Solar Production on Hot Summer Days

Today in July Guelph should be getting about a theoretical 5.8 kWh/m2/day compared to 4.5 kWh/m2/day in March. Cloud and atmospheric conditions affect the amount of energy of course. But our nice long days of 14.5 hours vs. March's 12 hours should be yielding more peak and cumulative power on any given day.
My SolarWorld 240 W mono panels are rated at 14.3% efficiency. And the extra energy striking the panels - where does it go? It is converted into heat. And heat decreases the efficiency of energy conversion. Essentially, the voltage decreases. The figure below shows three curves where the panel temperature is 25C, 45C, and 60C. This is an often borrowed image, I shall calculate it for my panel myself one day.

Effect of temperature on solar panel current
http://www.solarpower2day.net/solar-cells/efficiency/

This chart only goes to 60C, but my panels will be reaching over 70C on these hot July days. The current [amps] decreases and power output decreases [P = V.I]. I've read on websites a general rule of thumb [which means it is wrong] is a 0.5% decrease in efficiency for every 1C increase. They don't specifiy if it is a 0.5% decrease of the overall efficiency or of the 14.3%, not do they specifiy for mono/poly crystalline or for amorphous panels. Many good general descriptions of panels and output exist on the web. However, I found a great tool to calculate the panel derating effects by temperature where you can put your own panel data in. The temperature coefficient of my panels is -0.45 %/ºC. 
What this all means is the long summer days of July provide a lot of sunshine, but the total accumulated output is lower than the best days in March. Fortunately for my solar revenue and unfortunate for the farmers and plants and animals, there has been a lot of clear sunny days, albeit our grass is very brown. 
I show our cumulative daily output chart as of 19 July 2011 below. 

Daily power output [kW.hrs] to 2011-07-19.

Our average revenue from 1-19 July has been $21.23. This is pleasing as we've been averaging about $10.50 above our loan costs. We are now above our cumulative loan payments by about $250. We have just over a month to go until we can get a full year tally. I was also pleased that we received out first cheque over $1000 the other day from Guelph Hydro! Our cumulative loan, revenue, and MicroFIT payments are shown below.

Cumulative loan, revenue, and MicroFIT payments to date.