Most of Donn Tice's customers make $4 to $6 a day. What little money they have, they guard. But they're willing to part with some of it, Tice says, for a product that can improve their lives. Tice's company, d.light, sells solar-powered lamps in the developing world. The lamps charge on their own during the daytime, shine for at least four hours at night and are designed to last more than five years. The standard model costs $30 - a significant investment for d.light's core customers. But the San Francisco startup has sold about 3 million lamps in the last five years, mostly in parts of rural Africa and India with limited access to electricity. "What we've discovered, frankly, is there's a much bigger global problem around reliable power than we imagined," said Tice, d.light's chief executive officer. D.light is one of a growing number of companies trying to make money by selling to the "bottom of the pyramid" - the world's poor. They see a vast, often-ignored pool of potential customers for a wide range of products, so long as those products serve real needs and are affordably priced. For entrepreneurs like Tice, there's the added lure of doing something that can help people pull themselves out of poverty. "I can't tell you how profoundly meaningful it is, how inspiring it is, to go to a village with our customers and go to a school where recently the students weren't using lights to study," Tice said. "It doesn't take a lot of imagination to visualize how really transformative that could be in the trajectory of their lives."