The wind wavers. The sun sets. And the electricity they produce stops flowing.
In small-scale installations, this is not a problem. Batteries provide sufficient backup power, or the system is simply connected to the grid.
But what happens when wind and solar systems start supplying substantial amounts of our nation's electricity? Will today's batteries fill the need for gigawatts of power?
Not without some technology changes. As this story from the Christian Science Monitor explains, conventional lead-acid batteries aren't up to the job. Researchers are turning to newer technologies like sodium-sulfur and lithium-ion to meet our immense storage needs.
Some companies are going beyond these electro-chemical solutions, to technologies like compressed air, flywheels and molten salt that store energy from solar power.
While technology is important, so is size. Some of the batteries being built are as large as a double-decker bus and can accumulate megawatts of energy. (By comparison, the battery in your car supplies a maximum of about 1 kilowatt of power.)
But while some companies are betting on size, others believe salvation lies at the microscopic end of things.
Gary Rubloff, director of the Maryland NanoCenter at the University of Maryland, believes nanoscience is the answer.
Rubloff and his fellow researchers are using nanotechnology to increase the amount of electricity a conventional capacitor can store. "Batteries are too big, too heavy, too expensive," he says. He estimates his team can increase a capacitor's power by a "factor of 40 to a couple hundred."
The technological hurdles to ecomomical renewable energy are getting lower day by day.