WASHINGTON — ENERGY storage is crucial to transforming the electric grid into a clean, sustainable, low-emissions system, the experts say. And it’s happening already, just not the way most consumers would expect.

The simplest idea for storage — charging up batteries at night when there is a lot of wind energy and not much demand for it, or at midday when the sun is bright — is years from being feasible, according to the experts.

The reason? It costs hundreds of dollars to store a kilowatt-hour of energy in a battery, while nationally the average retail price of a kilowatt-hour is about 11 cents. On the wholesale market, even buying low at off-peak periods and selling high could earn a battery owner perhaps 25 or 30 cents for each $400 or so invested. For that kind of transaction, “storage is not profitable,” said Jay Apt, executive director of the Carnegie Mellon Electricity Industry Center.

Prices would have to fall by 90 percent, from the current range of $300 to $500 per kilowatt-hour of capacity down to $30 to $50, he said. Instead, electric companies and some users of commercial power are adopting storage in forms that many people would not recognize as batteries — big containers of ice in building basements, or vast tanks of molten salt, for example. And where plain old batteries that store actual electricity are being used on the power grid, they do very subtle, high-value jobs, like keeping the alternating current system in the proper rhythm, or smoothing out the flow of energy from wind farms that are prone to start and stop suddenly.