Increases in electricity rates for large commercial and industrial customers rose 45 per cent more than increases for residential users in the 18 month period ending March 31, according to a recent survey.

The national study, based on charges by the 24 largest electric utility firms, showed rate increases averaged 18.4 per cent for industrial and business users between December 1975 and last March compared with average increases of 12.7 per cent for residences.

According to National Utility Services, Inc., of New York, an energy consulting firm which conducts periodic surveys of electricity costs, the trend of higher rate increases for business is likely to continue.

Sarkis Soultanian, executive vice president of the firm, noted that an April 1976 survey reflected industrial and commercial rate increases of only 4.5 per cent for the same 24 utilities.

He also cautioned that time-of-day rate experiments, now being introduced or considered in a number of states, should be introduced on a gradual basis. Large users are being asked to accept these pricing concepts without having enough information to evaluate their merit, he asserted.

Soultanian advocated a "multi-tier" system of rates in which utilities would offer large customers alternative rate schedules so economic advantages could be assessed. The D.C. Public Service Commission will decide in the near future whether to order Potomac Electric Power CO. to begin time-off-day pricing this summer, for more than 200 large District-based users.

The Metropolitan Washington Board of Trade has asked the city agency to delay enactment of the new rates - which would include higher charges for consumption during peak-use daylight hours - for one year, while tests are conducted by customers affected.

Board president Foster Shannon said many large customers here will find it difficult to shift power use to off-peak hours.

According to Soultanian, utilities which had the greatest recent rate rises rely on natural gas as a primary or secondary source for generating power; the firms include Middle South Utilties and Texas Utilities in Texas and Pacific Gas & Electric in San Francisco.

Companies using nuclear energy or coal for most of their power have had the lowest percentage rate increases - including Baltimore Gas & Electric (up 1.8 per cent for large business and 3.3 per cent for residences in the recent 18 months).

Virginia Electric & Power Co., with a mixture of energy resources, increased industrial and business rates 26.1 per cent and residential rates 38.2 per cent in the 18-month period. Repco was not included in the new survey.