The Federal Energy Regulatory Commission is investigating the decision by Virginia Electric & Power Co. to delay for two years completion of a $1.7 billion hydroelectric storage project in Bath County, Va.

The agency's staff is trying to determine whether the delays violate the license the FERC gave Vepco to build the project, a press spokesman said after Virginia State Rep. Caldwell Butler (R) disclosed the investigation.

Butler, a critic of the decision, asked for a FERC investigation last month. In a letter this week, the agency told him it already is looking into the matter.

In May, Vepco said it was slowing down construction in Bath County, laying off 2,000 workers and moving the completion date of the job back from 1983 to 1985.

Vepco officials said at the time the delay was necessary because the company was short of funds and because the growth in electricity use had slowed so much that power from the project would not be needed before 1985.

The delays will save Vepco $275 million in 1980 and 1981 but ultimately will add $460 million to the total cost.

When it is completed the Bath County project will be the largest "pumped storage" facility in the United States. It will be used to store electricity for use at periods of peak demand, such as hot summer days.

In the mountains near the West Virginia border, Vepco is building a pair of lakes, one about a thousand feet above the other. To produce electricity, water will be released from the upper reservoir to run through a hydroelectric power plant into the lower lake. At night, when Vepco's other power plants are producing more electricity than its customers need, the surplus will be used to pump the water back into the upper lake.

Butler and many other politicians from southwestern Virginia have criticized the slowdown in construction work in Bath County.

FERC officials said their primary concern is whether the electric company is living up to the obligations of its license to complete the project within nine years after work starts.

The agency told Butler it is studying whether part of the Bath County project could be sold to American Electric Power System or Allegheny Power System, a pair of neighboring utilities.