Tesla CEO Elon Musk said Monday that his company made “significant mistakes” in its solar roof tile project that have led to further cost overruns and delays. The product was originally revealed back in 2016.

Tesla is currently still trying to meet significant demand for the solar roof, but has yet to widely roll out the product, Musk said in a conference call with investors following the release of Tesla’s financial results for the first quarter of 2021. Tesla reported $494 million in sales of its energy generation and storage products in the first quarter, but the overall division is still not profitable. Musk’s comments come as irate customers have told outlets like The Verge and Electrek that Tesla is increasing the price of the solar tiles when it comes time for installation.

Musk said Tesla has run into trouble “assessing the difficulty of certain roofs,” and said that the “complexity of roofs varies dramatically.” If an existing roof has protuberances, or problems with the underlying structure, or is not strong enough to hold Tesla’s solar tiles, then the cost can be two or three times higher than Tesla’s initial estimates, Musk said. Tesla is refunding customers’ deposits if they don’t want to pay the price increase, Musk said.

Musk unveiled the Tesla solar roof in 2016 on the set of Desperate Housewives. At the time, he was trying to acquire Solar City, a solar energy company formed by his cousins. Musk was also chairman of Solar City at the time.

The roofs he showed off at the event weren’t fully working, Fast Company later reported, and Musk allegedly had said prototypes of the tiles were a “piece of shit.” Still, the vision for the solar roof — replacing regular roof shingles with photovoltaic tiles that could collectively capture even more of the sun’s energy than traditional solar panels, and storing that energy in Tesla’s Powerwall home battery — was crucial to the merger, he wrote in his second “master plan” for Tesla.

“We can’t do this well if Tesla and SolarCity are different companies, which is why we need to combine and break down the barriers inherent to being separate companies,” he wrote. “That they are separate at all, despite similar origins and pursuit of the same overarching goal of sustainable energy, is largely an accident of history. Now that Tesla is ready to scale Powerwall and SolarCity is ready to provide highly differentiated solar, the time has come to bring them together.”

The merger eventually went through, though it’s still the focus of a shareholder lawsuit in Delaware. Since that 2016 event, the solar roof has gone through multiple revisions but has yet to be widely rolled out. Musk even once said that 2019 would be the “year of the solar roof.”

Musk now says Tesla will expand solar roof installations this year, and on Monday he talked up the company’s recent decision to more bundle Tesla’s home battery with its solar products. “This is the long term solution to a sustainable energy future,” he said.