So how do you figure out how many solar panels and batteries you need? The first step is to determine how much power the appliances and lights you intend to run draw. There are a couple of ways to determine this. The wattage of a UL listed/approved appliance can usually be found near the AC power cord. This may be in amps or watts. If it is in amps, a simple formula will let you convert it to watts: Volts x Amps = Watts. In other words if your appliance draws 4 amps, the formula would be 120 x 4= 480 watts.
Another method is to use a Kill A Watt Electricity Usage Monitor. This is an inexpensive device that monitors how much power your appliance uses. If you don’t have the patience for math calculations and want quick answers, this is probably the best way to go! I wrote a detailed explanation for how to use the Kill A Watt in this article.
The average refrigerator use varies from as low as 200 watts for a new Energy Star refrigerator to as high as 600 watts for an older unit. At startup, your refrigerator may have a higher draw, so you want to err on the high side when you figure how many watts of power you need. Also, determine the wattage of any lights or other items you intend to run.
How long will you run that fridge and those lights?
Once you have determined the wattage, you need to run all your appliances. Determine how many hours each day they will run. For instance, your refrigerator can run approximately 1/3 of the time during a 24 hour cycle, or 8 hours a day. The light that draws 75 watts may run 3 hours a day. Once you have determined how many watts you use a day from each appliance, add them up to give you the daily watt-hour results.
- Refrigerator 1600 watts
- Lights 400 watts
- Misc 400 watts
- Total 2400 watts daily watt-hours
Now, to allow for bad weather when the sun doesn’t shine, multiply the daily watt-hour result by three. This provides a buffer, in case you aren’t able to recharge the batteries on a daily basis.
Because you should not discharge the batteries below 50%, multiply this number by 2. This will give you the total battery capacity your system needs to store to run for three days. Or in this example, 14400 watts.
What size of battery bank?
Now we will calculate the size of the battery bank you will need in amp hours. Amp hours are used because this is the standard by which batteries are rated. This is determined by dividing the total battery capacity required by the voltage of your batteries.
For example, if your system requires 14400 watts, you would divide 14400 by 12, which would show that you need 1200 amp hours. Divide the total amp hours by the amp hour rating of your batteries and you will have the number of batteries you need. For example, if you have deep cycle batteries rated at 300 amp hours, you would need 4 batteries. The Vmaxtanks brand is one of the highest rated deep cycle battery brands that I have found.
Factor in the solar panels
With these calculations in hand, how many solar panels do you need? Again, this is just a matter of a math calculation.
Divide the daily watt-hours by the wattage of your solar panels, times the number of hours of sun. If you have a 75-watt panel and 5 hours of sun per day, you would produce 375 watts per day per panel. Now divide the daily watt-hours by the wattage produced by one solar panel. Using the 2400 daily watt-hour figure from the above example, you would divide the 2400 by 375 and need 6.4 panels. Always round this off to the higher number.
For this system you would need seven 75-watt solar panels and four 300-amp hour batteries. If you were using 200-watt panels, you would need only three panels and four 300-amp batteries.
If you’re intrigued with solar power and its uses, in this article I explain a simple way to turn a simple 12-volt battery pack into a small solar generator and more explanation for building a small solar system here. A portable solar sytem on wheels is another versatile option for solar power and in this article you can see photos and simple instructions.
I recommend learning the fundamentals of solar power on small projects like these and then using that knowledge on larger projects, such as a system to power a home or man cave. Over the years, my family and I have used a number of small solar-powered gadgets and batteries around the house, and you can read about them on this page.
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