Tesla's battery factory gets a lot of attention. When completed, the so-called Gigafactory will manufacture more lithium-ion batteries each year than were produced globally in 2013.

That will help push prices further downward. But a few other large producers -- LG Chem, Panasonic and Samsung -- are already making batteries at unprecedented scale. There are numerous giga-scale factories producing cells and battery packs for electric cars and stationary applications throughout Asia. And the recent wave of capacity is already impacting pricing in a big way.

According to Larsh Johnson, the chief technology officer of Stem, the company is paying 70 percent less for lithium-ion batteries than it was 18 months ago.

"It’s happening. The capacity is out there," said Johnson in an interview. "The momentum continues."

Stem has installed 68 megawatt-hours of batteries for commercial and industrial applications, mostly to shave demand charges for customers that consume a lot of power in the middle of the day. Johnson said the improvement in pricing is allowing Stem to think beyond traditional demand charge management.

"There are new markets opening up because of what we've seen in battery pricing," he said.

Traditional demand management typically requires systems that discharge for 1 to 2 hours. Stem is now getting customer requests for systems that can provide 4 or more hours of storage to support grid management services, such as frequency regulation or load-shifting to support renewable energy integration. The company is also looking at a broader geographic range, which includes Texas, Germany and Ontario.

The energy density of lithium-ion batteries continues to improve as well, helping vendors improve performance without adding new costs. "Double-density batteries are important," said Johnson.

As battery costs go down, more hours of storage can be packed into the same battery.

"I'm not surprised that customers are asking for 4-hour duration systems," said Ravi Manghani, director of GTM Research's storage practice. "As soon as you start using storage for something beyond demand-charge management, you're looking at a multi-hour project."

Many utilities looking to aggregate behind-the-meter storage services in New York and California are requiring more than 4 hours of discharge. And it's getting cheaper to provide that level of service every month.

There are a couple of reasons for the 70 percent drop in pricing. Expansion of worldwide production capacity played a role. Since much of the new capacity was designed for electric vehicle demand that never materialized, stationary storage vendors are getting a better deal.

According to the National Renewable Energy Laboratory, there was a total of 53 gigawatt-hours of lithium-ion cell production capacity in 2015 -- but only 40 percent of that was utilized.

Stem's large purchase orders give it an advantage as well. Of the 70 percent price improvement reported by Stem, Manghani estimates that roughly 40 percent comes from oversupply, and another 30 percent comes from bulk ordering.

Johnson said that Stem is expecting a similar drop in pricing over the next 18 to 24 months. He wouldn't give any exact numbers on pricing, however.

"We're trying to balance our purchasing decisions with this continuing price decline," he said.

By 2020, GTM Research expects average lithium-ion battery costs to hit $217 per kilowatt-hour. "But we're already starting to hear numbers in the $200 to $250 per kilowatt-hour range," said Manghani. And Tesla may already be well below those numbers, say some analysts.