This image shows the lengths investors go, to keep their investment producing at full performance:

http://upload.wikimedia.org/wikipedia/commons/2/27/Googleplex_Solar_panels.jpg

When the Mars rovers Spirit and Opportunity were only guaranteed to work for 90 days, because they thought the solar panels would be affected by Martian dust. This shows that even NASA knows that dirt and dust on panels affects performance.

Luckily for interplanetary knowledge, the Martian weather kept the dust to a minimum, but does show that keeping the panels clean does increase output.