Hizook

Bonus points for anyone who knows what this device is... I'll give you a hint, it's not a UFO (though it bares a striking resemblance).

OK, I just can't hold off the suspense any longer.  It is called a "pyranometer."  From the Wikipedia page:

Pyranometer clearly showing the instrument main components: glass dome, metal body, black sensor, radiation screen, level and cable. Dimensions: diameter of the dome is 40 mm. Photo shows model SR11 Courtesy Hukseflux Thermal Sensors.

These devices measure the incident solar irradiation by measuring the temperature change of an absorptive material (isolated by the glass domes).  They're used to tell you how much sun-light is hitting per square meter at any time throughout the day.  Luckily, a large dataset has already been compiled (worldwide) for many different locations in the TMY2 dataset.  All of this can be used to predict how much a PV system will generate in a typical year at a particular location, accounting for cloud cover, fog, etc.  For Atlanta (my residence), that works out to about 4.5 kWh/m²/day if you take the daily average. 

Of course, it might just be easier still to just use a NREL chart.

Anyway, I had never heard of a "pyranometer" before...  Good to know.