The S&P 500 Utilities index dropped 0.8% in May, as 13 of the 28 U.S. power and utility companies landed in the negative territory, according to data compiled by S&P Global Market Intelligence. Energy stocks were the biggest losers on the S&P 500 in the month, with a negative return of 11.1%, compared to a negative 6.4% return for the overall S&P 500 index.

New Jersey-based water utility American Water Works Co. Inc. continued to sit atop of the list, posting a total return of 4.9% in May. American Water Works was followed by Eversource Energy, WEC Energy Group Inc. and Sempra Energy with gains of 3.8%, 3.5%, and 2.7%, respectively. Sempra's gains come on the heels of Saudi Arabian Oil Co.'s decision to pursue a big supply deal with the proposed Port Arthur LNG project.

NRG Energy Inc., on the other hand, saw its total return decline by 17.3% in the month. On May 20, the Princeton, N.J.-based merchant generator and energy retailer announced an all-cash transaction to purchase Stream Energy's retail electricity and natural gas business for $300 million, subject to various regulatory approvals. Also in May, NRG sold approximately $1.83 billion of notes to fund a cash tender offer and to fully repay its senior first priority secured term loan.

AES Corp., CenterPoint Energy Inc. and Edison International also were among the bottom performers in May, with negative returns of 7.7%, 7.4% and 6.9%, respectively.

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