This experiment has already been done at Google, as reported on their official blog.
The company runs a 1.6 Megawatt rated solar installation at its Mountain View, Calif. headquarters. Those on its roof were installed with a tilt. Those on carports were installed flat.
Some 15 months after they were installed Googlers cleaned the carport panels, and energy output doubled. When they were cleaned again 8 months later, output climbed 36%.
This was not a problem with the rooftop panels. Rain cleaned them naturally. (It didn't get the corners, as this illustration shows, but the energy loss there was not great.)
Google has also crunched some numbers and found the installations will take 6 1/2 years to pay for themselves.
This impresses Google. It does not impress me. Does it impress you?
These are still early days in the evolution of solar energy, so points like this that should seem obvious are lessons that still need to be learned.
As the efficiency of solar panels increase, with new technology payback times should come down. Google has a lot of roof space, a lot more than the average homeowner, and it has a lot of money to spend. It can afford to conduct these experiments.
So let me leave you with a final question.
How long must the payback time be before you get interested in putting solar panels on your roof -- slanted, flat or otherwise?
And remember the lesson. Tilt your panels so cleaning them does not wind up on your "honey do" list.