With all the data collected so far we can say that during the summer months this portion of the roof can expect an average of around 3,100 watt hours per meter square of sunlight in a 24 hour period. To run the Raspberry Pi off sunlight alone on an average day would require a 30 watt solar panel, and an 8 amp hour battery. Cloudy days will require an external power source unless a larger battery and/or solar panel is used.
The days are getting shorter as summer draws on, but the fall equinox is not until late September. It will be interesting to see how the light is effected as the sun gets lower in the sky, especially with our fairly treeline. We could try some theoretical approaches to making this estimate. For example, the sunrise for today in Middleton, WI happened at 5:56 am and the sun set at 8:19 pm for a total of 14 hours, 14 minutes of daylight. The winter solstice will produce 8 hours, 59 minutes of light—37% less light time than today. By scaling the light curve to force it into this shorter amount of time will produce a number that would reflect the energy available from the shorter day. However, it will not take into account the loss of light from the sun's position in the sky (it will never get as high). So having the Operation Lux take the measurements is still the best way to get this data.