To win, they had to impress the judges, two energy investors, Colleen Calhoun, senior directory of energy ventures and GE Ventures, and Abe Yokell, a partner at Rockport Capital Partners. It was no easy feat because they’ve seen it all and are well aware of how tough it is to succeed in clean energy.

First up was Damian Beauchamp, co-founder and CEO of Kair Battery, a company working on batteries for other companies to store energy. The batteries rely on oxidizing potassium for more efficient energy storage without some of the usual toxicity. Kair Battery is trying to take on Tesla (TSLA), which aside from making electric cars, is starting to sell batteries for energy storage. Beauchamp claims that Kair Battery can do the same thing, but at half of the price of Tesla. The judges were impressed with Beauchamp’s pitch but questioned how the startup will make money in the battery business, which is known to have low margins

Next up was Etosha Cave, co-founder and chief science officer of OPUS 12, which takes excess carbon dioxide from oil refineries and corn ethanol plants and creates other chemicals from it. Based on technology developed at Stanford University, Opus 12 can take carbon dioxide and convert it into ethanol, which can be used by oil refineries and corn ethanol plants. The judges response was positive, but they cautioned that commercialization of such technologies can be challenging.

Erik Norwood, founder and CEO of Curb Energy, is taking on Google’s Nest thermostat (GOOG) to provide a smarter, Internet-enabled way to monitor your home’s energy. Curb’s monitor, which plugs into a home’s circuit breaker, tells you how much you are spending to power specific appliances and lets you know when an appliance isn’t working properly. Although the company already has 200 of its intelligent home monitors installed, the judges worried about how Curb could tackle commercial adoption and loyalty in a market already dominated by Nest.

The last entrepreneur to pitch was Claudia Wentworth, co-founder of Sunlayar, a cloud-based software that helps solar designers and developers design and install solar projects.

And the envelope please? The victory goes to Etosha Cave of OPUS 12, who wowed the crowd with her pitch to turn waste into usable chemicals.

Click here for more coverage of Fortune’s Brainstorm E conference.