I have been keeping meticulous records since I installed our solar array in 2009, and have been keeping all this information in a spreadsheet. One of the things I have been doing is comparing the estimated power I should expect with the actual power generated. The National Renewable Energy Lamps tool called PVWatts allows you to input the specifics of your solar array including its location, number of panels, and orientation etc. so that it can give you an estimate of how much power you should expect. At the end of every utility billing cycle, I access the records from the web interface for the solar inverters on my system to determine exactly how much power was produced by the array for each month. The chart below shows the calculated estimate vs. actual production. Notice that the estimated production bumps up each year as I have added panels to the array. The scale on the left represents kilowatt hours per month.