Average refrigerator load?

I like using 3.3 kWH per day for a refrigerator+lights+washing machine+water pump+laptop computer/tv/etc. for a "near normal" electrical electrical existence in a very conservation minded home.

• 3,300 WH * 1/0.85 AC inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 max discharge (better battery life) = 647 AH @ 24 volt battery bank (basically your calculation with 10% more storage)

Size of solar array--Two calculations, first based on minimum rate of charge for battery bank. 5% for weekend/seasonal usage, 10% to 13% for full time off grid--Your system may be better off with the 10% rate of charge--Basically you are running the fridge 24x7, so you are "stealing" some charging current during the day--The 5% minimum rate of charge assumes no concurrent loads:

• 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,218 Watt array minimum
  
• 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 2,437 Watt array nominal
  
• 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 3,168 Watt array "cost effective" maximum
  

And then based on the hours of sun per day--Assuming 4.0 hours of sun minimum for your time at the cabin:

• 3,300 WH per day * 1/0.52 off grid system eff * 1/4.0 hours of sun minimum = 1,587 Watt array "break even" @ 4 hours of sun per day
• 3,000 WH per day * 1/0.52 off grid system eff * 1/4.0 hours of sun minimum = 1,442 Watt array "break even" @ 4 hours of sun per day if you stick with 3,000 WH per day
  

Of course, a refrigerator uses less energy in cool weather (assuming you don't crank up the wood stove in spring/fall and run the fridge "hot")... But I would guess that 1,440 Watts of panel is a bit on the small side...

Now the questions about how you will maintain the system... If you are there the whole time running the system and watching battery bank SG / Voltages, and making sure the folks are "wasting power"--You can probably get away with 1,440 Watts of energy--Or at least catch when the battery bank is "not happy" and cut down energy usage and/or power up the backup genset in the morning.

If you will not be there and don't have somebody you can trust to monitor the system--I would tend to suggest you look at ~2,215 (3,000 WH per day) to 2,427 Watt array (3,300 WH per day)--It will give you that extra bit of energy to keep the battery bank happy without having to watch energy consumption/generation like a hawk.

Part of this will be a risk calculation too... Get ~4x series GC 6 volt @ 200 AH batteries * 3x parallel strings for 12 total batteries (~24 volts @ 600 AH) battery bank (mayby $1,200 USD) vs getting some very nice Rolls or other "expensive batteries"... Less expensive batteries, you may take the risk of a smaller array. "Expensive" battery bank that you want to last 8+ years and you not being there "full time" (or running somebody remotely by cell phone and text messages), you might want the additional security of the larger solar array.

Of course, you can build out the system and run it for a month or two--See how everything shakes out--And if more panels are warranted--Then add them when you have time and $$$.

I am sorry I cannot give you a more black and white answer--Just some guidance.

-Bill

For 7 hours of sun per day, you probably would need a two axis tracker:
http://pvwatts.nrel.gov/pvwatts.php

Requested Location

	sudbury canada 2 axis tracking
Weather Data Source	(INTL) NORTH BAY, ONTARIO 75 mi
Latitude	46.35° N
Longitude	79.43° W

Month	Solar Radiation ( kWh / m2 / day )
January	4.24
February	6.15
March	7.55
April	7.50
May	8.62
June	7.94
July	8.88
August	7.35
September	5.84
October	4.59
November	2.34
December	2.74
Annual	6.15

A fixed array system summer:

Requested Location

	sudbury canada fixed, facing south, 30 degrees from horizontal
Weather Data Source	(INTL) NORTH BAY, ONTARIO 75 mi
Latitude	46.35° N
Longitude	79.43° W

Month	Solar Radiation ( kWh / m2 / day )
January	2.98
February	4.45
March	5.61
April	5.43
May	5.93
June	5.48
July	6.02
August	5.28
September	4.17
October	3.37
November	1.87
December	2.02
Annual	4.38

June through August--You are getting closer to 5.28 hours of sun (August), and towards end of August into September, it drops to 4.17 hours per day...

Also, if you have "floor" (minimum daily power consumption)--Such as refrigerator, water, lighting (cannot turn off refrigerator for a day or two of cloudy weather, but you can put off washing, cooking with electricity, etc.)... You should plan on your "floor" to be something like 65%-75% of your "average daily output" (average--some days more power, some days less).

Also, remember that you are using power during the day (refrigerator, possibly other tasks)--5% is my suggested "minimum charging current" available to the battery bank. 5% of 647 AH * 29 volts * 1/0.77 panel eff = 1,218 Watts -- That leaves ~200 Watts of "additional array capacity" to run concurrent loads. The refrigerator will draw ~120 Watts running (25-50% duty cycle)...

Not to say that it cannot be done (smaller array)--Just you will need to manage your power usage vs sun more closely--Some people love doing that, other hate it. And run the genset at times to bump the bank from 50% State of Charge to 80% or so when needed.

At this point, if a fixed array, I would still use 5.0 hour per day for the array calculation--But that leaves you very little headroom for bad weather or a bunch of family and friends plugging in computers, running games on the TV, and such (yea--I know, back to nature to commune with the X-Box....).

For winter--Your batteries will be very happy in the sub zero weather--Assuming they where left >90% charged when you buttoned up the camp. If you have a 1-2% rate of charge:

• 647 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.01 rate of charge = 244 Watt array minimum
  
• 647 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.02 rate of charge = 487 Watt array nice for float
  

For example a couple ~240 Watt panels in series with a smaller ~15+ Amp rated MPPT charge controller (just two panels mounted high/vertical for winter charging) should keep the battery bank happy (rest of loads turned off, leave main array+charge controller connected or not--Your choice).

Your batteries will last longer in "cold storage" and will not freeze if charged. Plus their self discharge rate drops when stored cold--By the time you have snow over your array, your batteries will be in "cold storage" and not need any float charging until spring.

-Bill

My "cheap" fridge/freezers use a mechanical timer with (what seems to be) a 12 hour cycle... Turning off at night just delays whatever cycle it was in before continuing the next day.

Bigger issue is probably the swing in the average temperature of the refrigerator and freezers. An old thread with even older data on optimum freezer temperature (tested for 6 months of storage):

Question - Best Freezer Temperature

Quick answer, Zero F is optimum. Even 5F noticeable increase in decay of markers.

I also would watch too much swing in freezer temperatures--You can get freezer burn (as air is "re-chilled", plastic bag is colder than contents, get freezer burn/draws moisture from food and deposits on bag/air space as free ice--dehydrating your frozen food over time).

And in the above thread, some quick math on freezers and opening the door... Very roughly, if you assume all cold air is lost in vertical freezer of 15 cuft with only 5 cuft of food--You loose about 0.3% of the mass of "cold" in the freezer that needs to be chilled back down to zero F--And air does not have latent heat of freezing, so it is not as bad as water which you also have to turn to ice to get to zero F.

Open the door 10x per day for freezer it is the same effect as placing less than 3% (by weight) new food/water in freezer to freeze. Not a whole lot to worry about.

As a comparison, if you place 1 lb of room temperature food in the freezer, it cold warm 6 lbs of frozen food from 0F to 32F (not thawing). Another good reason to not put a freezer on a timer if you have a tendency to put new room temperature food in the freezer when it is shut down at night (i.e., only run the freezer during daylight hours to save on your battery bank cycling). You will raise the temperature of your frozen food and exacerbate freezer burn/dehydration because of the "wide" temperature swings.

And if you make lots of ice (drinks, for ice chests, etc.)--That significantly increases energy consumption too.

-Bill