Tesla said on Tuesday it began manufacturing its premium solar roof tiles at the company’s Buffalo, NY, factory last month and has started surveying the homes of customers who made a deposit of $1,000 to reserve the product last year.

The solar roof, which generates electricity without the need for traditional rooftop panels, is a cornerstone of the electric vehicle maker’s strategy to sell a fossil-fuel-free lifestyle under the luxury Tesla brand.

Tesla unveiled the product in October 2016 as it sought to acquire solar installer SolarCity.

At that time, Tesla said the product could be rolled out as soon as the summer of 2017. In November, Tesla founder Elon Musk said the product was going through a six-month testing process, saying “it just takes a little while to get this behemoth rolling.”

More than a dozen Tesla employees, including Musk and Chief Technical Officer JB Straubel, had the solar roofs installed on their homes last year as part of an initial pilot program.

Tesla started taking orders for the solar roof tiles in May by asking homeowners to put down a $1,000 deposit via its website. Tesla would not disclose how many reservations it had received for the solar roof. The product will be installed on some customer rooftops in the coming months.

The company has said its solar roofs would cost between 10 and 15 percent less than an ordinary new roof plus traditional solar panels.

Tesla is collaborating with its longstanding battery partner, Japan’s Panasonic Corp, to manufacture solar products at the Buffalo factory. There are about 500 employees working at the site currently, Tesla said.

The companies began production of traditional solar panels at the site last year, but they have not yet been installed on rooftops. The company said it will achieve more than 1 gigawatt of cell and module production in Buffalo, and possibly as much as double that, but gave no timeline for meeting the target.

Tesla said its primary focus is increasing production of its Model 3 sedan. The company said last week it would likely build about 2,500 Model 3s a week by the end of the first quarter, half the number it had earlier promised.