By Giulia Petroni

Siemens Energy AG made its debut on the Frankfurt stock exchange Monday with a market capitalization of nearly 16 billion euros ($18.60 billion) in one of Europe's biggest listings in recent years.

At 0751 GMT, the shares slipped to EUR21.36 from an initial price of EUR22.01, which Siemens used to calculate a market capitalization of EUR15.99 billion.

Earlier this year, German industrial engineering company Siemens AG decided to spin off 55% of its energy unit to shareholders, making the corresponding number of shares available for free-float ownership. Siemens Energy includes Siemens's gas and power business as well as its 67% stake in Siemens Gamesa Renewable Energy SA.

Siemens retained a 35.1% stake in the company, with the rest controlled by its pension unit, but said it aims to significantly reduce its equity stake within 12 to 18 months.

The spinoff represents a new chapter for Siemens, as it transforms Europe's biggest conglomerate into a leaner business more focused on digital transformation, smart infrastructure and sustainable transportation, as the company describes itself.

"The listing of Siemens Energy means that we've successfully reached a key milestone in Siemens's structural realignment," said Chief Executive Joe Kaeser. "The separately listed companies will be in a significantly better position to tap the individual businesses' value-creating potential than would be possible in a conglomerate."

Siemens Energy is the third major business unit to be parceled off.

The move also marks also a significant transition of power, as Roland Busch gets ready to start running operations next month, and then fully take over from Mr. Kaeser as CEO early next year. 



  Write to Giulia Petroni at giulia.petroni@wsj.com