Non grid connected solar to run a fridge?

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So, let's say if we don't want to deal with grid connect, regulation, but still want to run cheap solar to cut out some usage at minimum cost:

1. Existing fridge is my target because it is a constant electric usage and pretty predictable usage when the sun shine and regardless of season.
2. Minimum solar, battery, and inverter investment.
3. Switch over based on timer with a safety gap in between, so no accidental power crossing over between grid and inverter.

I see that a typical fridge needs 1200w starting draw and 200 continuous draw, so a Harbor Freight $120 2000w inverter seems to be ok. Then a typical 200w panel and control / battery power kit seems to run $200 on amazon (as cheap as $60 on wish but that's shady as heck, so no), then add a $120 12V battery with 5 year warranty, plus some smart power plug that has timer settings (i.e. $10-20 on amazon), or maybe 120V 15A normally open / normally closed relay to auto switch between power sources based on the inverter output / battery charge.

Will this work automatically or will it fry something along the way?


Seems like 200w * 8 hrs a day = 1.6kwh a day * 20c / kwh = 30c a day. $500 investment for 30c a day saving = $120 a year give or take, and $40/yr for replacement battery give or take afterward the first battery is dead.
 
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Doesn’t seem worth it to me but my electric is around $0.12/kWh. Does your fridge actually run 8 hours a day? I’m not sure any of mine do.
 

OVERKILL

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You are going to need significantly more than a 200W panel, that's 200W "peak", you are going to want to at least double it, maybe triple it, to keep the battery in a reasonable state of charge for when the compressor kicks on/off.
 
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You are going to need significantly more than a 200W panel, that's 200W "peak", you are going to want to at least double it, maybe triple it, to keep the battery in a reasonable state of charge for when the compressor kicks on/off.
also need more than 1 deep cycle battery.

FWIW my fridge runs a couple hundred watts but my 1200w/2400w surge coleman inverter wasnt too happy powering it.
this was with 2ft 4gauge all copper cables run to inverter from running car.
startup would make it squeal abit. but it worked.

OTOH my 700w(1200 surge?) storm power battery backup ran the fridge fine. for about 6 hours continuous.

This was after the fridge warmed all night so it ran awhile.. PITA was when the defrost timer kicked on.
 

PandaBear

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Doesn’t seem worth it to me but my electric is around $0.12/kWh. Does your fridge actually run 8 hours a day? I’m not sure any of mine do.

Around here it is about 22c to 28c for tier 1 and tier 2 rate that I can see, half of that is for grid and half for generation. Seems like someone has to pay for those hefty power line fire and gas pipeline explosion a few years back. I was thinking about 8 hrs of sunlight give or take, then auto switch to grid based on the power.

Looks like it is not worth the trouble keeping a solar off grid for our usage. I though it need at least to cover the fridge to make sense, and even that doesn't.
 
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You will never see 200W from a 200W panel. Those numbers come from a new lab cleaned panel under lighting conditions you can not achieve in nature. In other words they are the hypothetical max the panel could produce. If you had a tracking mount on a good sunny day without pollution or haze you might get 150W until dust settled on the panel. If a cloud passes over you wont get 20%. You need to scale your battery with the thought in mind that you might get a cloudy day or 2 and no charge and if you dont have a tracking mount figure 25% of panel output over the day light hours. If you plan on cycling the battery every day the cycle life will kill it long before the years and you wont find a true deep cycle battery warrentied for 5 years for less than $300 and even then they may not honor it connected to a solar array.
Solar can be nice but in a stand alone system it is always more expensive than grid power, kind of like running a standby generator. Its the reason the grid connected systems have dominated, maintaining batteries and chargers and switches kicks the price way up.
 
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I agree with all the responses above.
That, and there are many videos on YouTube where someone is trying to power a window AC with solar panels. This is going to be the closest thing that I have seen to what you are doing.
 

PandaBear

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You will never see 200W from a 200W panel. Those numbers come from a new lab cleaned panel under lighting conditions you can not achieve in nature. In other words they are the hypothetical max the panel could produce. If you had a tracking mount on a good sunny day without pollution or haze you might get 150W until dust settled on the panel. If a cloud passes over you wont get 20%. You need to scale your battery with the thought in mind that you might get a cloudy day or 2 and no charge and if you dont have a tracking mount figure 25% of panel output over the day light hours. If you plan on cycling the battery every day the cycle life will kill it long before the years and you wont find a true deep cycle battery warrentied for 5 years for less than $300 and even then they may not honor it connected to a solar array.
Solar can be nice but in a stand alone system it is always more expensive than grid power, kind of like running a standby generator. Its the reason the grid connected systems have dominated, maintaining batteries and chargers and switches kicks the price way up.

Good info, thanks. Was thinking about building the panel size just right (maybe if I put in 600W it would work), and just shallow cycle with car battery (keep replacing it pro-rated warranty). If there is a fridge that has variable compressor maybe it won't need a big battery to cycle on and off but then we'll be trading cost of the fridge and the cost of batteries.

In the end I guess with today's technology it is not cost effective. Probably an inverter based window AC would be ok but still, likely not worth it without grid connection.
 
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You are describing a full-on solar/battery backup system to run a fridge. One expensive item is the isolation switch to keep your system isolated from the grid. That’s a pretty expensive switch, not like the Frankenstein switch in the movies. I
understand it has to sense power in the grid to re-connect to the grid. This is so you don’t electrocute a lineman during a power outage. Anything short of that, you are jury rigging and liable. However, you have excellent solar insolence and fairly expensive grid power so carry on and let us know what the total cost was.
 
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If you are going to actually do it, probably the first place to start is to find out what refrigerator is the most efficient, get one, and then add a heck of a lot of additional insulation around it being careful not to restrict air flow to the condenser.
 
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If there is a fridge that has variable compressor maybe it won't need a big battery to cycle on and off but then we'll be trading cost of the fridge and the cost of batteries.
you also have to consider the fridge usually defrosts at least 2x daily. so there will be extra power draw for defrosting, then more for cooling it back down.

I'll get solar when they have shingles/roofing that are solar cells and it just works at 1/10 the cost of now.. oh wait maybe that will be after I'm dead ;)
 

PandaBear

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You are describing a full-on solar/battery backup system to run a fridge. One expensive item is the isolation switch to keep your system isolated from the grid. That’s a pretty expensive switch, not like the Frankenstein switch in the movies. I
Understand it has to sense power in the grid to re-connect to the grid. This is so you don’t electrocute a lineman during a power outage. Anything short of that, you are jury rigging and liable. However, you have excellent solar insolence and fairly expensive grid power so carry on and let us know what the total cost was.

If you have 2 switch that are set 5 mins apart using timer as a way to keep things separate, so your fridge is blacked out for 5 mins between connecting to the grid vs the solar, it should be fine.

This is one other reason why I was thinking using a fridge as the target, you don't need to worry about it blackout for a few mins a day. As long as it was cold before and after it is fine with enough heat sink inside. Another alternative is to plug in a UPS and use the 12V to "charge" the UPS, that would probably be very inefficient but will prevent any back powering.
 
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The inverter powering a residential fridge is not a novel idea.
Many have pulled it off, without having to get all that much more solar or battery power than a dedicated 12-24v DC fridge would require. Many have tried it and found it chewed up their batteries prematurely and required way more solar than expected.

Many convert chest style residential freezers, using a different thermostat that also triggers the inverter to kick on when the compressor needs to.

An inverter turned on, not actively running the compressor, will consume wattage, My 800 watt consumes 0.68 amps(@12.8vdc) ( 8.32 watts) turned on, powering nothing. This standby load is different for all inverters, but in general the larger the inverter the larger its standby draw is.

Most inverters are modified square wave inverters, not true sine wave inverters, which are more $$$.
The fridge might not care, or it might.

Residential fridges can use the skin of the fridge as the condenser, making adding more insulation there impossible.
The defrost cycle eats up a lot of wattage.

Dedicated DC fridges are $$$$!
Look for fridges with Danfoss/Secop BD35 and BD50 compressors. They are variable speed, 2000 to 3500 rpm. The Sawafuji swing compressor is good too, but noisier and less efficient, in my experience.

If one gets a DC fridge, then one can have their battery powering it directly, no inverter, no grid tie, no transfer switch.
One should have a solar controller which is programmable, as to absorption voltage Absorption duration and float voltage

Then during the night time, one need not cycle the battery to power the fridge, when they have the grid available, but can use an adjustable voltage power supply, set to just below the float voltage of the solar controller, to hold the battery at full charge overnight, while also powering the fridge. As soon as the sun rises then the solar starts taking load off of the DC power supply and for much of the sunny day, it will be drawing next to nothing from the grid.
Yes, some solar wattage can/ will be wasted as the battery is already full and taking next to nothing from it.

This way The battery remains fully charged, and should the power go out, it is already powering the fridge and will do so for however long it can automatically, how long is dependent on battery capacity and fridge's current draw.

Golf cart GC-2 batteries are 6v and true deep cycle batteries. a 12v flooded Marine battery is NOT true deep cycle.

Next time somebody tries to ascribe 'deep cycle' nomenclature to a common flooded/wet 12v marine battery, print out the following picture, and smack them across the face with it. Twice. One can deep cycle a starter battery, doing so does not make it a 'deep cycle' battery though.

08-What-Is-A-Deep-Cycle-Battery.jpg

source:



2 GC-2's wired in series are needed for 12v. You'll likely want 4, and they should be able to power a pretty large DC fridge in 75f ambient for 3 days without solar/Grid assistance. They are easy to get as sams costco or batteries+, and are usually cheaper than 12v marine batteries, while being heavier, having more capacity, and being able to be cycled 2 to 3 times more than the best marine battery, when treated well.

DC fridges do frost up, but have thicker insulation, and their condensers usually have efficient 12v DC brushless fans on them. Adding more insulation and insuring best possible airflow across/through/around condenser/compressor and compressor controller, can really increase the efficiency greatly.

DC fridges do not have huge start up surges. My Danfoss BD35f run at 2000 rpm draws ~2.5 amps/ 33 watts running, and the highest start up surge I've recorded is 56 watts. changing compressor speed is done via a resistor added in the thermostat circuit. I don't bother but would, if I were constantly filling it with warm items that needed to be cold ASAP.

Get no less than 1 solar watt for every 1 amp hour of battery capacity. More is better, for the cycling battery.
A pair of GC-2 batteries has 200 to 235 amp hours of storage capacity.

A LED power supply like this can be adjusted to 13.19v, and hold the flooded gc-2 battery at 13.19v all the times the solar panels can't directly power it.


This ^ is just an example to show how cheaply one can get a constant voltage adjustable voltage DC power supply. Burlier better built models are available. Look into Meanwell power supplies. This one rated for 30 amps^ can likely provide 36 amps, for about 10 minutes before it burns itself up, but perhaps they incorporated more safety features ( constant current on overload) than my experimental cheapo had.

I've since modified something similar, (meanwell rsp-500-15) to use as an adjustable voltage(13.12 to 19.23) 40 AMP battery charger. When I set it to 13.59, and my solar controller's float voltage is set to 13.6v, anytime the sun shines that is wattage the grid is not having to provide to the meanwell, and the battery is always fully charged and ready to go, and then late afternoon the grid can tale over where the solar leaves off, if I remain plugged into it.

I have a 100 watt monocrystalline panel that near the summer solstice on a good sunny clear day, aimed directly at the sun, when still cool, provides about 92 watts, and this tapers to about 82 watts as the panel heats up from ~70f towards 120f At 140f this is about 73 to 75 watts maximum.
It can only provide this 82 watts for about 2.5 hours either side of noon and when aimed well at the sun. It can only produce 100 watts when it is cold( sub 60F) and aimed well at the strong high sun and can only maintain that output if it remains sub 60f.

DC fridges are usually made for the RV or marine marketplace but there are some for the off grid community as well. All of these are designed around efficiency, whereas residential fridges are designed to be marketed to those who pay 12 cents kwh, and whine about it while leaving door open for 5 minutes while preparing food.

I use a Vitrifrigo c51is as my DC fridge
It's small. 1.8 cubic feet.
If I spent 100$ more I could have got the c51isAC which runs on AC anytime it has grid power and switches automatically to DC when there is no AC power available. I spent the 100$ on the adjustable 40 amp DC power supply instead, which effectively does the same thing, but is an extremely capable 40 amp charger too.

To this DC fridge I've added insulation to 5 of its sides, added another door seal inside the original, and use a better fan sucking coolest possible air from floor and pushing it through condenser, across compressor and compressor controller, and then out of the cabinet so the condenser cannot be bathed in its own heat.

It uses on average 0.42 AH each hour as I use it65f ambient overnight 75f day. My 12v AGM battery has 100 amp hours of capacity.
In theory, In an emergency, with no solar or grid power available, I could run it for over 200 hours on the fully charged battery alone.

Of course, I do not want to take the battery that low, regularly anyway.

Dometic/Waeco/ Norcold/Vitrifrigo/NovaKool( canadian), Isotherm/Truckfridge/frigoboat all make front loading dc compressor fridges for boats and RV's.

If one wants a chest style DC compressor fridge there are more Brand options these days, but if they are under ~350$ they are not a compressor fridge but a thermoelectric cooler which use no less than 4x the electricity and cannot cool to more than 40f below ambient.

These chest style DC compressor fridges can be used as an in car fridge, with a ciggy plug, though ciggy plugs are junk and unreliable and best bypassed. The can be used as a freezer too, when plugged into the grid, and even if not as long as one has enugh battery and solar.

If you get the DC fridge, and a DC powersupply for powering it overnight, the GC-2 batteries will be doing nothing but remaining fully charged at all times . They will age out in 10 years or so, even if never cycled, but if cycled to 50% and fully recharged to 100% promptly, they can be cycled ~1200 times. Might as well plan on cycling them once a week or every two weeks when the next day is going to be sunny, and that is KWH not being added to your grid's bill. All you'd have to do is unplug the power supply.

Return on this substantial investment is going to take a long while, but in case of emergency, you could have an operational fridge for however long it take them to restore power. YOu can always get a small, or large inverter for the batteries and be able to power laptops and other things too.
The DC power supply is a great manual battery charger too, with some caveats, depending on what power supply you choose.

I always thought about setting up just enough solar and battery to run only the residential fridge in case of extended power outages, but some residential fridges can be such powerhogs that one would need to have to deal with long duration power utages and lots of spoiled food, often before investing in such a system.

The people with panels on their roof, but no battery and inverter, are quite frustrated when the power goes out and they have 2000 watts on their roof they cannot use to power their things directly.
 
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