Pete,

There is usually an Energy Star tag that comes with the refrigerator (or can look online at the EnergyStar website).

Because you have a full size 120 VAC refrigerator--While pretty efficient, they just use both a lot of energy (Watt*Hours) per day, they also take a lot of current to start the compressor and run the defrost heater(s). I will do the "minimum" design--Just run a fridge, but you are getting to the point where you are looking at a larger system to run your whole home.

You did not say if this was an "emergency" system for a week, vs full time off grid--But if you are looking at weeks to months off grid, you really need a full size "off grid" system (not cheap).

And as Photowhit says--I will do this in Watts (a rate--like miles per hour) and Watt*Hours (an amount, like miles driven in a day).Amps and Amp*Hours are a "partial unit"--You need to know what voltage is is measured at... P=V*I:

• P=V*I= 12 volts * 6.5 Amps = 78 Watts
• P=V*I= 120 volts * 6.5 Amps = 780 Watts or 10x as much power at 120 VAC vs 12 VDC

You will need to decide how much "safety factor" you need for your system. The solar+battery system has to be relatively large because of the high starting surge of the fridge compressor & defrost heaters. Like a large truck to get the load moving, even though not a lot of power is needed to go 25 MPH down the country road. And a 12 volt battery bank is a bit on the "small side" to supply the starting surge--But it can work.

First, you need a 12 Volt TSW/PSW (true/pure sine) AC inverter in the 1,200 to 1,800 Watt range. Next sizing the battery bank. 2 "rules"... One is to supply the starting surge, the second to supply the daily energy usage--Typically 2 days of storage and 50% maximum discharge (longer battery life). First surge (note, using lots of design rules that work well--I will not go into the details "why" for now--We can talk about later if you are interested)

• 1,200 Watt inverter * 100 AH * 1/250 Watt inverter (at 12 volts) = 480 AH @ 12 volt minimum battery bank
• 1,500 WH per day EnergyStar typical fridge energy usage * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge (longer battery life) = 588 AH @ 12 volt battery bank

Your 4x GC battery bank (6 volt @ 210 AH) is  on the small side (2s x 2p) or 12 volts @ 420 AH. I will do the design based on 6 GC Batteries--It will be more reliable for you. You can get 2x more batteries and wire them with the first 4 batteries--Or you can get a full set of 6. The first 4x batteries could have been damaged (I don't know). At this point, you might just keep the first 4x batteries and get the rest (solar panels, etc.) configured and test the system... Once up and running, adding/replacing the batteries then (or before hurricane season) might still be a good idea.

Next, calculate the size of the solar array.. Two calculations--Minimum solar array based on rate of charge for battery bank, the second based on your loads and hours of sun per day.

First rate of charge. For a short term/emergency backup system, 5% rate of charge can work (especially in a sunny region). Otherwise, 10% to 13% minimum recommended for full time off grid. Batteries are "expensive" and easy to kill--Solar panels are historically "cheap" and having more will keep the battery bank "happier".:

• 12 GC Batteries (2s x 3p strings): 630 AH * 14.5 volts charging * 1/0.77 solar panels+controller deratings * 0.05 rate of charge = 593 Watt array minimum
• 12 GC Batteries (2s x 3p strings): 630 AH * 14.5 volts charging * 1/0.77 solar panels+controller deratings * 0.10 rate of charge = 1,186 Watt array nominal
• 12 GC Batteries (2s x 3p strings): 630 AH * 14.5 volts charging * 1/0.77 solar panels+controller deratings * 0.13 rate of charge = 1,542 Watt array "typical" cost effective maximum

Next size the solar array based on your load and "hours of sun" per day (solar harvest). Fixed array facing south:
http://www.solarelectricityhandbook.com/solar-irradiance.html

San Juan PR

Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 72° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
5.43	5.88	6.29	6.13	6.00	6.49
Jul	Aug	Sep	Oct	Nov	Dec
6.38	5.86	5.71	5.69	5.25	5.29

 Ok--You have "lots of sun" which is good.... Looking at November for minimum sun.

• 1,500 WH per day fridge * 1/0.52 off grid AC system efficiency * 1/5.25 hours of sun per day (Nov) = 549 Watt array November "break even"

Normally, I would be suggesting that your "base loads" (those loads that must run evey day--You cannot "not use" the fridge for a couple days of bad weather (as an example). Typically the base load factor suggested is 65% to 50% of predicted system harvest. Or the array would be:

• 549 Watt array ( 1/0.65 base load factor) = 844 Watt array (65% factor)
• 549 Watt array ( 1/0.50 base load factor) = 1,098 Watt array (50% factor)

While your 4x GC batteries and 600 Watt array could "just keep up" with your typical refrigerator loads--There is no safety factor. For a reliable full off grid system, I would be suggesting around 1,186 Watt array minimum and 6x Golf Cart batteries in 2s x 3p configuration.

The Renogy 2,000 Watt inverter is a bit on the large size--But it should work:

https://www.renogy.com/2000w-12v-pure-sine-wave-inverter/

It is not a particularly efficient inverter with a 2 amp no load draw (P=V*I= 12 volts * 2 amps = 24 Watts). If you run the inverter 24 hours pe day, that becomes:

• 24 Watts * 24 hours per day = 576 WH per day just to turn on the AC inverter

Compared to 1,000 to 1,500 WH per day for the fridge, that is about 1/2 of your fridge's energy usage... And bumps up the overall energy usage:

• 1,500 WH fridge + 576 WH inverter = 2,076 WH per day
• 2,076 WH per day * 1/0.85 inverter eff * 2 days storage * 1/0.50 max discharge * 1/12 volt battery bank = 814 AH @ 12 volt battery bank
• 2,076 WH per day * 1/0.52 off grid system eff * 1/5.25 hours of sun per day (Nov) = 760 Watt array Nov break even
• 760 Watt array * 1/0.65 base load factor = 1,170 Watt array (65% base load factor)

So even a 630 AH @ 12 volt battery bank is still on the small side because of the AC inverter's "tare losses" (24 watt * 24 hours per day losses).

When you add everything together--Too small battery bank, solar array at minimum size, etc. -- Just not a large enough system.

For the larger solar array (1,186 Watts), generally an MPPT type charge controller is a better fit... And you can purchase less expensive "large format" (typically 200 Watts or larger) solar panels (cheaper $$$/Watt than smaller "12 volt" solar panels).

Before you purchase any more hardware--Get a Kill-a-Watt type energy meter and measure the actually 24 hour energy usage of your refrigerator--The one you have may use more or less energy than the 1,500 Watt*Hours I used above:

https://www.amazon.com/s?k=kill+a+watt+meter&ref=nb_sb_noss

-Bill

Not sure exactly what your question is: - GC batteries can be used to 20% battery life? not just 50%

If you are are asking can you discharge the battery bank to 20% state of charge--Yes, lead acid batteries can be discharged that low and still "survive"... However, they must be quickly recharged to >80% state of charge and not allowed to sit for even 24 hours at 20% SoC. The batteries will sulfate and lose capacity if not recharged within hours.

And, the batteries--If, for example rated to 1,000 Cycles at 50% discharge, they will probably have closer to 500 cycle life or less if discharged to 20% SoC.

Another issue is that is it hard to recharge a lead acid battery bank, with just solar, in 1 day... The sun is just not in the sky long enough to bring the bank from 20% to 100% SoC in the ~14 hours (just a guess) in one day without the help of a genset... The batteries just need those Hours on Charge to fully recover.

Another issue is that most rechargeable battery chemistries do not like going "dead", and if discharging continues, the other cells acxtually begin to "reverse charge" the weak/zero charge cells (the cell voltage goes from roughly 2.0 volts to zero volts to -2.0 volts). This will destroy a lead acid cell/battery. That is why a 20% "floor" is suggested--To prevent permanently damaging one or more cells in a battery bank... The going to zero and reverse charging is more likely to happen on the higher voltage battery banks--Especially the 48 volt banks (one 2 volt cell going "dead" will not shut down an AC inverter).

So--If you have an emergency--Yes, you can take the battery down to 20% SoC (somebody hurt, need lighting & power tools to repair storm damage, etc.)==Yep do it.

However, do not plan on running a full time off grid power system this way... For "optimal operation" (longer battery life, less "fussing, monitoring ,and managing of loads", plan on daily 25% discharge (to 75% of state of charge) for normal usage... 50% discharge for those several days of "no sun" during stormy weather. And 20% state of discharge when you have no other choice.

Golf Cart type batteries are not known for long life... Probably in the range of 3-5 years. And hot weather (hot battery banks) age faster... For every 10C/18F over room temperature, the batteries will have 1/2 their life... 75F+18F=93F -- That would mean a battery with 5 years of general life, will die in ~2.5 years.

Using flooded cell/deep cycle GC batteries for smaller systems--They are cheap(er) and relatively forgiving. As the system gets bigger (more AH), then need to look ar larger AH capacity batteries (or even 2 volt cells) for longer and more reliable long term operation. For larger systems, don't just keep adding more GC batteries in parallel... It is a lot of "extra" maintenance.

Regarding the AC inverter--You probably will not be the first person to open the case and cut out the beeper... Things to double check include using a DC Voltmeter to measure the voltage at the DC input to the AC inverter. If your DC cables are too small of diameter (not enough copper) and/or too long, you could have a good size voltage drop from the battery bank to the AC inverter's DC input. Short/heavy cables and good bolted up connections are required--Especially for a 12 volt system. I would suggest that you see no more than 0.5 volt drop from battery bus to AC inverter's DC input under full load.

But at this point, a small battery bank, small solar array, and relatively large/inefficient AC inverter is part of the problem. Running to 50% of battery capacity is not great for long term system operation.

To setup the system for full time off grid refrigerator use--It needs to be larger (array, battery bank, etc.). And you need to budget battery replacement every 3-5 years (especially if GC or similar batteries are used). Even if you only use the batteries for emergency backup (float service)--They do age and eventually fail--Especially in hot climates (keep battery room as cool as possible, if you have a basement--Keeping batteries below ground level--With good ventilation--Helps too). Keep an eye on the batteries--Take Specific Gravity measurements+Voltage per battery, and log to a notebook. You are looking for "differences"... Too low or too high of voltage across one battery, one cell with low SG or all cells with low SG, etc. indicate further research and checking of connections required.

It is just the nature of the beast.

-Bill