For solar shoppers these days, the price is right. Panel prices have fallen about 40 percent since the middle of last year, driven down partly by an increase in the supply of a crucial ingredient for panels, according to analysts at the investment bank Piper Jaffray.

The price drops — coupled with recently expanded federal incentives — could shrink the time it takes solar panels to pay for themselves to 16 years, from 22 years, in places with high electricity costs, according to Glenn Harris, chief executive of SunCentric, a solar consulting group. That calculation does not include state rebates, which can sometimes improve the economics considerably.