To make a meter that reads actual values of solar insolation is a bit more complicated as you will need to use solar insolation tables to get an idea of what kind of intensity (usually in the USA measured in BTU/Ft^2/Hr) you can expect.  Now if you live in an area that always has some smog or hazy skys then these tables will not be correct as you will always measure less than what should be available.
I have put in a picture (below) of one example of these tables that works for me as where I live in Colorado totally clear skys (other than when its cloudy) with no haze or smog are the norm.  There are other tables that give approximate values of BTU/Ft^2/Hr or Watts /Meter^2 on map contours for the USA.  Possibly these are available for all over the world if you find the right sites on the internet.
If you use the approach in the second schematic you could make up a table of output voltage readings for various availability listings on the table to have a somewhat calibrated measurement.  I have found that calibrating these is best done with the cell in the horizontal position as you get less reflected light onto the cell.

Schematic three shows the actual circuit in my meter using an old analog meter movement I had laying around.  The meter is set up to read voltage and the corresponding variable resistor in series with the meter is of a value that works with my particular analog meter movement.  The potentiometer was adjusted to calibrate the output reading on the meter scale with the known incoming sunlight intensity.