SolarCity published its earnings for the fourth quarter and full year 2015 yesterday. The company’s stock price fell 25% following the results, mainly because of lower than expected guidance for the first quarter 2016 and a miss in installed solar capacity following its exit from Nevada, as well as difficulties completing a few commercial projects on the east coast.

On a more positive note, we learn from the company’s SEC filing that it more than doubled its battery pack business with Tesla Energy in 2015 versus the previous year, despite the market for home energy storage only being in its infancy.

SolarCity has to disclose its business with Tesla because the company is considered a related party due to sharing a chairman, Tesla CEO Elon Musk, as well as a few investors and board members.

The solar installer doesn’t breakdown the related party transactions by companies, but it disclosed that they were primarily purchases of batteries from Tesla Motors. The only other related party mentioned is SpaceX, obviously for the same reason (Musk) as Tesla, but SolarCity’s relation with the rocket manufacturer has to do with the sell of millions of dollar worth of ‘solar bonds’, making the inventories and equipment transactions mainly, if not entirely, with Tesla.

Related parties transactions:

solarcity related parties

As we discussed in our overview of the history of Tesla’s stationary energy storage effort this morning, SolarCity operated several pilot programs with Tesla, some for residential projects and others for commercial installations. These projects represented around $1.7 million and $3.4 million in 2013 and 2014 respectively.

Even though most of the company’s projects in 2015 would have also been pilot projects or early adopter installations, SolarCity still doubled its inventories and equipment transactions with Tesla.

SolarCity’s energy storage projects are not always made public, nor does the company always disclose its battery suppliers, but we reported last year on a few important project the solar installer completed, including a 6 MW solar project with 2.6 MWh of Tesla Powerpacks and a 52 MWh energy storage system in Kaua’i.

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