Your friend tries to evaluate the idea of installing solar roof and asks for your advice. The initial costs including materials and installation are estimated to be $20,000. It will generate enough electricity for the entire household for the next 30 years. The monthly maintenance costs about $10 for the solar roof. Without the solar roof, your friend pays about $120 on electricity bill every month. Assume the annual cost of 4%. If your friend likes to have the lower monthly cost, would you suggest installing the solar roof? What is the monthly saving or additional cost if going with solar.