AT THE Konokoyi coffee cooperative on the edge of Uganda's Mount Elgon national park, Juliet Nandutu is trying out a new toy: a solar-powered cellphone charging station that is activated by text message. She is offering the service to her village. "I charge 18 phones a day, sometimes 20," she says.

How many phones she charges depends on the local electricity supply. When it's there, people can charge their phones at home, but that's not very often. "It's not so reliable," she says. "It's on and off."

A patchy or absent power grid poses a conundrum for rural areas in the developing world, particularly in Africa and Asia, where the use of cellphones is rapidly rising. Farmers, for instance, use cellphones to get up-to-date pricing information for nearby and distant markets, allowing them to better manage the sales of their crops.