For homeowners in 46 of the 50 largest cities in the U.S., investing in a residential solar power system would yield better returns than putting money in thestock market, according to an analysis by the North Carolina Clean Energy Technology Center at North Carolina State University. For 21 million owners of single-family homes in the U.S., solar energy already costs less than current local utility rates, the report says, as long as the system can be purchased with low-cost financing of 5 percent interest over 25 years. Residents of New York City, Boston, and Albuquerque would likely see the largest benefits from investing in residential solar, the report says. The findings assume , however, that government incentives encouraging solar investments — such as tax exemptions and policies allowing homeowners to sell excess solar power to utility companies — will continue, which is highly dependent on federal, state, and local politics.