Attendees take pictures of the new Tesla Energy Powerwall Home Battery during an event at Tesla Motors in Hawthorne, California April 30, 2015.

A new study published by EnergySage and the North American Board of Certified Energy Practitioners has found that just 12 percent of solar panel installers in the U.S. have Tesla Powerwall in-home batteries to install.

This isn't due to lack of demand. Instead, the study suggested over 55 percent of solar installers reported that customers looking for in-home batteries are specifically requesting a Powerwall, which stores energy from solar panels and allows a home to operate entirely off of the grid, even if the power goes down.

Tesla is selective in which suppliers are able to sell its Powerwall products, however, and it certifies the ones who do. A majority of its sales comes from Tesla's website instead of through suppliers, Tesla said in its Q3 shareholder letter.

Tesla recommends that a 2,200-square-foot home have at least two Powerwalls, which costs about $14,500 before installation costs. Customers can place reservations on Tesla's website for $500, but that means the Powerwalls need to be ordered separately from the companies doing solar panel installations.

Tesla CEO Elon Musk explained during the company's earnings call in January that Tesla was "cell-starved for vehicle production" and had to switch its Powerwall production lines to vehicle battery lines. Tesla said it has since added additional capacity to its Powerwall production lines.

CNBC reported in January 2018 that Tesla was still manually assembling batteries, at times borrowing employees from Panasonic and causing delays for the company's Model 3 vehicle.

"A new manufacturing line made by Tesla Grohmann is further increasing production of Powerwall and Powerpack modules at Gigafactory 1," Tesla said in its Q4 shareholder letter. "With a better supply of cells and new manufacturing equipment, we are aiming to more than double energy storage deployments to over 2 GWh in 2019. Through various operational efficiencies, our average sale-to-installation time also decreased by about 50% in 2018."