Solar panel company SolarCity today announced what it calls the "most efficient rooftop solar panels". The company claims it "generates more power per square foot and harvests more energy over a year than any other rooftop panel in production, and will be the highest volume solar panel manufactured in the Western Hemisphere."

While there have certainly been solar cells that have achieved a higher efficiency than the 22.5 percent SolarCity offers, the highest-efficiency solar cells, and the panels that incorporate them are not used in residential or commercial applications because they're far too expensive.

SolarCity acquired Silevo, another solar technology company, last year, and are using the solar cell architecture developed there. They continued to work on the solar cell to bring it to an efficiency of 22.5 percent.

"Think of the solar cell as the engine that powers the solar panel," says Ben Heng, senior vice president for product engineering at SolarCity.