The Pennsylvania Public Utilities Commission yesterday gave the firm that owns the two idle Three Mile Island nuclear power plants 20 days to say why Pennsylvania should have to continue to pay rates that finance the undamaged plant.

The three commission members ruled unanimously that the company will have to defend the inclusion of Three Mile Island Unit 1 in its rate base, since the plant will not be "used and useful" at least until February.

The decision is an indication that the commission looking with disfavor on claims by General Public Utilities, which owns Three Mile Island, that costs of the accident there last March should be passed along to the public.

The commission took the damaged Unit 2 out of the rate base in June, ruling that the utility customers did not have to continue helping to finance some $49.2 million since they are getting no power from Unit 2. The regulators said then that they were leaving Unit 1 in the rate base on the assumption it would be returned to service by Jan. 1.

But the Nuclear Regulatory Commission announced it would hold extensive public hearings before allowing Unit 1 to reopen. The process could take at least a year.

"We would hope that the PUC will recognize that the delay is a direct result of the NRC order," said General Public Utilities chairman William G. Kuhns. He said the firm still thinks Unit 1 ought to be in the rate base.

Ken McKee, a spokesman for GPU, said the ruling yesterday could force Metropolitan Edison Co., the GPU subsidiary that runs Three Mile Island, to the brink of bankruptcy for the second time since the March accident. An arrangement of long-and short-term loans to tide the company over could fall apart as a result of this decision, he said.