NEW YORK — A Washington, D.C., regulator approved Exelon’s nearly $7 billion takeover of rival utility Pepco, but the deal only will go through if Exelon agrees to the regulator’s terms.

The District of Columbia Public Service Commission said Wednesday Exelon has 30 days to agree to the way it wants more than $70 million in payments to be used for customer discounts and energy conservation.

Exelon said it is reviewing the order.

Pepco is the parent company of Delmarva Power, which has 506,000 electricity customers throughout Delaware and on the Eastern Shore of Maryland. It also provides natural gas to 126,000 customers in northern Delaware.

Delmarva, first incorporated in 1909, has 925 employees.

A combination of Exelon and Pepco would create the largest electric utility company in the U.S., according to the U.S. Energy Information Administration. Exelon first offered to buy Pepco nearly two years ago.

Chicago-based Exelon owns ComEd in Illinois and PECO in Pennsylvania. Pepco, based in Washington, powers homes through Atlantic City Electric in New Jersey and Delmarva Power in Delaware, Maryland and Virginia. Together, the companies said they would have about 10 million customers.

The District of Columbia Public Service Commission is the last regulator Exelon needs approval from to cinch the deal. The Federal Energy Regulatory Commission and commissions in four states have already approved it.

Shares of Pepco Holdings Inc. soared 27 percent to close at $26.93 Wednesday. Exelon Corp. shares slipped less than 1 percent to close at $34.72.