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Solar maker SunPower is pulling back the veil (a bit) on a long-discussed plan to sell batteries in conjunction with its solar panels.

On Tuesday SunPower provided more details about a pilot project it’s running in California with home builder KB Homes. SunPower and KB Homes will provide a solar panel and battery combo product to new home owners in the cities of Irvine, El Dorado Hills, and San Diego.

The pilot project is small at this point. Sunpower doesn’t expect “more than a handful of homes in three KB Home communities will include the technology” this year, according to a company spokesperson. But next year it sees potential for a broader rollout to additional homes and communities. SunPower is also piloting solar and battery combos in Germany and Australia.

SunPower said it’s testing out lithium ion battery technology from several companies to provide the storage part, but it wouldn’t disclose which vendors it’s working with. The company also wouldn’t comment on the cost of the systems to customers in the trial.

Clearly SunPower’s battery plans are at an early stage. But just the fact that SunPower is moving forward with this strategy highlights how the market for pairing solar panels and batteries is truly starting to emerge. Batteries can enable homeowners with solar panels to store the sun’s energy during the day, to be used at night, or at other times of the day when using grid energy is expensive. SunPower said its battery goals are about giving its customers total energy management options, as well as more control over their energy costs.

The barrier to this market has long been the high cost of the batteries. But with new business models — like SolarCity’s lease programs — and lower cost battery technologies, lithium-ion batteries are starting to become a more reasonable options for a home energy storage system.

Some day the market for solar and batteries will be huge. Navigant Research predicts that worldwide revenue from residential distributed generation (solar and other generation technologies) plus energy storage will grow from $52.7 billion annually in 2014 to $71.6 billion in 2023.

Solar financier and installer (and soon to be manufacturer) SolarCity has been one of the leading faces of pairing solar panels with batteries. Its relationship with Tesla (through Elon Musk and investors) has enabled it to pair Tesla batteries with the solar panels it sources. SolarCity is already selling batteries and solar panels to WalMart stores and Tesla has said it will use a portion of its batteries from its future gigafactory for grid storage.

Other companies in the U.S. eying this market include One Roof Energy, which is working with battery maker Silent Power to roll out products. Korean conglomerate Hanwha Group, which runs a solar panel manufacturing subsidiary, is an investor in both OneRoof and Silent Power. SunEdison has done a pilot project with a battery system from startup Seeo.