The value of water as a liquid is obvious. We can drink it, use it to clean ourselves and our things, swim in it to cool off and to play, and build dams across it to harvest its energy. Water as clouds are also essential: they are key for the way they deliver precipitation to cropland and forests. But scientists now say we may have been overlooking the most useful stage of the water cycle.

Ahmet-Hamdi Cavusoglu and his colleagues at the Sahin Laboratory of Columbia University published a study earlier this week in the journal Nature Communications theoretically proving that harnessing the latent energy contained in the process of evaporation from all of the US’s bodies of water can generate 325 GW of electricity. That’s 70% of current US electricity production. And unlike other renewables, energy from evaporation will be available whenever we want it, whether or not the wind is blowing or the sun is shining, and with minimal environmental impact.

Cavusoglu’s work builds on an earlier study out of the same Columbia lab. That paper described and demonstrated the world’s first machines powered by water evaporation. As reported in Popular Mechanics, the “Evaporation Engine” works using what the study’s lead author Ozghur Sahin calls HYDRAs. It’s short for ”hygroscopy-driven artificial muscles” which Sahin describes as “muscle-like plastic bands that contract and expand with tiny changes in humidity.” As seen in the video below, as the water content in the air changes, the HYDRAs expand or contract, transferring the evaporation energy into mechanical energy that can be harnessed to power a lightbulb or even a small electric car.

Cavusoglu and his co-authors calculated the average rate of evaporation of all the US’s fresh inland waterways (excluding the Great Lakes and coastal waterways, because the motion of these oceans makes it harder to estimate their rate of evaporation). They then extrapolated to estimate the potential energy created if the Evaporation Engine was used to harness these waterways’ latent power. They found an overwhelmingly powerful renewable energy source.

Almost as important, utilizing the natural heat-storing abilities of water can make for more efficient energy production and use. When there’s too little demand to use all the electricity produced by the Evaporation Engine, that excess energy can be used to heat up the water below the engine, effectively storing it for later use.

As if that wasn’t enough, the researchers also found that harnessing energy in this manner could have a huge impact on water conservation, possibly saving trillions of gallons a year. A good thing too, as the areas with greatest potential for producing this kind of energy are the inland lakes and waterways of hot, arid, drought-prone states like Arizona and California.

All of this is still theoretical, of course. The Evaporation Engines in existence are very small and exist only in a lab. Also, as noted in the study, “Using evaporation driven materials and devices on lakes or reservoirs could affect freshwater resources (e.g., altering the water withdrawal rate, gas exchanges, water quality, and recreational use).” And it’s hard to enjoy the picturesque splendor of a lake under a plastic cover.

That said, even utilizing just a small portion of the nearly 37,000 sq miles of the US’s available inland water, combined with the on-demand potential of the technology, could add huge value to the renewable energy picture.