Specific Instructions for DIY Solar

Re: Specific Instructions for DIY Solar

Refrigerators use a lot more power than most people realize... For example, a common Energy Star rated full sized US Refrigerator can use ~365 kWH per year (call it 1.0 kWH per day for our math below).

In general, an off grid cabin/small home with lights, refrigerator, small well pump, laptop computer can get away with as small as a ~1 kWH (1,000 Watt*Hour) per day system. Add a refrigerator and a clothes washer load ever day or so--closer to 3.3 kWH per day (~100 kWH per month).

Since you asked specifically about a refrigerator... Lets start with a 1,000 Watt*Hour per day appliance. And a minimum of 4 hours of sun per day (typically in much of the US for ~9 months of the year--The other three months, get a small Honda eu2000i genset or equivalent to make up for bad weather).

First the size of the battery bank. An "optimum" system will work out to be around 2 days of storage and 50% maximum discharge (for long life) on a flooded cell lead acid battery bank. The math:

• 1,000 WH per day * 1/0.85 AC Inverter eff * 1/12 volt battery bank * 2 days of "no sun" * 1/0.50 maximum discharge = 392 AH @ 12 volt battery bank

That would be roughly 4x 6 volt @ 200 AH "golf cart" batteries in series/parallel (two batteries in series for 12 volts, then the two strings in parallel for a 400 AH @ 12 volt battery bank).

Next the solar array... There are two calculations we like to do... The first is the minimum charging current to keep the battery bank "happy". That ranges from 5% to 13% typical for a solar array. 5% an work for a weekend cabin, short term emergency power. 10%-13%+ is better for full time off grid power (5% is the minimum recommended rate of charge and 10-13% is a healthy rate of charge for flooded cell Lead Acid batteries):

• 14.5 volts charging * 400 AH battery bank * 1/0.77 panel+controller deratings * 0.05 rate of charge = 284 Watt array minimum
• 14.5 volts charging * 400 AH battery bank * 1/0.77 panel+controller deratings * 0.10 rate of charge = 569 Watt array nominal
• 14.5 volts charging * 400 AH battery bank * 1/0.77 panel+controller deratings * 0.13 rate of charge = 739 Watt array "cost effective" maximum

Next, is sizing the panel for the amount of power you will be using. Again, in much of the US, you can start with ~4 hours a day of sun minimum (long term average) for ~9 months of the year (use a generator and/or larger array for winter power needs):

• 1,000 WH per day * 1/0.52 end to end system eff * 1/4 hours minimum sun = 481 Watt array minimum

So, based on our back of the envelope calculations--For somebody that wants "reliable" off grid power for ~9 months of the year, they would be looking at a 481 to 739 Watt solar array on 4 golf cart batteries (12 volt @ 400 AH battery bank).

AC inverter wise--Probably a minimum of 1,200 to 1,500 Watt 120 VAC (TSW--True Sine Wave inverter) minimum recommended.

There are some other details to look at that may increase the size of the system a bit (tare losses on AC inverter, does the refrigerator need 120 VAC 24 hours per day * 7 days a week--Most "frost free" models have a timer that needs 120 VAC on all the time).

And there are other options--Such as the Chest Freezer Conversion that can use about 1/4 the AC power (chest freezer conversions frequently do not make a spouse happy to have to keep moving baskets of food around).

Anyway--This gives you an idea of what an "absolute" minimum/but reliable Off Grid Refrigerator Sized system could look like.

-Bill

Add missing link:

Chest freezer as a chest refrigerator

-Bill