"solar energy is actually a financial investment with recent returns more solid than stocks and bonds."
"The price what he was being told" may have nothing to do with reality, that is, the price at which he could actually purchase solar panels. I know this to be the case, having dealt with many Solar Distributors and Contractors.
You are correct in that the dollar amount saved would depend largely on one's cost for electricity. A savings of $77.40 per month, as in your example, would yield a 6.2% return in the first year - not bad when considering the current uncertainty in financial markets. My point is that investors would be well served to rebalance their portfolios in favor of safe investments that actually pay a good rate of return, especially in light of historic low interest rates and volatility in the stock market (where anyone can get clobbered at any time). When factoring in future expected utility rate increases, let's say a conservative 3% per year on average, your 6.2 % rate of return increases to 7% in year 5 and 8.1% in year 10. Now, if we assume a 6% per year increase, your rate of return would increase to 7.8% by year 5 and 10.5% by year 10. Also of note, as Peter mentioned, your savings would be tax free.