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Discussion Starter #1 (Edited)
I put a post on this forum about building a walk in fridge using an a/c unit with a modified controller. I would like to add a dimension to that. I would like to build a trailer to be the fridge, with walls that are r30 and a ceiling with r60. Furthermore, I want the unit to be self contained, with solar panels on the top to keep the room cool. I will install batteries, but hope to have to not buy more than 4 days worth of batteries to keep things chilled. The area is high plains desert in northern Nevada. There are 231 days of sunshine. Does anyone know of a formula that I can use to calculate electricity needs? I want to find a better way than to use hunches.
 

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I put a post on this forum about building a walk in fridge using an a/c unit with a modified controller. I would like to add a dimension to that. I would like to build a trailer to be the fridge, with walls that are r30 and a ceiling with r60. Furthermore, I want the unit to be self contained, with solar panels on the top to keep the room cool. I will install batteries, but hope to have to not buy more than 4 days worth of batteries to keep things chilled. The area is high plains desert in northern Nevada. There are 231 days of sunshine. Does anyone know of a formula that I can use to calculate electricity needs? I want to find a better way than to use hunches.
There are plenty of off-grid solar calculators on the web that are accurate. All you need to know is energy consumption of the AC unit you plan to use. You already know the sun-hours.
 

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For estimating heating and cooling are the same thing, you would have to multiply your final number by the efficiency of your AC unit.

One pound of air is around 10 cubic feet(1'x1'x10'). That is a rough rounding to make calculations easier but in this case errors on the side of caution.

One BTU can heat 4 pounds of air 1 degree F.

So 1 BTU can heat 40 cubic feet 1 degree F. (2'x2'x10')


From there you would have to figure out the number of cubic feet you are trying to cool and how many degrees you are trying to cool it. That would give you the number of BTU's you need to remove to bring the trailer to your desired temp one time. Once you have that you can start guessing at how much heat gain you will have through leaking, radiation and conduction over time. That is all beyond me but could be a starting place.


You can make it more efficient by parking the trailer in the shade(panels not on trailer), Wetting the outside of the trailer, Using the panels to shade the trailer, Burying the trailer, Using radiant barriers, super insulation, preventing leaks and thermal mass.

Keep in mind batteries are VERY expensive ways to store power. I would consider setting it up so the AC only runs when the sun is shining, that way you need very little battery storage to make it work. Then when the sun isn't shining rely on tight seals and thermal mass to continue to keep the interior cold throughout the rest of the time. If you have days and days of clouds in a row and the inside starts to warm up that would be a perfect time to pull out the generator and run it for a few hours.(a generator and a couple years worth of fuel to just help keep your trailer cold would be probably be about 1/10 the cost of the cheapest battery that would allow you to go several days without sun.

Depending on how serious you are about this I would build a room about the size you are thinking out of several layers of foam insulation, stick your AC in it and run it for a couple weeks and measure exactly how much power it uses. Once you know that you have a realistic idea of how much power your more insulated trailer will require to keep it running.


I personally am a big fan of solar, but I think you are asking a lot of it if you are trying to do it on the cheap. Heating(and cooling) are about the most power hungry ways you can use solar electric power and therefore the most expensive to set up.
 

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Discussion Starter #4
I am not sure a battery less system would work with a/c Most compressors need significantly higher startup wattage than is necessary to keep them running.
 

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I put a post on this forum about building a walk in fridge using an a/c unit with a modified controller. I would like to add a dimension to that. I would like to build a trailer to be the fridge, with walls that are r30 and a ceiling with r60. Furthermore, I want the unit to be self contained, with solar panels on the top to keep the room cool. I will install batteries, but hope to have to not buy more than 4 days worth of batteries to keep things chilled. The area is high plains desert in northern Nevada. There are 231 days of sunshine. Does anyone know of a formula that I can use to calculate electricity needs? I want to find a better way than to use hunches.
I built my house. I built r47 perimeter walls and r86 , 1st and 2nd floor ceilings. I think your insulation recommendations should be increased.
 

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A quick way to get some idea of what would be required, both in terms of machinery and power use, would be to check out the stats for the trailer mounted fridges that can be bought or hired and run with grid power.

You would then have a baseline guide of how big the compressor would need to be and how big a space it could cool. Increase or decrease the amounts according to how big you want your walk-in fridge to be. Super-insulating should help reduce these numbers but it is an easy place to start.

I think it was @Aerindel who made a very good point in another thread. Your fridge will only be as good as the lowest insulation.
I'm guessing here but it may work out better to have a high R value for insulation all around the fridge and stop direct sunlight from hitting the fridge somehow, instead of higher value insulation on the top with the sunshine hitting it. This might keep it cooler with less energy. Not sure, not a refrig tech.
From my experience in hot weather, the number one step is always to avoid direct sunshine by shading when trying to keep something cool.
 

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Years back, we contemplated building a DIY walk-in cooler & freezer at our BOL.
Decided NOT TO for a very fundamental reason.

If a big walk-in brakes down & you cannot get parts and/or repair it VERY QUICKLY.

The content will THAW OUT.

We stuck with multiple chest & uprights, as if one breaks down, its not such as issue & you can generally can the content, before any spoils.
 

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And the grand dreams of solar strike again.

You will spend 10x what you expect on this project for so so results. First off an AC unit is not made to be that cold.
 

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Discussion Starter #10
And the grand dreams of solar strike again.

You will spend 10x what you expect on this project for so so results. First off an AC unit is not made to be that cold.
I hear you there! I have followed dozens - maybe even hundreds of threads over the past decade, of people "going solar" or "going off the grid". You are pretty spot on with the overruns, though my own guess would be 3x the original cost - which is still a lot!

I appreciate the input from all of you. I have found a different way to think and plan this after reading your feedback.

The trailer/root cellar that I want to build will be 12x7, or 84 sq. ft. This is the limit to the size of solar panels I will buy. 84 sq ft will produce, on the average 1,260 watts

Bunkerbuster, my objective is not to keep fresh food for prolonged periods of time. The objective is to buy LTS food and create an environment that will allow them to be good for the expected lifespan, rather than having it cut in half because of 90-100 degree temps in the summer, etc. I would also like to store some equipment that might be temperature/humidity sensitive Maybe you can think of it as a portable, above ground root cellar. I will not be in a position to begun construction on my home for about 2 years, but I would like to move forward on food preps. But I feel that I am very limited in my choices without some temp controlled room/box to store the food in. Some people would just say "wait", but I feel very unsettled about the situation now in the US, and, while I don't think I can be "100%" ready in the next couple of years, or at least to where i want to be, I want to devote more of my resources to getting as far as I can. I don't know about all of you, but my spider sense is telling me that, at best, acquiring materials, even as harmless as powdered eggs, is likely to become more and more difficult in the future, no matter who sits in the oval office, difficult as in more expensive and not so likely to be able to buy in the quantity that we are used to.

There are external thermostats for portable a/c units, similar to those that are used to turn chest freezers into refrigerators, that will allow you to run an a/c down to 50 degrees or so. In a normal home, that kind of set up would probably burn the a/c up in short time, as well as consume lots of watts, hence the need to insulate. Window a/c units are cheap - it might be worthwhile to use two and alternate use every week. If things progressed well, the trailer would become an integral part of my bov and bo plan, since it would already be loaded with food. If it looks like I will be able to generate enough juice with 84 sq ft - have a surplus, I might consider putting an actual freezer in it and freeze fresh foods - specifically meat...but one step at a time! :eek: The unit has to be pretty self sustaining in the near future because it will be parked in a friend's year, behind a chain linked fence. It will be secure, but I don't think I can count on my friend paying much attention to it. Even though retired, he is pretty busy.
 

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Discussion Starter #11
For estimating heating and cooling are the same thing, you would have to multiply your final number by the efficiency of your AC unit.

One pound of air is around 10 cubic feet(1'x1'x10'). That is a rough rounding to make calculations easier but in this case errors on the side of caution.

One BTU can heat 4 pounds of air 1 degree F.

So 1 BTU can heat 40 cubic feet 1 degree F. (2'x2'x10')


From there you would have to figure out the number of cubic feet you are trying to cool and how many degrees you are trying to cool it. That would give you the number of BTU's you need to remove to bring the trailer to your desired temp one time. Once you have that you can start guessing at how much heat gain you will have through leaking, radiation and conduction over time. That is all beyond me but could be a starting place.


You can make it more efficient by parking the trailer in the shade(panels not on trailer), Wetting the outside of the trailer, Using the panels to shade the trailer, Burying the trailer, Using radiant barriers, super insulation, preventing leaks and thermal mass.

Keep in mind batteries are VERY expensive ways to store power. I would consider setting it up so the AC only runs when the sun is shining, that way you need very little battery storage to make it work. Then when the sun isn't shining rely on tight seals and thermal mass to continue to keep the interior cold throughout the rest of the time. If you have days and days of clouds in a row and the inside starts to warm up that would be a perfect time to pull out the generator and run it for a few hours.(a generator and a couple years worth of fuel to just help keep your trailer cold would be probably be about 1/10 the cost of the cheapest battery that would allow you to go several days without sun.

Depending on how serious you are about this I would build a room about the size you are thinking out of several layers of foam insulation, stick your AC in it and run it for a couple weeks and measure exactly how much power it uses. Once you know that you have a realistic idea of how much power your more insulated trailer will require to keep it running.


I personally am a big fan of solar, but I think you are asking a lot of it if you are trying to do it on the cheap. Heating(and cooling) are about the most power hungry ways you can use solar electric power and therefore the most expensive to set up.

I have looked at reefers but the ones that I saw were only insulated to r10. The food in them is only mean to be in them for the duration of transit, so I guess engineers have decided that being able to carry more goods (adding more insulation would mean reducing the volume of what the trailer can carry) is more important than conserving energy, so they make the trailers very air tight and use a bigger fridge compressor.

A friend of mine built a well insulated house in the Sacramento Valley that is about the same size and floor plan of the house that he used to live in. New R values wall - r30+, ceiling r50+ Electricity bill is obviously much lower, but I was impressed by one thing that he said. Just a casual observation, he says his central a/c unit only runs about 5 hours on the hottest days -100+.whereas his last house (poorly insulated) the a/c was running 8-16 hours a day. I have been using that piece of info as a rough guide to get me through the planning stage. I need to keep the panels on top of the trailer, but I could mount them 6" or so higher than the roof and let a fan run in the empty space to carry some of the heat away. That would also help to cool the panels, which would increase their efficiency. There are also some combo pv and hot water solar panels. One of the claims of the hybrid system is that it carries a significant amount of heat away from the house underneath. At this point, however, I have no need for hot water. I also have never hear an in depth interview with a person who has used such hardware.
 

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I understand your feel of need at the moment but your also rushing into this with less than 3 weeks left until a very large election day. Can you even find enough food to fill that area? 84sqft of solar is not much at all.

If you own the land why not use screw top 55gal drums and burry them?

Another prob very un popular method could be renting a close by climate controlled storage locker. I would figure the cost of a setup like you want to do vs 2yrs of storage fees.
 

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why not just buy a used reefer box?
 

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To minimize the amount of W/day needed keep the refrigerated box out of direct sunlight. The solar panels of course need to be in direct sunlight.
 
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