Solar energy has become the energy of choice in some of the world’s rural markets, helping to bridge the digital divide in developing countries. Daniel Kammen of the University of California at Berkeley said 2 billion people across the world don’t have access to electricity and 1 billion of them could afford to pay for power. “There’s a lot of money to be made in converting those people to solar,” said Allen M. Barnett, chief executive of AstroPower, based in Newark, N.J. With no moving parts, off-the-grid solar energy is easier to maintain than hydropower and wind power. The upfront cost of solar panels and batteries is a challenge for solar, but prorated over 10 years, the cost of solar is competitive with any off-grid power source, at about 18 cents a kilowatt-hour.