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Old 19-10-2020, 06:50   #1
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Seeking 12V guidance

Looking ahead to spring 2021(winter haulout looms this Saturday), I have purchased a Dometic 30CD drawer fridge as a replacement for my icebox. It evidently draws 3A but sundry literature is a bit vague. My house bank is two new Firefly Oasis Group 31s which I will use in parallel to provide 132AH. My starter battery for an Atomic4 is a Fall River AGM. My planned solar source is two Renogy monochrystalline 160W panels feeding an EPever DuRacer 40A MPPT controller. One side of the controller will supply the Fireflies, the other the Fall River. Bottom line: I should get at least 8-10A from the panels on a sunny day; will that be sufficient for the Dometic? I could add a semi-flexible 100A atop my dodger but would rather not. All lighting is LED, so the Dometic would be the main load. Any advice/observations would be greatly appreciated -- many thanks in advance.
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Old 19-10-2020, 07:07   #2
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Re: Seeking 12V guidance

The specs say “rated input power 36W”, I would read that as 3A when running. Hopefully it doesn’t run 100% of the time, but if it does that’s 36Ah/day and maybe 20Ah of that is overnight while the solar is not working.

The Fireflies should be 116Ah =232Ah total?

I would expect more than 8-10A from 320W of solar, but you know your installation, so maybe shading cuts that down. Generally we use 4-5 hours of rated output for total solar production per day, so 8x4=32 (most pessimistic calculation) says the solar might almost keep up. Realistically unless you have a lot of shading I would expect more from the solar and you will likely be OK. If I were estimating a nice, clean installation of 320W I would expect around 80-100Ah/day. Most of that should go to the Oasis, the start battery shouldn’t take much.

Caveat being, what other loads do you have? The LEDs are fine for several hours, but how about music, or TV, or charging a computer? Those are likely larger and can eat into power budgets but get overlooked.
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Old 19-10-2020, 09:51   #3
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Re: Seeking 12V guidance

From my experience with similar solar panels, you should see approximately 20A for 4-5 hours per day on a good sunny day with diminishing amounts before and after those hours.


A fridge that uses 3A continuously will use 3x24=72 amp hours per 24 hours. However, once fully cooled down it is common to see a fridge run approximately 1/2 of the time, which would bring your daily (fridge only) draw down to 36AH. Your 320 watts of solar should yield 80-100AH on good sunny days which will cover your fridge needs. You then need to factor in enough battery capacity to cover any additional loads and hold through the night and probably some additional for less than sunny days.


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Old 19-10-2020, 09:55   #4
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Re: Seeking 12V guidance

kchace,

thanks for posting that, I had slipped a 0.5 in there as average amount of runtime for fridge and then failed to show it Can't edit now, so my text doesn't make sense, oh well, that was my intent.
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Old 19-10-2020, 10:31   #5
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Re: Seeking 12V guidance

3A-24hr is 72Ahr.
320W solar should average about 100Ahr/d using MPPT controller.

You probably want another battery and some more solar.
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Old 19-10-2020, 10:33   #6
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Re: Seeking 12V guidance

Oops. Part of my answer has been addressed.
If you have an autopilot you are probably still under powered.
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Old 19-10-2020, 18:38   #7
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Re: Seeking 12V guidance

We have 200W solar array & have an Engel fridge that draws similar I think.
We get 8 to over 10 amps in peak sun ( Meter only goes up to 10 ). Solar panels out perform their rating down here I believe but boy the sunburn is bad. Only have a cheap Chinese PWM solar controller that cost 1$. Our 24 hr consumption is 55amps running GPs/fishfinder LEDS & VHF a bit. Sometimes a bit of laptop too.
Have no trouble covering power draw if the sun shines.
Think you'll piss in covering your needs but I'm no electrical guru, just giving you a real world comparison.
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Old 20-10-2020, 05:52   #8
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Re: Seeking 12V guidance

Thanks all. No TV but I do have VHF, AM-FM and Raymarine W/D/S/AP but the latter sees minimal use, i.e. to hold course into wind while raising the main. As an aside, the boat is lakebound so mostly single-handed daysailing is the game. BTW, I mistyped by AH; they are 232, not 132.
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Old 21-10-2020, 19:48   #9
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Re: Seeking 12V guidance

for a quick guide to how much power you can expect from your solar panels - in terms of a daily yield in amp hours (at 12V):

- for polycrystalline panels divide the panel wattage by 5
- for monocrystalline panels divide the panel wattage by 4
- for SunPower monocrystalline panels divide the panel wattage by 3

eg. your 320W of Renogy panels could provide around 80Ah in a day at 12V

Maximize the 'roof space' you have aboard for solar
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