A PhD in electrical engineering would help.

Google recently announced a million-dollar contest to reinvent the power inverter, the devices that turn the direct current coming from solar panels and batteries into alternating current you can use in your home to play Xbox and keep your beer cold. A smaller, more efficient inverter could help solar power finally go mainstream and save energy throughout our existing electricity grids. Here's a crash course on inverters and what it would take to win the prize.

Last month Google teamed up with the Institute of Electrical and Electronics Engineers to announce something called the Little Box Challenge: Build a power inverter that's size of a tablet, 95 percent efficient, and capable of powering a house, and the million bucks is yours. Today's inverters are 10 times bigger, about the size of a cooler. A smaller, denser inverter would force engineers to experiment with materials and circuit designs that the industry has been skittish to use, mostly because the current technology works well enough.

But Google thinks it could work better. The field of power electronics—the family of devices that help distribute electricity, which includes inverters—is ripe for innovation. New materials and circuit designs could have power-saving implications for any component that helps move electricity from one place to the other.

Winning the contest is going to require new materials, innovative design, and loads of electrical engineering experience.

Every inverter is built around a switch. When direct current electricity—essentially a torrent of electrons all flowing in the same direction—from a solar panel or battery reaches that switch, it splits the flow by rapidly flicking between two ends of a circuit. These two streams of direct current meet head on in a transistor a little further down the line and become a crude alternating current. Other parts smooth this into a sine-like wave that works more efficiently with our appliances.

The yellow boxes on the back of each panel are power inverters. Google believes they can be much smaller, and more efficient. The yellow boxes on the back of each panel are power inverters. Google believes they can be much smaller, and more efficient. Sputnik Engineering AG

The biggest problem with the inverters we have now is that the switch is made of silicon. Silicon is cheap and easy to work with, but inside an inverter it can only flick so fast before it starts shedding energy in the form of heat. This isn't just wasteful, it's dangerous because too much heat will destroy electrical parts. So, inverters require finned blocks of metal called heat sinks, cooled by fans, that ferry heat away from sensitive parts.

To shrink an inverter down, engineers are probably going to have to figure out how to build them around switches made out of semiconductors that can flick a lot faster. Two likely candidates are silicon carbide (SiC) and gallinium nitrate (GaN). Faster switching means they would shed less heat, so the inverter wouldn't need so many fans and heat sinks.

"When you can switch at a high frequency, then everything else falls into place," said Brad Lehman, an electrical engineer at Northeastern University in Boston, and editor of the journal *IEEE *Transactions on Power Electronics.

SiC and GaN semiconductor switches already exist, and they're generating a lot of excitement. These materials would also be better at smoothing the crude alternating current that comes out of the first part of the circuit, which would save space as well.

Another problem with today's inverter switches is that they introduce a little ripple on the direct current side of the inverter. This can degrade the terminals attached to the solar panels, the battery that stores energy from the panels, and other components on the DC side of the circuit. Today's solution is to add a little, temporary storage components called capacitors that suppress the ripple. But, capacitors take up a lot of space. A smaller inverter wouldn't have room for capacitors (good riddance!), so it would have to solve the problem through innovative circuit design.

Building inverters with new materials and designs seems likely to improve their efficiency, though it's hard to predict by how much. And putting all the parts together in a way that stays inside the contest's strict size requirement—and the FCC's electromagnetic compliance rules—is going to be more than just a circus of plug-and-play experimentation. "You can have a very good switch, but you have to be able to put all this shrubbery around it," said Steve Colino, vice president of Efficient Power Conversion, a company that makes GaN semiconductors.

Teams have until July 22, 2015 to deliver their entries. Google already has thousands of acres of solar panels powering its servers, and it wants to move to 100 percent green energy, but it isn't calling dibs on the winning device. Instead, it will be up to the developers if they want to file for a patent or put the winner out into the wild.