My efficient refrigerator uses very little electricity. During the period of 10/1/17 through 11/2/17 the average consumption was 10.2 amp hours per day. The highest 7 day period average was 12.13 amp hours per day with temperatures in the low to mid 90s. The lowest 7 day period average was 6.78 amp hours per day with temperatures in low 70s to mid 80s. This is actual in a van use, not an always cool place.
I do three things to make this happen. It has an electronic thermostat, it has additional insulation, and I use it as a self icing ice chest. The freezer compartment contains 5 pounds of water in a couple of zip lock bags and I only allow the compressor to run when the sun is shining. It freezes at a speed of 5 pounds in 3.5 hours and melts at a speed of 2.5 pounds per day depending on temperature.
Since I use it like an ice chest I don’t put hot left over food in it and I don’t use it to chill drinks. I prefer water at room temperature anyway.
This is a link to a Bob Wells video about my ice chest from the 2016 RTR.
The ice cost is not why I did this fridge. The problem with this ice chest is the small food space capacity. The volume it can hold is about 3 quarts. That is enough to upgrade eating from the no refrigeration level but I wanted a bit more space. Now I have solar electricity so the fridge is possible.
The efficient refrigerator has insulation similar to the ice chest. I started with a 1.7 cubic foot Haier fridge. It had 1 inch insulation all around with the hot condenser tubes heating the outside skin. I added 2 inches (4 half inch layers) of polyiso to the inside surfaces. Since the back and bottom don’t have condenser tubes those parts got insulation on the outside. The insulation I added inside is about 1 cubic foot leaving 0.7 cubic feet of space. That’s about 20 quarts.
I power this with a 100 watt panel, a $10 PWM controller, a $10 fuse box, a Harbor Freight 400/800 ($20 with coupon) inverter and a 75 amp hour group size 24 “deep cycle” battery that is also the engine starting battery. This is what I had been using to charge my laptop and cell phone, and run led lights, MaxxFan, and my electric shaver. The main reason for the solar power system was to have the MaxxFan. I modified the inverter by connecting a pair of wires to the back side of the on/off switch so that the thermostat can turn off the inverter when the fridge has frozen all the water.
It is my opinion based on my experience that regular starter batteries can regularly give 5% and occasionally 10% of the capacity according to the reserve capacity minutes without seriously shortening the battery life. With this group size 24 battery I think it is good for 10% and occasionally 20% of the 75 amp hour 20 hour rate specification, not 50% like golf cart batteries. That means I can start with the ice all frozen and have one no solar rained out day using ice and the next no sun day using the battery. So, 2 days of no sun and then I have to have good sun or start the engine for the third sunless day. If that happens I’ll add another 2.5 pounds of water or maybe switch to salt water.
The refrigerator was $80 at Walmart. The 4 x 8 sheet of polyiso was $10 at Home Depot. I used 2 cans of Loctite spray foam for $12 also from Home Depot. The electronic 12 volt thermostat was $5 from eBay. The fridge total is $107. The solar power system was about $150 plus the $80 battery that I needed to get anyway because the prior starter battery got to the end of the warranty period and died.
Some details: When the inverter turns on the surge current is about 55 amps. When the inverter is up to speed and running it takes about 0.2 amps. When the compressor starts the surge is about 75 amps. While the compressor is running the inverter takes about 6.5 amps. To measure these currents I use a 0.001 ohm store bought shunt resistor and a Fluke model 189 meter. The meter has a handy max / min / average feature. You set up what you want to test, start the meter then turn on the load. Measuring 1 millivolt = 1 amp with the shunt, 75 millivolts max is 75 amps max. The average feature is what I used to calculate amp hours. I have an old time electric clock. When the compressor runs, the clock runs. So average amps times elapsed hours gives amp hours.
While the compressor is running the inverter takes about 6.5 amps. The solar panel produces 3 to 5 amps. After the fridge has run for an hour and a half and shut off the battery gets charged to put back what was taken while the compressor was running. The battery ends up getting cycled twice each day. In the morning the fridge runs. Over night I use led lights and the MaxxFan. The next morning I don’t start the fridge till the battery is up to 14.5 volts.