For more than a year now, an enormous solar thermal power plant has been humming along in the Arizona desert, sending out power as needed, even well after sunset. The plant, called Solana, was developed by the Spanish energy and technology company Abengoa and has succeeded in meeting an elusive solar goal — producing electricity when the sun is not shining — and displacing fossil-fuel-based power in the grid.

“With the sun going down at 6 or 7 o’clock at night, all the other forms of solar production are essentially going to zero,” said Brad Albert, general manager for resource management at Arizona Public Service, the state’s main utility, “while Solana is still producing at full power capability. It just adds a whole lot of value to us because our customer demand is so high even after the sun goes down.”

Indeed, Abengoa opened another mammoth plant on Friday in the Mojave Desert in California that uses a similar approach. But despite the technology’s success, Abengoa and other developers say they do not have plans at the moment to build more such plants in the United States.

And that is largely because of uncertainty surrounding an important tax credit worth 30 percent of a project’s cost. Although the subsidy, known as the Investment Tax Credit, is to remain in place until the end of 2016, when it will drop to 10 percent, that does not give developers enough time to get through the long process of securing land, permits, financing and power-purchase agreements, executives and analysts say.