Apple announced Wednesday that it’s building a big battery storage project at a Northern California solar farm it spearhead in 2015. But what the company didn’t share is that the battery packs will come from Tesla, The Verge has learned.

The newly-announced setup, which will store up to 240 megawatt-hours of energy, was approved by the Monterey County Board of Supervisors in 2020, according to documents submitted last year. It will consist of 85 Tesla lithium-ion “megapacks” and be used to help power the company’s corporate headquarters in Cupertino. Monterey County’s planning chief confirmed that Apple will use the Tesla batteries in an email to The Verge. Apple declined to comment. Tesla did not respond to a request for comment.

Tesla first announced the megapack battery system back in 2019. The 60MW storage setup Apple will be using is not Tesla’s biggest, though. The company has built bigger overall battery storage solutions in Australia and south of Houston, Texas of around 100MW in size. Still, Apple touted it as “one of the largest battery projects in the country” in a press release, saying the battery system could power more than 7,000 homes for a whole day. The Tesla batteries will make it possible for Apple to store energy generated by its 130-megawatt solar array at the farm, which is called California Flats.

“The challenge with clean energy — solar and wind — is that it’s by definition intermittent,” Apple VP Lisa Jackson told Reuters on Wednesday. “If we can do it, and we can show that it works for us, it takes away the concerns about intermittency and it helps the grid in terms of stabilization. It’s something that can be imitated or built upon by other companies.”

While Apple uses lithium-ion batteries in many of its products, it’s not known to be working on any grid-scale projects. The company is reportedly developing a lithium iron phosphate battery for its electric car project, though.

Apple and Tesla don’t have much overlapping history, though each company is notorious for poaching talent from the other. Tesla CEO Elon Musk also said in December that he tried to pitch the idea of Apple buying his company back in 2018, but that Apple CEO Tim Cook “refused” to take the meeting.

Tesla is best known for its electric cars, but it’s spent years trying to build up an energy storage business to compliment the solar products it acquired when it bought Solar City. It has gotten increasingly involved in large-scale energy storage projects like Apple’s over the years in addition to its home battery business.

While it’s still modest compared to the billions of dollars generated by Tesla’s car business, the energy storage division’s products has already netted at least one other strange bedfellow customer: in 2019, Volkswagen announced it was using Tesla batteries at some of its Electrify America charging stations.