If you keep track of green technology companies, you may have heard rumblings about Bloom Energy, a secretive company that has raised nearly $400 million from investors like Kleiner Perkins for its supposedly game-changing fuel cell device. Now the eight year old company is finally emerging from the shadows with the Bloom Box, a $700,000 to $800,000 machine that 60 Minutes calls “a little power plant-in-a-box.” So what exactly is the Bloom Box?

The box consists of a stack of ceramic disks coated with green and black “inks.” The disks are separated by cheap metal alloy plates. Methane (or other hydrocarbons) and oxygen are fed in, the whole thing is heated up to 1,000 degrees Celsius, and electricity comes out. Bloom estimates that a box filled with 64 ceramic disks can produce enough juice to power a Starbucks.

As of right now, Bloom isn’t angling for the residential market–the box is far too expensive. But major companies like eBay, Google, Staples, and FedEx have already secretly started using the boxes. So far, the Bloom Box has been a success–eBay has already saved $100,000 in electricity costs since its 5 boxes were installed nine months ago. EBay even claims that the boxes generate more power than the 3,000 solar panels at its headquarters.

Of course, fuel cells aren’t new. They have just been too expensive to be viable until now, and Bloom still has to prove that its box can produce energy at a cheaper rate than other power sources. The box also produces carbon dioxide as a byproduct–a potential downside depending on how much it generates.

Bloom Energy founder K.R. Sridhar estimates that a Bloom Box for the residential market could be out in 5 to 10 years for under $3,000. That’s a big improvement from the $800,000 box of today, but only time will tell if Sridhar is being overly optimistic. And in the coming years, big name competitors will probably catch up to Bloom with cost-efficient boxes of their own. Will the Bloom Box and fuel cell devices like it eventually replace the power grid? Probably not, but they have the chance to one day at least partially free homeowners from the grid–along with solar panels, wind turbines, and other alternative energy sources.

Check out the 60 Minutes segment on the Bloom Box below for more info.