need but I have the following question. Under worst case scenario with minimal sunlight how long would the 2 12 V batteries? Would you get 24 hours out of it? Just trying to determine if I need to add another solar panel.
 

DonTom

Well-known member
Joined
Apr 21, 2005
Posts
7,114
Location
Auburn, CA or Reno, NV
We just bought a 2022 transcend by grand design end it comes with a 12 V fridge. There are 2 6 V batteries on it and a 165 W solar panel. I know there are many scenarios that will impact how much solar you need but I have the following question. Under worst case scenario with minimal sunlight how long would the 2 12 V batteries? Would you get 24 hours out of it? Just trying to determine if I need to add another solar panel.
Do you have a generator? If not, you probably need all the solar you have room for plus a bit more. You can never have too much.

There are many variables, as you know. But all the guesswork can be removed with a SmartShunt and then you will soon learn what your needs are. It will keep track of everything such as how many AHs you have left and what SOC (State Of Charge) you're at. It's nice to know when fully charged and how much you have left when not.

I find in a normal night boondocked, I will use around 10% of my SOC. That means a couple of hours watching TV, using my ham radio stuff and a bit more. But I have a 300 AH lith battery that I charge back up to 100% every day by both solar and my genny when boondocked. I have a 300 AH Lith battery and about 150 watts of max solar on a long a sunny day. I think of solar as a way to use less gasoline from the genny. Genny still has to be ran, but not for as long. I should be good for ten nights on a full charge, but I never let it get down that far.

-Don- Reno, NV
 

Mark_K5LXP

Well-known member
Joined
Nov 17, 2018
Posts
2,466
Location
Albuquerque, NM
Under worst case scenario with minimal sunlight how long would the 2 12 V batteries? Would you get 24 hours out of it? Just trying to determine if I need to add another solar panel.
In very coarse terms you have a max of 2600 watt hours of battery, and looking around online it seems a number of 12v fridges need 1000wh a day. So if the fridge was the only load then the math says 2 days and change. Throw in some house loads, batteries that aren't the best, less than a full charge, on and on I wouldn't count on more than a day. At a minimum you'd need a battery monitor to really know the net power available at any given point. Taking this to the next step you need to replenish 1000Wh a day plus house loads. Again, very coarse numbers is you get 50% of your panel for 6 hours a day so a 165W panel is around 500Wh a day. A 2nd panel gets you about net zero with the fridge. In full sun you're still in deficit mode with house loads, so you need even more panels or some other power input in the form of shore power, genset or vehicle. Duration of operation is a factor too, say if you only need 4 days of use you can operate in deficit mode, as long as you have enough solar and battery combined to last that time. Nothing says you have to leave the campsite with full batteries. A few trips out will give you a much better feel for power usage for the way you use it, and from there a system configuration can be more precisely determined.

Mark B.
Albuquerque, NM
 

Lou Schneider

Site Team
Joined
Mar 14, 2005
Posts
11,798
In very coarse terms you have a max of 2600 watt hours of battery, and looking around online it seems a number of 12v fridges need 1000wh a day. So if the fridge was the only load then the math says 2 days and change. Throw in some house loads, batteries that aren't the best, less than a full charge, on and on I wouldn't count on more than a day.
That's about what I saw when I installed an apartment sized 120 volt refrigerator in my Arctic Fox trailer 20 years ago. It's power consumption was only slightly higher than the current 12 volt refrigerators and I was able to get about 48 hours out of (4) 6 volt batteries.

Like DonTom suggested, get a SmartShunt or a similar battery monitor and let it track the refrigerator's power consumption.
 

Ex-Calif

Well-known member
Joined
May 15, 2020
Posts
3,077
Location
NorthCentral Florida
Short answer is yes you need a second panel.

In Mark's post he talks about the budget deficit. I am a little more optimistic than Mark and reckon you'd get more like 700W per day from one panel.

2 X 165 could get like 1400 a day.

Assuming the 6V batteries are 200a/h you have 1200W stored. If you had a day with no sun the batteries would be drained in 24 hours, likely.

When the sun came out the RV still wants 12-1400W a day so there is nothing left to charge the battery up.

My general rule of thumb is that you want 1/2 the solar number in amps for the battery. So 400 a/h battery and 800W solar. This is pretty much how we set up most boats as a basic solar install to run the house pretty much off grid.

Your next step probably wants to be 200 ah battery and about 400W solar. This probably lets you go 24 hours with no sun and leaves a little solar to catch up the batteries when the sun does come back out.

Depending on your latitude you could harvest a lot fewer watts, especially in early spring and fall.

Like RVs boats have generators and engines and many have a wind turbine. So when you get 3 days of rain you basically are running a genny or the engine to keep topped up.
 
Last edited:
Top Bottom