An estimated 1.2 billion people worldwide live without electricity. Many depend on kerosene lamps to light their homes-a fuel that has ill effects on health and is costly. Solar power might seem the better alternative in sunny parts of the world, except the poor can't afford the upfront cost of the equipment, which starts at around $50 for a modest rooftop system. "Even though photovoltaics have become much more affordable over the past 10 years, it's still too expensive to be competitive with other sources of energy," says Paul Polak, a social entrepreneur who has spent the last 25 years creating affordable tools used by millions of poor farmers and is now working on a solar water pump.
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