You can build a "poor man's" pyranometer that works pretty well out of inexpensive and readily available components. Although it will not be of laboratory quality, it will suffice for comparative measurements and educational purposes.

Making the Meter
The obvious component to consider as the basis for our meter is a silicon photovoltaic cell. When sunlight strikes it, it produces electricity, and the more sunlight strikes it, the more electricity it produces.

At first glance, you might think that you could just hook a voltmeter up to it and measure the output voltage. Unfortunately, if you do this, you will get very erroneous results (visit the Appendix if you want to understand why).

Instead, you will need to measure the cell's output current. If you short out (hook a perfect wire between) the positive and negative terminals of your cell, a current flows through that wire. That is the current that you would like to measure. It varies linearly with the amount of sunlight striking the surface of the cell. This is actually a little trickier than one might think, for reasons that are explained in the Appendix.