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Old 03-14-2015, 04:45 PM   #1
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Join Date: Aug 2010
Location: Minnesota
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Default Solar output prediction calculator

As we have messed around with the solar, I have always been uneasy about all the "predictions" of just how much power you should be getting out of the panels at any give time and place. Time of year, time of day, daylight savings, latitude, all have an effect, and that doesn't even bring into consideration of clouds and trees.

You can't put a number on the clouds and trees, unless you have a solar intensity meter, which is probably over the top, even for me

It does, however, appear that you can get quite close with some very simple calculations. It allows you to predict what the theoretical maximum output of your panels would be (with them sitting level) at any place on earth, at any time of day, any day of the year. You then can see how close you are to the theoretical max, and what your efficiency is. If there are clouds and trees, that will be a loss on top of the basic loss.

You start with this solar calculater that I found online

The only things you really need to enter are your latitude and longitude, which are easy off your GPS, and your time offset from Grenwich. The rest of the settings you can do with slide bars at the bottom of the sun position animation. You can move month and day on one, time of day on another. You can watch the sun move on the animation curved path as you change settings to get a maximum and then see the time of day (which I was surprised to not be noon or the same time even every day). The center of the animation is straight up sun and the outer ring is the horizon, with the circles being 10 degree changes in elevation. The data listing to the left of the animation shows more details, with most important one being what they call altitude, which is what I would call elevation. It is the angle of the sun from the horizon.

Once you know the altitude, or elevation, of the sun, the calculation of theoretical output gets pretty easy. I had read that the panels generally react in a way that they are putting out watts based on the 90 degree to them portion of the light hitting them. These guys seem to have found similar results.

Using that theory, all you need to do is take the sine of the altitude you get on the calculator and multiply it by your panel rating. Done.

An easy one is 45 degrees. Sine is .707. 100 watt panel. 70.7 watts theoretical.

I did the calculation for our location a few days ago when I did the first clear day testing on ours and got 188 watts theoretical. We were reading in the 170-180 watt range on the controller, so we are losing in the 5-10% range.

Finding that good solar calculator really made all this a lot simpler than I ever thought it would be.

I would recommend that anyone who installs solar do the calculation a couple of times to confirm that you are running well. I also think it would be a very good idea to run the numbers if you hire an installation done. All you need is a clear day, so you can do it in the parking lot of the shop, no matter where they are, or the time of day.
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Old 03-15-2015, 06:38 PM   #2
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Location: New Brunswick, Canada
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Default Re: Solar output prediction calculator

Good idea & good info.

The solar controller in my van is pretty old. The methods you describe would let me check to see if I can get what could be reasonably expected out of the panels.
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Old 03-15-2015, 08:28 PM   #3
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Default Re: Solar output prediction calculator

I did Homestead Florida on the June equinox and it maxed out at 88 degree sun angle and 99.9% of rated solar energy. No wonder it get hot down there!
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