Potomac Electric Power Co. is urging its customers to take advantage of a $15 professional home energy audit -- without mentioning that the same service will soon be available in the District of Columbia for free.

"Put your home through analysis," the company recommends in its August "Lines" publication, a regular insert that accompanies monthly bills. The mailing, which is arriving in many area homes now, describes Pepco's home energy audit program and the two types of audits offered.

One is a free do-it-yourself audit. The other is the electric company's "waste watchers" professional audit for which the company now charges $15.

In July, D.C.'s Public Service Commission, overriding objections by the utility companies, told Pepco and Washington Gas Light that they must begin to provide such audits without charge -- a fact that goes unmentioned in the Pepco mailing.

"Whether it's fair or not is a judgment question," said Pepco spokeswoman Nancy Moses. "But until we can gear up to make program modifications for the free audits, we're still functioning under the old program, and it still costs." She added, though, that consideration is being given to whether future mailings should say that such services are expected to be offered for free in 1983.

Both utilities said yesterday that the audit program must be approved by the U.S. Department of Energy before it takes effect and that they expect to begin to offer free audits in the spring. As things stand now, WGL performs no audits in D.C.

Moses said that the company normally pushes its energy audits at the beginning of fall and again in the spring and that the mailing went to Maryland and Virginia as well as to D.C. customers.

The company has performed approximately 14,000 energy audits in all the jurisdictions it serves since the existing program began in 1978, she said.

Moses said that performing the audits actually costs the utility company $90.