LOS ANGELES (AP) -- The largest solar power plant of its type in the world - once promoted as a turning point in green energy - isn't producing as much energy as planned. One of the reasons is as basic as it gets: The sun isn't shining as much as expected. Sprawling across roughly 5 square miles of federal desert near the California-Nevada border, the Ivanpah Solar Electric Generating System opened in February, with operators saying it would produce enough electricity to power a city of 140,000 homes. So far, however, the plant is producing about half of its expected annual output for 2014, according to calculations by the California Energy Commission. It had been projected to produce its full capacity for 8 hours a day, on average. "Factors such as clouds, jet contrails and weather have had a greater impact on the plant than the owners anticipated," the agency said in a statement. It could take until 2018 for the plant backed by $1.6 billion in federal loan guarantees to hit its annual peak target, said NRG Energy Inc., which operates the plant and co-owns it with Google Inc. and BrightSource Energy. More via News from The Associated Press.