What size solar panel

I was wondering is it possible to run a Dometic fridge, 185 L on 12 volts,using solar panels rather than using gas all the time. If so, what size panels would I need? Can anyone help me p[lease?
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Reply By: Pete Jackman (SA) - Monday, Feb 15, 2010 at 04:10

Monday, Feb 15, 2010 at 04:10
Anything is possible, but according to Dometic that fridge draws about 18 Amps on 12V and at a rough calculation you would need around 400W of panels and a large battery bank (300 AmpHours - ish) to keep up with the power draw. It would be much cheaper to refit with a compressor fridge that would run off a much smaller solar system.

There are others on here who are much better versed in Solar that can give you a better answer (and probably will).

The Dometics are normally very efficient on gas so why the need to change?

Regards

Pete
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Follow Up By: Member - Boobook - Wednesday, Feb 17, 2010 at 08:12

Wednesday, Feb 17, 2010 at 08:12
Pete, it is probably a mute point but I calculate that you would need about 2100w + of solar panels to keep up with that fridge in normal sunlight conditions. Just not an option, but I thought it worth mentioning in case OP was considering 400w of panels.
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Follow Up By: Pete Jackman (SA) - Wednesday, Feb 17, 2010 at 09:07

Wednesday, Feb 17, 2010 at 09:07
Agreed. They are a power hungry sucker.
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Follow Up By: Maîneÿ . . .- Wednesday, Feb 17, 2010 at 10:34

Wednesday, Feb 17, 2010 at 10:34
Wondering how you two guys calculate the 2,1OO Watts ?

Maîneÿ . . .
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Follow Up By: Pete Jackman (SA) - Wednesday, Feb 17, 2010 at 11:24

Wednesday, Feb 17, 2010 at 11:24
Hi Maîneÿ,

I did say "at a rough calculation" in my post.

My rough calculation to get 400W was:

Fridge draws 18 amps when the 12V is heating - from the dometic site and that is probably best case.

Then the assumptions started:

Fridge duty cycle of 50% = 216 amps per day. This seems resonable given that my chescold could flatten my N70 starting battery to less than 6 volts inside 3 hours. This assumption is also dependant on the 12 V having a themostat - my 50 litre chescold did not on 12V. If this is the case then the figures below are doubled.

So we now need 216 amps from the panels.

Set a peak solar day at say 8 hours, so we need 27 amps per hour - 324 watts.

Allow 75% efficiency from the panels unless you spend all day keeping them right angles to the sun = 432 watts nominal.

Thats where my rough guestimate of 400W came from. And it would be best case.

I have sold my absorbtion fridge and now have a Waeco 50l, 70 AH of gel cells and 160W of panels which all works fine even if I get a few cloudy days or have a shady campsite where I can't get the panles in the sun for the morning or afternoon.

Cheers

Pete
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Follow Up By: Maîneÿ . . .- Wednesday, Feb 17, 2010 at 11:32

Wednesday, Feb 17, 2010 at 11:32
Pete,
Umm, the question was:
"Wondering how you two guys calculate the * 2,1OO * Watts ?

Maîneÿ . . .

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Follow Up By: Pete Jackman (SA) - Wednesday, Feb 17, 2010 at 11:57

Wednesday, Feb 17, 2010 at 11:57
Hi Maîneÿ,

My posts do not mention *2,1OO* watts and I qualified my response with the words "at a rough calculation"

You seem to consider yourself an expert on solar. Rather than criticize Boobook and I why not provide some constructive feedback, answer the original question and tell all of us what size solar setup you believe is needed to run a 185 litre Dometic fridge.

I look forward to your solution.

Regards

Pete


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Follow Up By: Member - Boobook - Wednesday, Feb 17, 2010 at 16:11

Wednesday, Feb 17, 2010 at 16:11
I mentioned 2100 watts plus and here are my calculations.

The Fridge uses 18A at 12.8V.
Unlike a compressor type fridge these are 100% duty cycle on 12v mode. So that is 18 Amps times 24 hours or 432 Amp Hours per day!!!!

To generate 432 AH in a day with say 4 hours per day of full sun that is 108Amps per hour you need to charge into the batteries.

108 Amps x 19V =2052 Watts.

How much do you think it would need Mainey? with your calculations too would be good.


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Follow Up By: Member - Boobook - Wednesday, Feb 17, 2010 at 16:14

Wednesday, Feb 17, 2010 at 16:14
Oops

To generate 432 AH in a day with say 4 hours per day of full sun that is 108Amps per hour you need to charge into the batteries.

Should read

To generate 432 AH in a day with say 4 hours per day of full sun that is 108Amphours over the 4 hours you need to charge into the batteries.
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Follow Up By: Maîneÿ . . .- Wednesday, Feb 17, 2010 at 19:03

Wednesday, Feb 17, 2010 at 19:03
Pete, I did not criticise either of you fine gentlemen, my question, yes, it was a question, see the *?* at the end of the sentence, was:
"Wondering how you two guys calculate the 2,1OO Watts ? "

Pete, as Boobook states, a 3 way fridge does not have any 'off' period on 12 Volts, so it is being powered 24/7 by the battery system, sorry but your "rough calculation" of 400 Watts is therefore not relevant at all.

To generate the 432 ah required in a day, with say 7 sun hours, that is 62 Amps per hour over the 7 hours you need to place charge into the batteries.
The Time (7 hours) can be verified on this Australian Bureau of Meteorology map: *Average daily* SUN hours, there is no area in Australia, or even Tassie, that has only 4 Sun hours, as can clearly be seen most of Australia has 8 or 9 sun hours, so 7 hours is realistic.

Using your "19v" my calculations are; 62 Amps x 19 Volts =1,178 Watts, but where in hell do you put all of them panels, and they just run one fridge?

Maîneÿ . . .
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Follow Up By: Member - Boobook - Wednesday, Feb 17, 2010 at 21:23

Wednesday, Feb 17, 2010 at 21:23
Mainey, it may be 7 hours a day over the entire year, but that won't help you much in winter ( unless you have enough batteries to last from summer to winter).

In winter the average daily sunshine looks more like this.

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Follow Up By: Member - Boobook - Wednesday, Feb 17, 2010 at 21:26

Wednesday, Feb 17, 2010 at 21:26
Doh, wrong button.

......Like this Average daily sunshine June

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Follow Up By: Maîneÿ . . .- Wednesday, Feb 17, 2010 at 23:06

Wednesday, Feb 17, 2010 at 23:06
Hell I'm not looking to make points here but the Link I've supplied is for average sun hours *annually* so yes, some months will be higher and some lower, you have used just one month in the middle of winter, however you will see even in that mid winter month more than half of Australia has 7 or in excess of 7 sun hours.
I believe it would be difficult to run a 3 way fridge on solar, even on a generator it would be difficult too, that is why they run them on gas, and also efficiently and by the alternator when actually travelling.

Maîneÿ . . .
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Follow Up By: Member - Boobook - Thursday, Feb 18, 2010 at 07:30

Thursday, Feb 18, 2010 at 07:30
Whatever Mainey.

I originally just wanted to point out to the op how impractical it was, but once again the thread turned into a electronics lesson for one individual.
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Reply By: Member - Heather G (NSW) - Monday, Feb 15, 2010 at 07:42

Monday, Feb 15, 2010 at 07:42
Hi ronros,

We tried powering the 190 litre waeco (compressor) fridge fitted in our new van with 2 X 64watt panels and two 100 ah batteries and had great difficulty keeping it running for more than 24 hrs. It was very frustrating and we had to resort to buying a generator which we hated using!
And these fridges are supposed to be more economical on 12v than the 3-ways.

We calculated that we would need about double the solar and battery power we had to power that fridge and decided to change to one like yours, run it on gas when we werent powered, and havent had any problems staying unpowered for over two weeks in the one place since. The batteries are usually full after a couple of hours of sunshine in the mornings but even in wet and cloudy weather dont seem to have any problems getting back to almost full.

Why do you want to run the fridge on 12v instead of gas?

Cheers Heather
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Follow Up By: Member - Heather G (NSW) - Monday, Feb 15, 2010 at 07:44

Monday, Feb 15, 2010 at 07:44
forgot to add that we get about 11 days o a 9kg gas bottle when we run the fridge on gas, plus the hot water system, oven and cooktop. So with 2 full bottles when we get to any location we know we can stay for up to three weeks if we want.
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Follow Up By: Maîneÿ . . .- Monday, Feb 15, 2010 at 10:28

Monday, Feb 15, 2010 at 10:28
Heather,
* Unisolar * panel is: 64 Watt per Sq Mtr

Sharp ND130T1J is: 130 Watt per Sq Mtr
Sharp is more than *double* the output of the Unisolar panel per Sq mtr.

Both panels are similar physical size but with vastly different power numbers
Sharp is: 149 x 67 = ~1 sq Mtr
Unisolar is: 136 x 74 = ~1 sq Mtr

With 2 x Sharp panels I believe you could stay in one place indefinately
with a 190 Lt Waeco fridge, and you would have also saved money too

Maîneÿ . . .
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Follow Up By: Member - Heather G (NSW) - Monday, Feb 15, 2010 at 10:49

Monday, Feb 15, 2010 at 10:49
thanks Mainey,

we had one of the panels fitted as part of the deal at time of van purchase and weren't nearly as knowledgeable about solar and battery setup in those days unfortunately. As soon as we did a two day trip away to test it all we realised that we needed more solar and just had a second one the same as first fitted.

In those days I didnt know about ExplorOz and you - another mistake eh!

I found the website when we were experiencing our problems on our trip to Qld in 2008 and detailed some of our woes in my blog!

Yes it was expensive for us - probably cost us $1600 to change the fridge over to the dometic - but we have been very happy with our current set up. Have travelled in the past 18 months than we have been at home.
Not sure what it would have cost for us to change the solar panels to the sharp ones as we didnt enquire.

Cheers,
Heather


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Reply By: ronrox - Wednesday, Feb 17, 2010 at 00:35

Wednesday, Feb 17, 2010 at 00:35
Thanks Pete,

Great info, thanks, we thought if we could run the fridge on solar we wouldn't have to change the gas bottles every two weeks. Just a cost consideration really.

Regards

ronrox
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