An energy performance rating for new houses has been developed by Technology and Economics, a Cambridge, Mass., consulting firm. The mathematical forumula utilizes 13 variables, ranging from insulation efficiency to the rate of air changes per hour, as applied to any one of seven different types of single-family houses in a given climatic zone.

This generic system was created for National Homes Corp. but is said to measure the energy efficiency of houses by any builder.The higher the rating, the more BTUs saved.

The system is intended for use by buyers seeking to compare energy efficiency of several prospective purchases.

With information suppled by the builder, a buyer can easily figure out a house's energy performance rating,the consulting firm said. The same scoring system can be applied to ranch, two-story, bi-level and tri-level houses. Technology and Economics, which is preparing a brochure for the Department of Housing and Urban Development on how to buy an energy-efficient house, has used the average area of components like windows and doors in order to make it possible to compare houses of different layout and elevation.

The object of an energy performance rating is not to find the house with the highest possible rating, but to select the right balance between construction cost and energy efficiency, the consultants said.

For example, a bi-level home in Washington should have R-22 ceiling insulation, according to David R. Price, president of National Homes Corp. It should have R-17 exterior wall insulation and solid door to provide an optimum combination of heating and cooling.

The same house in Chicago should have R-30 ceiling insulation, R-17 exterior wall insulation and a solid wood door with storm door to provide the same combination of indoor climate conditions. The R factos is the measure of resistance of the insulation to heat flow-through; the higher the R number, the greater the insulating power.

Technology and Economics president David Mac Fayden predicted that the rating system would become "an important consumer benchmark in the same vein as the automobile EPA and appliance EER appliance ratings." Unlike the gasoline rating, however, the energy rating system is not intended to set goals or norms.

In the example given, the figure 135.5 cannot be regarded as either an optimum or average rating, Mac Fayden said. Pressed for some basis of comparison, he concluded that 135.5 represented a "reasonably good" energy rating for a house.

But he cautioned that energy rating of bricks and mortar does not take the residents' habits into account, so it is possible that homeowners who leave windows open or keep the temperature at 75 degrees in winter may get higher fuel bills than careful owners who house is structurally less energy efficient.

A free copy of the energy performance rating booklet can be obtained beginning in January from these National Homes Corp. builders in the Washington area: Battlefield Builders in Manassas, Gaddy Construction in Burke and Harmony Homes in Wheaton.