A pyranometer or solar irradiance meter is used for measuring sunlight intensity at a given location. It reads in watts per square meter or W/m^2. This DIY project isn't technically a pyranometer because it doesn't use heat in any way to measure light intensity. Instead, it uses a solar panel. This could be any small solar panel, like the ones in old calculators. Normally, a pyranometer and even a lux meter will do a cosine correction. What this means is that the meter collects light coming from all angles and measures it. It usually does this with a hemisphere shaped diffuser. But if you are testing for solar intensity for solar applications, then you don't need it. In fact, it is more useful without the cosine correction.
For instance, if a solar panel is perpendicular to the sun, then that is its max power. It's angle would be zero degrees and the cosine of zero is 1. If you turn it to 90 degrees off axis to the sun then you would get no power because the cosine of 90 is 0. (Well, you actually get some power because of reflected light hitting the solar panel)
Let's say you are testing a spot for solar and you use one of the expensive pyranometers. You place it on your roof and get 800 watts per square meter. Then you use the homemade version using a small solar cell. It shows 800 w/m^2 as well, but only if you hold it perpendicular to the sun. If you lay it flat on the roof like you plan on mounting the solar panels it shows only 480 w/m^2. So, in a way the homemade is better because it shows you which angle is the best for the most power.
You can't just measure voltage because it isn't linear on a solar cell. You have to apply a set resistance and then as the light goes up, the current goes up and the voltage goes up, but linear in this case because of the set resistance.
I start off by using an old volume control (potentiometer) and hook it up between the two solar cell wires. I then set my meter to milivolts. I want 1 milivolt to equal1 watt per square meter.
I then find a site that shows local weather statistics. I found this one that shows the data from a school very close to me. http://www.victoriaweather.ca/station.php?id=109&type=day&size=large You are looking for the Insolation value.
You should wait for a sunny day with no clouds. Let's say that it says 600 W/m2 on the site. Go outside and aim the small solar cell at the sun. You must be outside, a window cuts down the numbers quite a bit. Now turn the volume control up or down until the meter reads 600 mV or 0.6 volts. Milivolts is easier to read though.
You can leave it like this or you can make it have a finer adjustment if you have a lower value potentiometer. You can carefully disconnect the potentiometer and measure the resistance across the two leads you were using. Let's say it is 100 ohms. Then use a resistor that is under that like a 47 ohm for instance. Then if you have a 100 ohm potentiometer it would be great. I happened to have a precision 100 ohm pot. It can be turned about 25 times around or so. Makes for a super fine adjustment. But I think just a regular pot is fine for most things.
This device is really great for estimating energy. Just remember you have to know your surface area and efficiency. For instance, if I read 500 w/m2 at a certain angle and I mount my solar panels at that angle and they are 2 meters square total, then that would be 1000 watts times my 15% efficiency, or 150 watts.
Or you could solve for efficiency in a real world example. You could also use it for solar hot air or water. You can see what difference glazing or double layer glazing makes...all before you build it.