In 1994, NASA tapped professor KR Sridhar to create a device that turned water into oxygen for Mars. Afterward, Sridhar was left wondering: If we reverse the basic process, could we put oxygen in and get electricity out, right here on Earth? The result is the Bloom Box, a refrigerator-size, emissions-free power station that uses natural gas or biofuels to generate as much electricity as a coal plant. The handmade box costs some $750,000, but Sridhar, 49, hopes to slash that price to $3,000 within a decade. Some folks can't wait. Google, Walmart, and eBay were among Bloom Energy's earliest clients, and eBay estimates five Bloom Boxes have saved the company $100,000 in just under a year.