Maryland customers of Washington Gas Light Co. will begin paying an average 4 percent more for natural gas consumption today under a Maryland Public Service Commission decision Monday to approve about half of what the company requested.

A WGL spokeswoman said "the company is obviously disappointed we came up short" and indicated that a new request for higher rates probably will be filed in the near future.

Information about the price boost, which has not been announced by the regulatory agency, was confirmed last night by Maryland People's Counsel John K. Keane Jr. and by WGL Vice President Edmund Smallwood.

Washington Gas revenues from 233,000 customers in Prince George's, Montgomery and Charles counties will increase by $9.36 million a year, under the PSC decision--about 54 percent of the $17.3 million rate increase requested last July 31 by the D.C.-based utility.

According to the WGL spokeswoman, an average Maryland residential customer with gas heating uses 114 therms of month of gas, for which average monthly bills will jump to $60.31 from $57.93. For nonheating residential customers using 31 therms a month, average monthly bills will rise to $19.75 from $18.95.

The last rate increase in Maryland for WGL customers was at the end of 1980, when the PSC approved a $7.5 million (3.8 percent) hike.

Smallwood said it is too soon to compute the impact of the PSC decision on the gas company's earnings. "We're looking at it now . . . we will be filing" a new rate increase request, he said, noting that the level of expenses used to make this week's decision was for the 12 months ended last March 31, after which interest costs and other prices soared.

According to lawyer Tom Gorak of the People's Counsel staff, a large portion of the higher rates denied by the PSC this week reflects complex accounting disputes.

But the agency also agreed with the People's Counsel that costs associated with a residential conservation program, under which WGL and other utilities make energy audits of houses under a federal program that doesn't cover all costs to the firm, should be spread throughout all customers and not just residential users.

Gorak said that because conservation efforts benefit all users of gas, they all should pay. The amount of money involved is a relatively minor $158,000, but Gorak said it is an important precedent.

The PSC also denied about half of the advertising expenses claimed by WGL to be necessary as part of costs customers should cover. Information advertising is allowed, but promotional ads and "image" ads are not supposed to be paid for by customers. The PSC decided that about half of the ads were of the latter variety.