Off-Grid Solutions: How to Build a Solar Power System to Run a Chest Freezer During a Power Outage
One issue that many people face, or worry about, is how to keep their perishable food safe during a power outage. Whether it’s a short-term grid failure or a long-term off-grid lifestyle, the ability to freeze food is essential.
I can help.
One reliable solution is to build a small solar power system designed specifically to power a chest freezer. Done correctly, it’s a set-it-and-forget-it system that offers peace of mind and food security.
Step 1: Choose the Right Freezer for Your Needs
First, assess what you’re working with:
Do you already have a freezer you’d like to continue using? Great! You’ll be designing your solar power system around that model.
Don’t have one yet? Consider your food storage needs. How many people rely on the food that you store? Based on that, how much space do you need? Will a small freezer suffice? Or, do you need a larger chest freezer that will provide for a large family or for an extended length of time? Is your decision based on a move toward food independence? For instance, a change from immediate food purchases to a more long term food focus, like buying a full or half beef from a local farmer often triggers the need to increase your storage capacity. Regardless, take your unique circumstances into account and then decide on the size and type of freezer that makes the most sense.
Next, let’s discuss the difference between:
Standard AC Freezer (like a Frigidaire 7 cu. ft. - common in most modern home applications - plugs directly into a standard wall outlet)
12V DC Freezer (like the SunStar 8 cu. ft. - less common and links directly to a 12v power source, like a solar battery bank)
AC vs. DC: Understanding the Difference
DC freezers typically consume much less electricity than AC models of the same size. For example, our 21 cu. ft. DC freezer consumes about the same amount of power as our 7 cu. ft. AC freezer (~100 watts while running). The reason? DC freezers are built for efficiency, with thicker insulation, better seals, and fewer compressor cycles. In today’s marketplace, DC appliances are typically viewed as a niche product, purposed for off-grid or emergency situations. They’re just built differently, with resilience and efficiency in mind.
That said, AC freezers are more affordable upfront, and their function is usually more consistent with people’s common expectations for their home appliances. You can often find small models for under $200. Our 7 cu. ft. AC unit was $140 (purchased a few years ago, so prices will vary).
Whatever you choose, it is important to make your power system decision holistically. A cheaper freezer may cost more to power long term, or require more expensive system components like a larger battery bank or an AC power inverter.
Inverter or No Inverter?
Here’s a key technical difference between AC vs. DC powered appliances:
DC Freezer: Runs directly off your solar-charged battery bank, utilizing the 12v DC current that your batteries already supply.
AC Freezer: Requires a power inverter to convert the DC current (from the batteries) to AC current (for the freezer).
If DC power vs AC power current is a foreign concept to you, then I would highly recommend doing a quick review of this topic. There are hundreds of good videos on YouTube that explain this concept clearly, and if you’re considering investment in a solar power system, then knowing this key difference is a must!
With this idea solidified, you will see that a cheap inverter may work in a pinch, but I wouldn’t trust it for continuous freezer duty. While you can usually find inverters that offer the wattage you’ll need for less than $100, many of these types of inverters are meant to be more like a glorified car outlet for road trips or temporary device charging, not for the reliable, consistent use that you need for confident food security. When building our solar power system, I chose the Giandel 3000W pure sine wave inverter (about $350 and well worth the investment). Another primary consideration for sizing your inverter is the fact that AC appliances, upon startup (when the cooling process first cycles on) will often draw up to several times their standard running wattage for a few seconds. For instance, when our 7 cu.ft. AC freezer cycles on, it can draw up to 1000W for a few seconds (the average running draw is about 100W). So, size your inverter for this initial start up draw, not just the average running power load.
Comparing Power Draw: Frigidaire vs. SunStar
Let’s compare the actual power usage of each unit so we can size the rest of the solar power system accordingly.
The Frigidaire 7 cu. ft. AC freezer provides us with an estimated annual energy use of 250-280 kWh. We can break this down to estimate the daily energy usage. We’ll be conservative and use the 280 kWh estimate, which equates to roughly 770 Wh (watt hours) per day.
The spec sheet for the SunStar 8 cu. ft. DC freezer notes a daily energy draw of 385 Wh.
Battery Bank Sizing (48-Hour Autonomy)
Let’s assume we want 48 hours of backup power (no sun), as a resilience buffer. When sunlight is plentiful this stored energy will be more than enough to keep things running overnight, but when we have periods of limited sunlight we will be counting on this buffer to keep foods cold until we get another sunny day to replenish the bank. I find that this is especially important during winter as well as during summer storms, when the temperature remains high (causing the freezer to run more consistently) but solar input is limited for an extended period of time.
The AC freezer will consume roughly 1540Wh and the DC freezer will consume roughly 770Wh over 48 hours. So for each freezer, this tells us how much stored, usable electricity we will need to have in order to meet our 48 hour resilience buffer.
But most batteries shouldn’t be drained to zero.
This is especially true for lead-acid and AGM type batteries, while LiFePO4 (often referred to as Lithium Ion or Lithium Iron Phosphate batteries) can withstand much lower depth of discharge. For our purposes here, let’s assume that we’re using lead-acid batteries (this is what we use in our own solar battery bank). Draining the battery bank to zero can damage the batteries and cause limited recharge capability. So let’s plan for a maximum 50% depth of discharge, meaning we need to size the storage capacity of our battery bank to double the desired resilience buffer.
Frigidaire AC Freezer requires a battery bank with a storage capacity of ~3000Wh
SunStar DC Freezer requires a battery bank with a storage capacity of ~1600Wh
To estimate a battery’s total watt hours we simply multiply the voltage by the amp hours (Ah). So, a 100Ah, 12v battery has a total of 1200Wh.
This could mean:
The AC freezer requires ~3000Wh storage: 3x 12V 100Ah batteries = 3600Wh capacity
The DC freezer requires ~1600Wh storage: 2x 12V 100Ah batteries = 2000Wh
I’m using 100Ah batteries in this example for simplicity. As you are making your battery choice, simply use the Ah rating to determine how many batteries you will need.
Solar Panel Array Sizing
When sizing the solar panel array, we need to take into account that the panels won’t receive full sun all day long. We can typically assume ~4.5 peak sun hours/day (adjust based on your location). The remaining daylight hours will provide some input, but it will be lower than the full panel wattage. And, the goal of our solar panel input is to first provide for the freezer’s electricity usage during the daylight hours and second, to recharge the watt hours that were consumed from the battery bank overnight. If we can’t recharge our battery bank on a consistent basis, then we won’t have our 48 hour resilience buffer when we need it.
We know that our AC freezer consumes 770Wh daily. We also have a battery bank that holds ~3000Wh. So, if we’ve just had a couple of cloudy days, there is a potential that we will need to supply the daily need, plus replenish the battery bank for a total of ~3800Wh. We can roughly accomplish this with 2x 400W solar panels receiving ~4.5 hours of full sun (800W x 4.5 hours = 3600Wh collected).
Our DC freezer consumes 385Wh daily and our associated battery bank holds ~1600Wh. We can roughly supply the total ~2000Wh with 1x 400W solar panel receiving ~4.5 hours of full sun (400W x 4.5 hours = 1800Wh collected).
In all cases, I tend to oversize the solar array to account for shading, cloud cover and to accommodate for reduced wattage over time as the panels age. Solar panels are likely the least expensive component in the solar power system, so it is less prohibitive to add one panel more than the minimally sufficient array. Based on this, I would add 3x 400W panels for the AC freezer and 2x 400W panels for the DC freezer.
Solar Charge Controller: A Critical Component
Your solar charge controller is the component that takes the solar input your panels are collecting and delivers it to your battery bank for effective storage. Solar charge controllers vary widely in quality, battery-type compatibility, input/output rating and other convenience features. This component also helps to maximize your panel input and preserve the safety and longevity of your batteries, so it is important to get this right. I have found that VictronConnect charge controllers are compatible with most battery types, provide Bluetooth-enabled analytics and offer a broad range of voltage/amperage capacities.
How do you size your solar charge controller? We need to consider both the voltage coming in from the solar panels as well as the amperage going out to the batteries. The solar panels you choose should offer a spec sheet that will tell you the amount of volts produced by the panel at maximum output. The 400W panels that we use produce 33.3v with maximum sunlight. So, if we’re connecting 3x panels to our charge controller (wired in series - the voltage increases with each panel), the total volts coming from the panels will be ~100v, and the output to the batteries will max out at around 100 Amps (3 panels collecting 400W = 1200W total/12v = 100A). Two panels will produce ~67v and about 67 Amps output to the batteries. So, for the AC freezer, we will need the VictronConnect 250|100 (capable of managing 250v|100A) and for the DC freezer, the VictronConnect 150|70 (150v|70A) charge controller.
Putting It All Together
The Frigidaire 7 cu. ft. AC Freezer will need:
Freezer - est. $200
3x 400W Solar Panels - est. $525
VictronConnect 250|100 Charge Controller - est. $615
Giandel 1500W pure sine wave inverter - est. $210
3x 100Ah lead-acid batteries - est. $350 (based on locally avail. in our area)
Wiring, connectors, fuses - est. $100
Total Cost Estimate - $2000
The SunStar 8 cu. ft. DC Freezer will need:
Freezer - est. $1288
2x 400W Solar Panels - est. $350
VictronConnect 150|70 Charge Controller - est. $380
The DC Freezer does not require a power inverter - $0
2x 100Ah lead-acid batteries - est. $240
Wiring, connectors, fuses - est. $100
Total Cost Estimate - $2358
As you can see from the cost estimates, while the AC freezer has a significantly lower initial cost, the additional components and power usage bring the grand total for each build to within just a few hundred dollars.
One additional thing that you may want to consider is that if you end up in a situation where you have to rely on your battery bank for a longer period of sunless weather, the DC freezer, built with increased insulation and more robust seals will likely retain cold more efficiently, meaning that your perishable food items will be better protected. The alternative to this is the fact that having an AC enabled system means that you have a power inverter already set up. In times of abundant sunlight, you can easily use the AC system to power other items that you may need to power or charge (mobile devices, power tool batteries, fans, or other small appliances).
Ultimately, the build you choose to proceed with should accommodate the needs that your unique circumstances make most practical.
Frigidaire AC option: More affordable upfront, but requires a more robust system (bigger battery bank + inverter). Also allows for ease of use if you want to power other AC-type items with this system.
SunStar DC option: Expensive freezer, but much more efficient. Smaller system required = better for limited space, solar input, or battery capacity. Also provides peace of mind in longer periods of sunless weather.
That said, when it comes to keeping food safe and cold off-grid, you have options.
The best setup is the one that suits your lifestyle, climate, and storage needs. Whether you start with an AC freezer and build the system to match, or invest in a purpose-built DC unit, the key is to plan thoroughly, and build for resilience.
Let me know in the comments if you'd like me to run the numbers for your specific situation. And if you're building a solar freezer system, I'd love to hear what you chose and why.
—Adam
Our Off Grid Life