Portable Solar Power --Need Advice--

Discussion in 'General Technical Questions and Answers' started by W4ARW, Oct 6, 2019.

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  1. W4ARW

    W4ARW Ham Member QRZ Page

    I am trying to run a dorm/mini fridge on a homebrew solar power system for long term camping (not an RV). I have purchased a 100 watt solar panel which charges 2 35ah batteries run in parallel. The fridge I am trying to run uses 1.5 amps at 115V, the compressor duty cycle is approximately 82% running for every 18% off. Originally I had thought my batteries would be large enough, But after testing it the result is that every time I try to run the batteries for any considerable time, they go dead if the panel is not in full light (very annoying at night or on cloudy days). My main questions are:
    • Is there an exact equation that will tell me how many 12volt DC amps I need for every 120volt amp AC?
    • What should my total ah required amount to after I account for running without sun?
    • Once above is determined, how much solar wattage should I use to keep those batteries charged?
  2. AG6QR

    AG6QR Premium Subscriber QRZ Page

    The exact answer depends on the efficiency of your inverter at that particular power level, which you can measure, but which the rest of us don't know. I guarantee your inverter is less than 100% efficient. It's probably more than 60% efficient. I'll make a wild guess of 70% efficiency:

    Power out = power in * efficiency

    Vout * Iout = Vinput * Iinput * 0.70

    120 * Iout = 12 * Iinput * 0.70

    Iinput = Iout * 10 / 0.70

    Input = Iout * 14.3

    So you need a bit more than 14A input at 12V for each amp output at 120V.

    Over a 24 hour day, the refrigerator needs

    Hours * (Amps at 120V) * (conversion factor from 12V to 120V) * (duty cycle)
    24H * 1.5A * 14.3 * .82 = 422Ah at 12V

    Since lead-acid batteries won't tolerate deep discharge well, you probably need to double that to size your batteries. True, in an ideal world, the batteries don't need to run the refrigerator for 24 hours; they only need to carry the refrigerator overnight, but there will be cloudy days sometimes, so there will be times when your batteries need to carry the refrigerator more than 24 hours.

    There's another efficiency question here -- how many Ah can you draw out of the batteries during discharge for each Ah you put in during charging. I don't know the efficiency rating, but I guarantee it's less than 100%. Again, I'll take a wild guess and say 85%

    The total power you need to draw from the batteries is 422Ah * 12V = 5064 Wh. To account for the battery efficiency, the power the panels need to provide during charging is 5064Wh / 0.85, or 5957Wh. The wattage required depends on the number of hours of full sunlight you expect, but to charge the batteries in ten hours, you'd need 600W of panels.

    Although I made a pessimistic assumption about running the refrigerator on batteries for 24 hours straight, I think I've probably made some optimistic assumptions here about assuming you'll have sun every day. I know the overall result is not exact, but I believe it to be somewhere in the ballpark. To make the system really work well in all weather conditions, you'll probably need more than 600W of panels, and more than a thousand amp-hours of battery capacity.

    I used to have a van camper with a propane fridge. I lived in that van full time for about four months of summer weather (split over two summers) using about a pound of propane per day to cover the refrigerator, stove, and furnace combined. There's a reason propane refrigerators are commonly used for off-grid situations.
    KA2IRQ, KD0CAC and KM4LKC like this.
  3. W4ARW

    W4ARW Ham Member QRZ Page

    Wow, I was way off. Thank you for that information. I am beginning to think we will go a different route but I will shop around and see if I can build an adequate system for a reasonable price.
  4. KD0CAC

    KD0CAC Ham Member QRZ Page


    It use to be with professional alternative energy / solar techs , that the 1st thing is to look at loads - measure , THEN HOW TO REDUCE CONSUMPTION .
    In your case a low consumption refrigerator , then with the numbers build the electrical system .
    You can make almost any cooling / heating system consume less by adding more insulation .
    Get some foam panels and make a box for all sides --- but the coils on the back , they need to remove heat and be properly vented / air circulation .
  5. AG6QR

    AG6QR Premium Subscriber QRZ Page

    KD0CAC is right. Many don't realize that a refrigerator is the biggest energy consumer in many people's homes (sometimes heating or A/C is more, depending on climate). Sure, a microwave or hair dryer consumes more power while it's running, but a refrigerator runs many more hours per day. A little dorm refrigerator is typically poorly insulated and inefficient, and uses almost as much energy as a full-sized refrigerator. It's designed for college students who don't have individually metered electric service.

    If you want to power a refrigerator on solar, consider a Stirling cycle refrigerator, like those made by Engel. They're much more expensive than a dorm fridge, but they are about the most energy-efficient available, and the total system cost of the refrigerator, batteries, and panels will be less than that of a dorm fridge, I believe. Most of them will run directly off of 12V, eliminating the inverter and its efficiency loss.

    If you don't have your heart set on solar, a propane fridge is portable, and will run for a very long time in all weather with only an occasional refill of a propane tank.
  6. KF5LJW

    KF5LJW Ham Member QRZ Page

    You are not even remotely close to having what is required. A small dorm fridge is about a poor of a choice you could possible make because they are extremely inefficient. They use roughly 1 Kwh of electricity per day or about 10-cents worth of electricity.

    Minimum configuration assuming you actually live somewhere where solar can be used and absolutely no shade issues from sunrise to sunset is:

    Panel wattage = 400 watts
    MPPT Charge Controller = 30 Amps
    12 Volt Battery Capacity = 400 AH or a 300 pound battery.
    Inverter - 500 watt TSW

    In all around $1500 to make 10-cents of electricity per day. Now here is the fun educational part. That 12 volt 400 AH battery will cost you roughly $500 to $600 and will need replaced in 3 to 5 years if you give it a lot of TLC. So lets say it last 4 years and you use the deigned 1 Kwh per day. In 4 years would be 4 x 365 = 1460 Kwh or round up to 1500 to make it sound better than it really is. If the battery cost you $550 means just in battery cost alone the electricity cost you $550 / 1500 Kwh = $0.37/Kwh. Where I live electricity cost 8-cents per Kwh. I would be a fool to consider off-grid solar like most of you out there. Why on earth would you want to pay 5 times more than you do now?

    To do this with Solar properly would require using a fridge made to run off solar at 12 or 24 volts directly. Something like Danfoss uses half the energy of a dorm room fridge and slightly larger. That would require 200 to 300 watts of panels and a 12 volt 200 AH battery. Does not change the economics as the battery Kwh cost do not change. Additionally the Danfoss unit will set you back $1500 to $2000.

    So unless you have several thousands of $$$$ to throw around, forget it.
    Last edited: Oct 6, 2019

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