There is one question that is impossible to answer in any investment analysis of solar or other renewable energy source of power. That is how much the cost of electricity purchased from the grid will increase in the future.
Everyone agrees it will increase. The average increase over the last several years has been 2.5¢ per year. However, since no one knows how much it will increase in the future, use several different inflated numbers to figure your return on investment.
Remember IRS and other taxes: The savings on your electric bill (your "profit"), unlike the profit on a stock market investment, will not be taxed. By investing in a renewable energy system and an energy-efficient home you may get a higher rate of return on your investment in renewable energy than with the average stock market investment or when leaving your money in a CD.