solar panel

Hi all,needing help again,am looking at purchasing solar panel to hook onto deep cycle battery in 4wd,will be running 1 and sometimes 2x 40 litre engel fridges (1 freezer,1 fridge) and when we repurchase another caravan to maintain charge in 1 12 volt battery,question..... what size wattage would i need,expect to stop 3 to 6 days at times,thanks,peter.
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Reply By: ABR - SIDEWINDER - Wednesday, Jun 02, 2010 at 20:45

Wednesday, Jun 02, 2010 at 20:45
Please advise what batteries are fitted to the 4wd.


Derek from ABR
AnswerID: 419289

Reply By: Battery Value Pty Ltd - Wednesday, Jun 02, 2010 at 21:05

Wednesday, Jun 02, 2010 at 21:05

two fridges might draw a mean current of 3 amps.
Over 24 hours, this gives 72Ah (amp hours).
So your daily solar requirement is 72Ah plus 20% (covering for all sorts of losses and inefficiencies), make it 90Ah.

For calculating solar panel wattage, divide this figure by 3.5 ('peak sun hours' which is derived from average sunshine hours per day, and then a correction factor is applied).
Now you receive 25.7A, which is what your panels will have to supply on current.
If you multiply this by 14.5V you have the necessary panel wattage, namely 372W.

Note that this installed wattage will be enough to get you through cloudy days, but your batteries will have to be capable of bridging the gap too. I recommed 100Ah extra per cloudy day. So if you want to power through 2 cloudy days, you'd have to install 3x 100Ah batteries.
If you install less solar wattage and energy storage you'll have to run the generator more frequently.

Best regards, Peter
AnswerID: 419299

Follow Up By: Ann 40 - Saturday, Jun 05, 2010 at 09:48

Saturday, Jun 05, 2010 at 09:48
hi,again,have just read tips and again most helpful,am going to have a look at some panels this morning,thanks to all,peter.
FollowupID: 689808

Reply By: Maîneÿ . . .- Saturday, Jun 05, 2010 at 12:36

Saturday, Jun 05, 2010 at 12:36
When you start driving, leaving home, the fridge and freezer will be cold and frozen respectively, they will be maintained this way because the vehicle Alternator will be charging all batteries connected to it while you’re driving to your first destination.

When you arrive at your destination, assuming sun is still shining the solar system will take over charging the already charged Aux battery system till nightfall.
The Aux battery system will then power the fridge & freezer that night.
The solar panel will recharge the Aux battery system next morning and run the fridge & freezer simultaneously till nightfall again, day after day - only if you buy solar panels that are capable of supplying the power your fridge & freezer uses each 24 hours in about 7 to 8 hours.
Any shortfall in power going *into* the Aux battery system has to be made up by the vehicle Alternator when driving or idling, if parked for prolonged periods.

To replace ~90ah in 7 hours you will need >200 Watts when done numerically.
My fridge as an example, because I've measured all the numbers often is:
draws 2.8ah and charged by a 200 watt solar system (12 Amp capacity) without the 200ah battery system ever getting below 12v (50% DOD).

I believe you need ~140ah battery capacity to last for about 3 days without going below 50% DOD, maybe 120ah if you keep an eye on it, but bigger is better as they say.

Check out “Mandrake's Solar Power” web site, the 140 watt mono panel shown there.

You will need an efficient solar regulator also to make this all work for you.

Maîneÿ . . .
AnswerID: 419638

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