S&P Dow Jones Indices said it would add Tesla Inc. to the S&P 500 in December after leaving it out of its September rebalancing, sending the electric-car maker's shares soaring in after-hours trading Nov. 16.

Tesla will replace a company that will be disclosed later, with SPDJI saying it was still determining how Tesla, which has a market cap of $386.8 billion, can be best added to the benchmark index.

"Due to the large size of the addition, S&P Dow Jones Indices is seeking feedback through a consultation to the investment community to determine if Tesla should be added all at once on the [Dec. 21] rebalance effective date or in two separate tranches ending on the rebalance effective date," S&P Dow Jones Indices said after market close.

The company's stock surged about 13.2% as of 3:47 a.m. ET. on Nov. 17.

Tesla shares slumped 10.1% on Sept. 8 when it was unexpectedly left out of the S&P 500 reshuffle that instead saw Etsy Inc., Teradyne Inc. and Catalent Inc. added to the index and H&R Block Inc., Coty Inc. and Kohl's Corp. dropped.

Tesla's market cap peaked at $464.3 billion on Aug. 31, rising from this year’s low of $66.5 billion on March 18, according to S&P Global Market Intelligence data.

A requirement for joining the S&P 500 is reporting four straight quarters of profitability, which the company met in July. Some analysts had said that Tesla's high valuation and volatile stock price may have caused the committee to leave it out of the shake-up in September.

"Tesla has been a conundrum for the [Index Committee]," Michael O’Rourke, chief market strategist for JonesTrading, wrote in a Nov. 16 note.

The committee's "snub" of Tesla was a relatively major event back in September, he said.

"Now, a couple of months later, the committee has had a change of heart," O’Rourke said. "It is hard to understand why it was unsuitable in September but is appropriate in November."