Measuring  Sunshine

Every hour the Earth intersects 1.4 Kilowatt hours of solar energy per square meter... AND since the radius of the Earth is 6,000 kilometers the surface area intersected with solar radiation is  about 3.14 x 6,000,0002   or  113,000,000,000,000 m2. This means that about 160,000,000,000,000  KWH of radiant energy intersect the path of our earth every hour. Of course not all of this energy makes it through our atmosphere. HOWEVER where sunlight is direct and not blocked with clouds about 1KW of power is available per square meter. The question is how can we measure sunlight to make a sound investment calculation. Weather  you're planning on installing solar panels to generate photo-voltaic power or solar collectors to generate solar thermal heat you'll need a tool to measure sunlight to predict the payback period of your investment.

Professionals use something known as a Pyranometer to measure solar insolation or solar flux that heats up a blackened thermopile incased inside a set of clear hemispheres.  The thermopile generates a current proportional  both direct and indirect radiant energy. A data logger is used to log flux variations throughout the day. From this data a graph may be plotted that gives a clear picture of the sunlight availability over a period of time.  More affordable Pyranometers may be made with silicone sensors incased in white plastic capsules. Alternatives to the Huckse Pyranometer and the Apogee Pyranometer are the Sky Eye and the photodiode assemblies below.

Over the course of 12 hours on a clear sunny day on Long Island in the month of November a solar insulation curve generated with a Sky Eye and a photodiode looks like this...

At first glance the plot of the graph made with the photodiode seems a bit crude BUT it still accurately represents the radiant energy available within a limited range. The 600  point is off the chart but  If this point were extrapolated I believe it would read a light intensity half the peak light intensity at solar noon.  Solar Noon peak voltage is 3.3 mV. and the  600  point  is about 1.6mV.. I'm sure you could plot the rest of this curve more precisely if you had the sun for the job.... Remember the most important points on the curve are the:

PEAK ENERGY  SOLAR NOON                                                        DIRECT RADIATION         angle of incidence 00
HALF ENERGY  (.66 x (time between sunrise and solar noon)        HALF RADIATION             angle of incidence 600
ZERO ENERGY ( at sunrise and sunset sun horizontal to sensor)   ZERO RADIATION            angle of incidence 900

The following videos may not be 100% accurate but they will give you the basic idea of what's involved with measuring sunlight.

The photodiode assembly is a nice tool that may be used to estimate the intensity of solar flux. It's simply pressed into DVM probes and the meter is set to the mV scale. To calculate the intensity of light in terms of watts/m2 the user multiplies the output voltage by a factor of 80 or so....  Unfortunately I can not remember where I purchased these PV cells.