The percent of homes heated with electricity in the area served by Potomac Electric Power Co. has more than doubled since 1974, from 5.3 percent to 11.7 percent in 1980, the company reported.

In addition, electricity now is used in a higher percentage of homes for water heating in Pepco's service area, and the number of certain kinds of electrical appliances have increased substantially in the homes, Pepco said a survey had indicated

At the same time, the utility found encouragement in the increased level energy conservation efforts its customers said they were taking.

Of 6,391 customers responding to the survey, about 90 percent said they turn off lights when they leave a room, 78 percent said they lower the thermostat in the winter, 46 increase thermostat settings in summer, and 52 percent answered that they use appliances less than they used to.

In addition, the responses indicated that 53 percent have installed storm windows or thermopane glass, 32 percent installed or added insulation, and 50 percent have caulked and weather-stripped.

At the same time, however, the survey showed substantial increases the number of electricity-eaters, including central air conditioners, self-defrosting refrigerators, electric ranges and clothes dryers. About 12 1/2 percent of the survey respondents said they buy energy-efficient appliances.