Yea, this part was on my mind when we were considering this as we have very nice, fairly new ceramic tiles on our roof that are both durable (to the elements) and delicate (to humans stomping on them).Some of the installers are real fly-by-night operations and will destroy the integrity of your roof to install their panels.
Your numbers align with my calculations so that's a nice sanity check.
Net20? Is that NEM?