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.
source:
Light Cycle vs. Deeper Cycle NOTE: This article deals with FLOODED marine batteries, not AGM or GEL batteries. The Problem: The problem, as I see it, is one of dishonest marketing where multiple battery types wear the same clothes (sticker) and will perform differently when
marinehowto.com
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.
Nominal power: 360W. Output Power: 12V DC 30A. Green Power On LED indicator light. Design for all LED 12V needs and others. Built-in Cooling Fan for longevity. LED Modules for demonstration only, they are not included in this sale.
www.ebay.com
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.