What Can You Power with One 120 Watt Solar Panel

Hello guys.

In today’s video, we’re going to take another look at solar panels, we’re going to come at it from a different angle. Traditionally, what you do is to start from with your loads, then you calculate the size of your solar panels from there. But today I’m going to look at it the opposite way around. I’m going to take a small 120-watt solar panel and then see what we can actually run with this small photovoltaic panel.

I’m going to work out the calculation back the other way. The reason for doing it this way is you might only have room on your roof for one small scale solar power system, or you might only have the budget for one small scale solar system. So this might be your deciding factor. 

For this example, I selected the Coulee 120 watt 12 volts solar panel. Solar panel ratings are given under the standard test conditions. The light source calibrated to deliver 1000 watts per meter squared onto the surface of the panel and the surrounding ambient temperature is kept at 25 degrees C. These are the standard conditions that all solar panels are rated at. We know that taking the volts times amps equals to the watts. So if we use the maximum operating voltage 18.4 volts and multiply that by the maximum operating current 6.52 amps, we get 120 watts.

The rating of the solar panel is based on one hour at standard test conditions. Obviously, during the day we are going to have a variant amount of sunshine. Typically in the morning in the early hours between 8:00 and 9:00, the sun’s irradiance is about half of what it would be at midday. So we need to look back at some historical data for the areas that we live in to find out how many average daily sunshine hours (peak sun hours) we can expect. We are mainly going to be traveling in Vancouver, Canada. So if we look at this power data, we can typically expect to get around 3.1 peak sun hours. We simply take the 120-watt solar panel to multiply it by 3.1 and come up with a figure of this 372 watt-hours. That’s the total amount of power that we can expect to generate during one day.

However, we won’t be able to use all of that power because there are some losses to take into account. We could have some dirt on the surface of this small 12 volts solar panel. There will be voltage drops on the cables between the solar panel and the charge controller. And there is also some inefficiencies in the way the controller works when it regulates the power from the solar panel to charge the batteries. So if we take all of these things into account, we can expect some system losses of about 30%. In this example, if we take our total power of 372 watt-hours minus 30% system losses. We come out with a net value of 260 watt-hours. And this is the value that we can actually use to decide what we can power within our van.

Now we know how much we can generate in one day, realistically what we can use.  I’ve taken some of my own devices, plug them into the watt meter, and have recorded the readings here. So we can see that I could use my laptop which uses 40 watts for three hours, giving me a total watt-hours 120. I can run some LED lights for a few hours, might charge my iPhone, charge my Kindle, Ipad, and some other devices. So if we total all of these, it comes to our figure of 263 watt-hours. So we can use quite a lot of devices for quite a few hours during the day with one Coulee 120 watt 12 volts solar panel. Obviously, you wouldn’t necessarily use all of these devices on the same day. So there’s quite a bit of diversity there. And I think you can see this demonstrates that you don’t actually need to put a lot of solar power onto your van to make it usable.

But generally, you can’t use the power generated by the solar panel directly. It needs to go via some leisure batteries with a charger controller. So let’s look at the calculation to size a leisure battery suitable for this single Coulee 120-watt small solar panel. The leisure battery runs on a 12 volts system. So we take our total generated power of 263 watt-hours divided by 12 volts. And that gives us 21.92 amp-hours, and this is what we will use to size our batteries. Lead-acid batteries don’t like to be discharged entirely. At best you don’t want to go any further than 50% discharge. They do say that AGM battery can go up to 80% discharge. But the deeper you discharge the battery the less average cycles you’re going to get out of it. It’s far better to keep the depth of discharge to a minimum, and then you will get a much longer life out of your battery. In this example, we are going to use a 50% depth of discharge. We take the 21.92 amp-hour divided by 50% effectively doubles the amp hours of our battery, and we get 43.84 amp hours. So because we can only use half of the battery power, the battery effectively needs to be twice the size.

One more thing to take into consideration is the ambient temperature that the battery is going to be stored in. The temperature of the surroundings does affect the output of the battery. We can see here in this table, at 10 degrees C is a multiplying factor of 1.11. So we take our 43.84 amp hours multiplied by 1.11, and we get 48.7 amp hours, that would be the battery power required for one day.

Now let’s say that we didn’t have any sunshine for a couple of days. It was really cloudy, bad weather, and we couldn’t recharge the battery. We want probably a couple of days autonomy on the battery so that we could go maybe a weekend without having any solar. We simply oversize the battery again so that we can have a couple of days of spare capacity. So we will take the 48.7 amp hours multiplied by three days, and we get a total battery power of 146.1 amp hours. So something like a 150 amp hours battery (or battery group) will be plenty for this single 120-watt solar panel. Now we could have one 120 watts Coulee small solar panel, one 12 volts 150 amp hours battery(or two 12 volts 75 amp hours batteries), and a charge controller, that would be our solar system ultimately.

One last thing that I want to discuss is the tilt angle of the solar panel. A solar panel will produce its maximum power when it’s facing directly at the sun so that the surface is at 90 degrees to the angle of the rays of sunlight. Now if you’ve got your panels laying flat on the roof of your van, there will be a different angle to the sun. They won’t be at 90 degrees to the sun. And that angle is directly related to the lines of latitude on the earth. Here in Vancouver, we are typically at degrees 49 North. So our solar panels ideally need to be tilted to an angle of 49 degrees. This will make sure that the sun’s rays at the middle of the day are directly 90 degrees to the surface of the panel. And it does make a huge difference with the output. Also, we need to take into consideration the time of the year, because in the winter the sun is much lower on the horizon, and in the summer the sun is very high in the sky. So we need to make a little adjustment to the angle of the tilt, depending on what seasons. Roughly speaking, during winter we can increase the angle by 15 degrees because the sun is much lower on the horizon, we need to tilt the panel up further to get it to be 90 degrees to the sun. And in the summer when the sun is directly overhead, we can afford to decrease that angle, lay the panel more flat to get that 90 degrees angle. By tilting your panels, you can expect to achieve up to 30% more power out of your solar panels than having them just laying flat on the surface of your van. 

Therefore because we are traveling in Vancouver, Canada. I’m going to engineer a way that I can use my solar panels on the roof of my van. I’m going to secure them with some aluminum angle and a couple of aluminum braces. And then I’m going to drill a series of holes into that angle which corresponds to the angles for summer, winter, autumn, and spring. So during the normal time of the year, it will be at 49 degrees, during summer I will lay it back to 34 degrees, and during the winter I will tilt it up further to 64 degrees. Because ideally, I just want to maximize the most I can get out of the solar panels. Obviously, with this arrangement, I’m not going to lift these panels if the weather conditions are poor. But if it’s nice and sunny and in calm conditions, I’m going to elevate my panels, and I’m going to maximize the amounts that I can generate. 

OK, that’s all. We hope this video has been helpful. If you have any questions, please feel free to contact us.

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