Thanks to remarkable drops in prices for solar panels, mainly from China, simple market forces seem to be driving an all-out unstoppable solar boom. Globally, some 40 percent of solar’s growth is in the form of people powering their own homes and businesses, and in the United States, solar accounted for more than half of all new power last year. Solar seems to have passed a tipping point: In many countries, the low cost of the technology is propelling its own growth, despite little government help.