Potomac Electric Power Co., which last week was granted only a fraction of the rate increase it sought, yesterday filed a new petition to increase District of Columbia electric bills by $24.7 million.

Pepco has asked the D.C. Public Service Commission to raise rates by another 7.9 percent.

The power company estimated that the new rates would increase the average electric bill in the District from $19.98 a month to $20.25 a month, or a little less than a penny a day.

Last Friday the PSC agreed to let Pepco raise its rates by about 2 percent - roughly 6.5 cents a month for the average customer.

But the PSC ruled that Pepco could not raise the rates of small electric customers at all and gave the power company a total increase of only $5.8 million a year rather than the $44.8 million Pepco sought.

In the new request filed yesterday, Pepco said another $24.7 million increase is needed to update last Friday's decision.

The PSC ruling was based on Pepco's operating costs in 1977, the company said, and inflation has pushed costs up since then.

Pepco said the additional $25 million is necessary if the power company is to earn the profit margin authorized by the PSC in last week's decision.

A 9.25 percent return on Pepco's investment in power plants and facilities was authorized in the PSC ruling. Pepco claimed it could not make the profit margin the commission said it deserved without higher rates now.

The utility company noted that the Consumer Price Index - popularly called the cost of living index - has gone up 26 percent since 1976, but Pepco's electric rates have increased by only 2 percent.