Both a method and apparatus are provided for sizing and operating enable groups of thermal elements in a thermal printer to allow the printer to be operated by power sources (7) having outputs too small to operate all of the thermal elements simultaneously. Prior to the printing operation, the maximum number of thermal elements that can be actuated by the output of the power source (7) is determined, and then divided into the total number of thermal elements. Next, the resulting quotient is rounded up into the nearest integer in order to ascertain the number of enable groups. The number of enable groups is then divided into the total number of thermal elements to determine the size of each enable group. During the printing operation, streams of non-actuating data are multiplexed into the stream of image data to create virtual enable groups wherein the number of thermal elements actuatable by the image data is no greater than the number of thermal elements in any of the enable groups at any time during the printing operation, thereby preventing the power source (7) from being overtaxed.