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Solar Pantry Part 1: Feasibility

One of our goals for 2019 is solar back-up for the refrigerator and freezer in my pantry. If we ever lost power for more than several days, I'd have to scramble to save what I could and lose the rest. In summer, a lengthy power outage would likely be due to hurricane damage or an intense lightning storm. In winter it would be from an ice storm. So far, we haven't lost power for long enough to worry about food stored in these appliances, but these scenarios are very real possibilities.

I got the idea for a solar pantry from a book I reviewed awhile back, Prepper's Total Grid Failure Handbook. (An excellent book; you can read my review here.) The authors bring solar energy down to a small-scale, realistic level for someone like me. My thinking has been that if I can at least have solar energy for my fridge and freezer, I won't have to worry about losing the food I have stored in them.

My first step was to measure how much electricity these appliances use. From that I would be able to calculate how many solar panels I'd need and how big to make my battery bank. To find that information I used a Kill-A-Watt meter.

Recording watts used by my 400-watt food dehydrator.
Obviously it's a good idea to check wattage for yourself

Recording kilowatt hours.

I measured each appliance separately and added them together. From that I learned:
  • Refrigerator uses 2.6 kilowatt hours per day
  • Freezer uses 1.6 kWh / day
  • Total for both appliances is 4.2 kWh / day

Well, compared to my monthly electrical usage that didn't sound too bad. Then I started running the numbers.

We already have the solar panels. I found them on Craigslist about the time I reviewed Prepper's Total Grid Failure Handbook. They were leftover from a larger job, and we were able to get several new 345-watt Sunpower brand panels for $240 each. What I needed to know was how many panels I'd need and how to size a battery bank for several days of no sun. For that, I looked for an online calculator.

There are a number of online calculators for this, but I found that most of them are developed by businesses geared toward the brands and services they sell. They may give results as packages they offer, or want you to contact them to get the results. Wholesale Solar has one that I thought quite straightforward to use and gave me a ballpark estimate online. The most helpful step-by-step guide was at Preparedness Advice blog. I used both and my results from each were similar. However, what I discovered was quite dismaying.

Assuming I receive at least five hours of sunlight daily, here's what I'd need to make and store enough energy for three days of cloudy day use:
  • 1.32 kilowatt size system
  • 4, 345 watt solar panels
  • 11, 200 amp-hour 12-volt batteries
  • Or 8, 260 AH 12-volt batteries.

[NOTE: Batteries used for solar battery banks are deep cycle batteries, not cranking (starting) batteries. Deep cycle batteries are measured in amp hours (AH). This doesn't refer to actual time, because performance varies with conditions (such as temperature). Rather, AH is a way to compare the relative storage capacity of deep cycle batteries. The greater the amp hours, the longer they last before needing recharging.]

Back to my results. Discouraging because I would need more batteries than I assumed! Considering that 200 AH deep cycle batteries start at $350 for the cheaper ones (and 260 AH start at about $500), I have a budget problem. I only have $1500 for this project, and besides batteries I still need to get a charge controller and an inverter, plus all the miscellaneous items like racks, wiring, etc.

The pantry refrigerator, however, is an old one. Out of curiosity, I ran the numbers again with an Energy Star fridge. The following is based on the advertised energy rating and not my actual usage, which of course, could vary. I used an average estimate for low-end energy efficient refrigerators, and here's how the numbers changed:
  • Energy★ fridge 0.94 kWh / day (Quite a significant drop!)
  • Same freezer, 1.6 kWh / day
  • Total for both appliances is roughly 2.6 kWh / day
Total usage for both is the same as my current fridge uses in one day!

For three day's electricity storage with that fridge I'd need:
  • 0.8 kilowatt size system
  • 3, 345 watt solar panels
  • 7, 200 AH 12-volt batteries
  • Or 5, 260 AH 12-volt batteries. 

As you can see, energy efficient appliances make a big difference! The cost of such a low-end fridge is about the same as a 260 AH battery, so if I had a bigger budget I could buy a new fridge and still save over $1500 on the batteries. But the budget is what it is, so at this point, feasibility is near impossible. However, that doesn't mean that the mental wheels stop turning. More on that in Solar Pantry Part 2: Analysis.