One thing is perfectly clear: you would be foolish not to use at least a conventional energy-star refrigerator, and even with those, you need to be aware that there is a wide variation, mostly dependent on style and features of the unit.
This article is excerpted from the Spring 2003 newsletter of EORenew, the Eastern Oregon Renewable Energies Non-profit, which is the sponsor of SolWest Renewable Energy Fair.
In most households, refrigerators consume more energy than any other use except heating and hot water. For on-grid users looking to save energy on refrigeration. the choice is pretty straightforward. New Energy Star refrigerators provide energy savings without sacrificing features by using high efficiency compressors, improved insulation, and more precise temperature and defrost mechanisms to improve energy efficiency.
Energy Star qualified models require about half as much energy as a 10-year old model, at least 10% less energy than required by current federal standards, and 40% less energy than the conventional models sold in 2001. Replacing a 10-year old refrigerator bought in 1990 with a new Energy Star qualified model would save enough energy to light the average household for over three months and over 300 pounds of pollution each year. This represents a pretty impressive energy savings.
But if you are off-grid, is "impressive" enough? Refrigerators range from conventional to Energy Star to very high-efficiency. The very high-efficiency units cost more, due to the quality of the components and construction, but they can save energy in terms of kilowatt-hours. Just how much should you spend in order to save a kilowatt-hour of energy? John Richter and Larry Schlussler decided to find out, and came up with this article.
SUMMARY (some conclusions by Jennifer Barker):
What I am seeing is that the best conventional Energy Star refrigerators use at least one-third more energy per cubic foot of storage space than the SunFrost RF-16 DC model (the gold standard for off-grid refrigerators). SunFrost now has three models on the Energy Star-qualified list.
If you are using a SunFrost powered by DC, there will be no inverter inefficiency, and no inverter cost in the system! For your refigerator and freezer, that portion of your inverter's capacity has to be available at all times. You cannot, say, run your washing machine if it pushes the total over your inverter's capacity because your refrigerator and freezer may come on automatically at any time. Your inverter needs to be able to supply the full surge capacity at any time, as "brownouts" shorten the life of motors.
The value of conservation is obvious. However, you may be able to decrease the cost of the solar array by doing the work yourself and fabricating your own rack. You may be able to decrease the cost of batteries over time by buying better quality batteries and taking good care of them (is this consistent with buying a less-efficient refrigerator?).
Conventional Energy Star refrigerators are improving rapidly, meaning that this year, for that extra KWH per day, you can also get up to 65% more space if you choose carefully. Study the table below. I have chosen the best refrigerators I could find in each class to be represented here. The ultimate "bottom line", dollars per cubic foot to own and operate for 24 years, is in the right-hand column.
You will notice that some models give you much better efficiency per cubic foot of storage space than others. Beware as you peruse the table that some fridge-freezer combos have a different distribution between fresh and freezer compartments, and that the freezer carries a heavy energy penalty (compare the energy use of the SunDanzer fridge and freezer models, which are exactly the same size).
I have used Larry's estimate of $5606 per daily kilowatt-hour delivered from the batteries. Cost comparisons in the table assume that your inverter and load centers are large enough to handle the increased load from an AC refrigerator.
About Fridges and Other Things
Side-by-side fridge/freezers have an energy penalty because they have more surface area proportional to interior space. They all seem to have ice makers, another feature that has an energy penalty (about 5-10%). In fact, you should avoid all extra penetrations (ice makers, water dispensers, etc.). Automatic defrost refrigerators also have an internal heater to melt ice off the coils, which can draw 600 watts and more for a short period during the defrost cycle. There will be more usable freezer space in a chest freezer or bottom-freezer (if it is configured as a drawer) than in the freezer compartment of an upright refrig/freezer.
The amount of energy used is directly related to the ambient temperature in the area where the refrig/freezer is kept. Steve Willey reports that his Conserv freezer was tested to use 893 watt hours per day at 80°, 500-600 watt-hours at 70° (its rated use at 77° is 540), and under 450 watt-hours at 50°. So the penalty for each 10° of additional temperature is as much as 70% more energy use! Keeping your refrigerator and/or freezer in a partially or completely unheated space can really save the watts! Our Conserv freezer in an unheated shed has been measured (using a Brand watt-hour meter) to use as little as zero watt-hours in some 24 hour periods!
Another point of interest is that the faster you withdraw energy from a battery, the less total energy you will get out of it before the battery is at a voltage considered "dead" (Peukert's Equation). It is worth noting that the larger and less efficient a refrigerator's compressor is, the more it will tax your batteries.
They also assume that you have, and choose to run, a generator. If you choose not to run a generator, only the most efficient appliances you can find will reduce your base loads enough to allow you to survive an extended cloudy period.
Doing this little spreadsheet made something obvious: the choices are not clear-cut, and they will be different for everyone depending on your goals. One thing is perfectly clear: you would be foolish not to use at least a conventional energy-star refrigerator, and even with those, you need to be aware that there is a wide variation, mostly dependent on style and features of the unit (see sidebar).
For serious energy savers, DC models are an obvious choice. I have reviewed DC models where available, because they save the 10% inverter loss. Which model you will buy depends on how much cold storage space you need, and how much energy you have to devote to it. Starting with a chest refrigerator, and converting it to a freezer later, is a great way to get going for very little energy and dollars (both the SunDanzer and Conserv are available with conversion kits).
These costs are the best estimates I can make in order to allow you to make reasonable comparisons. It's up to you to lay out your personal long-term strategy, which may include learning to use your expensive refrigerator space more effectively. Our behavioral choices, as well as our purchases, affect our bottom-line costs - in refrigeration, as in everything else!