It's estimated, based on meteorite finds in desert areas, that meteorites larger than 10g reach the surface of the earth at a rate of 1 per million km^{2} per year. Dave's solar panel installation has an area of 1.93e10^{-5} km2, so the probability of an impact on the array in any given year would be 1.93e10^{-11}. That's 1 in 52 billion, if my math is correct. Meteorites smaller than 10g comprise the vast majority of mass that falls to earth, but have very little kinetic energy when they reach the surface; they are essentially dust.