Hawaii has the highest cost of electricity in the U.S., largely because it's expensive to ship oil in from elsewhere, which also contributes to the Aloha State's hearty carbon footprint. That's part of why legislators there have passed an ambitious mandate to get 100 percent of energy from renewables by 2045.

To meet those goals, Hawaii is tapping into its most plentiful energy resource: the waves. The nation's first wave power generators, two state-of-the-art buoys, have just been placed online. The system is already generating roughly 22 kilowatts of energy, and that's just the start. Officials estimate that wave power like this could eventually supply 20-28 percent of the nation's — not just Hawaii's — power.

"When you think about all of the states that have water along their coasts ... there's quite a bit of wave energy potential," said Jose Zayas, a director of the Wind and Water Power Technologies Office at the U.S. Energy Department, which helps fund the Hawaii site.

A surprising amount of power is contained in the natural sway of the waves. Buoys can translate this power into electricity through bobbing in a variety of ways. One of the buoys, the Azura, converts the waves' vertical and horizontal movements into electricity. Meanwhile, a second, doughnut-shaped device called the Lifesaver produces electricity via an anchor and cable system that wobbles with the movements of the sea, turning a generator's wheels in the process. These devices will pave the way for more efficient buoys of the future that can capture all of the ocean's movements at once.

It's a non-invasive method that has minimal, if any, impact on the surrounding environment. The trick is just in building buoys that can withstand the natural elements: wild storms, constant pounding of waves, and the corrosive effects of saltwater. The engineering breakthroughs are behind those in the solar and wind industries by a few decades, but the potential inherent in wave energy is just as stark as those other renewable stalwarts. Some officials estimate that wave energy could be competitive with fossil fuels within a decade.

The technology could also offer military benefits, which is why the U.S. Navy has been a major player in the development of the Hawaii test site. Offshore stations could generate their own power thanks to wave energy systems.

"More power from more places translates to a more agile, more flexible, more capable force," explained Joseph Bryan, deputy assistant secretary of the Navy. "So we're always looking for new ways to power the mission."