Low cost batteries could be a game changer for clean energy sources like solar or wind. One tesla return on investment solar and wind fall short of natural gas and coal on reliability is that these renewable sources only produce electricity when the sun shines or the wind blows. In contrast, gas and coal can produce power at any time, without fluctuation. But if the cost of batteries falls low enough, clean energy can be much more competitive.

The rain pauses, the clouds part, and the sun finally shines down on a sprawling 65-acre solar panel farm, surrounded by bright green grass and tropical foliage, on the plush Hawaiian island of Kauai. The close to 55,000 solar panels, which are still being installed on the site, aren’t yet converting the sun’s light into electricity, but something else quite remarkable is happening at the project on this Wednesday morning in mid-November. When the project is finished early next year, the batteries will store the sun’s energy during the day to be used at night, when the local utility, Kauai Island Utility Cooperative, usually starts turning on fossil fuel-consuming generators. When the farm is switched on it will be one of the first ones where a utility is using big blocks of Tesla batteries, and it will also be one of the first utility systems to use solar and batteries to displace power-hungry generators. Elon Musk, the CEO of Tesla and Chairman of both companies. The companies will hold a shareholder vote on Thursday afternoon, which will determine if the two firms will merge into one. Fortune around the project, a first by a media entity.

Tesla’s massive battery factory, itself under construction outside of Reno, Nev. Last year, Tesla officially announced that it would sell batteries for the power grid, buildings, and homes after building a business on battery-powered electric cars. The crew then string wiring along the rows of panels, connecting the rectangular electricity generators to inverters that convert the solar energy into usable electricity. The issue is, KIUC can’t add any more solar panels to the island without adding more energy storage, says KIUC engineering manager John Cox.

To take the next step, KIUC needed technology that could store the energy for when fast-moving clouds cover the solar panels or when night falls. The island sees peak energy demand start around 8 p. In other words, the utility needed the batteries if it was to continue along its path to more clean energy. There’s another key element driving the pioneering project: low battery and solar costs.

9 cents per kilowatt hour for the energy and storage capability from the solar and battery project. Importantly, the energy from the project is cheaper than energy from some of KIUC’s older fossil fuel generators, says KIUC’s Cox. With solar panels are at their cheapest moment in history, battery costs have recently started to drop as well. Originally spurred by massive factory investments by battery giants like Samsung, Sony, LG, and Panasonic, lithium-ion batteries are starting to break out of the market for cell phones and gadgets and become firmly planted in the power grid, basements of buildings, and electric cars. Working with Panasonic, Tesla plans to lower the cost to make its batteries by a third with its Gigafactory in Nevada. Tesla confirmed that the Kauai batteries were manufactured at Gigafactory. For an inside look at Tesla’s Gigafactory watch our video.