Recently, the city of Denton passed a resolution to move to 100% renewable energy by 2020. This will be the second city in Texas to do so, and the 58th in the country.

But what does 100% renewable energy mean? This question was asked a lot when Georgetown, the first city in Texas to move to 100% renewable energy, decided to take this step. According to David Spence, a professor at the University of Texas at Austin, for now it “means that those cities purchase as much renewable electricity as they can get.”

Neither Denton nor Georgetown can guarantee that the electricity will always be 100% renewable. Saying so is “misleading,” says Charles McConnell of Rice University’s Energy and Environment Initiative.

Without an economical way to store energy, 100% renewable energy is not a reliable or sustainable energy source because these sources aren’t always producing energy when it’s needed. According to Spence, “it is prohibitively expensive to build a large enough system of wind farms, solar arrays, transmission lines and batteries necessary to ensure a reliable supply.”

The move to 100% renewable energy is accompanied by the launch of the Denton Energy Center (DEC), a natural gas plant. This addition is explained as a hedge against spikes in electricity prices, a potential revenue source, and as a backup if Denton needs additional energy. Back in 2015, Denton Mayor Chris Watts said, “You don’t know exactly when the sun is going to shine or when the wind is going to blow. To maintain that reliable power, you must have backup power.” Building non-renewable generation at the DEC while claiming “100%” renewable exposes the claim’s challenges.