Heat loss measurements have been performed on a V-trough collector model with concentration ratio 1.56 and with flat absorbers consisting of five parallel reflector troughs aligned east-west. The collector was tilted 45 degrees. Depending on the similarity in geometry between V-troughs and compound parabolic concentrators, the results should in general be valid also for low-concentrating CPCs. The absorbers were heated electrically and the heat losses were calculated from the input power to the absorber surface. Several geometrical and material properties that affect the heat losses from the collector were investigated. It is concluded that the use of transparent insulation, such as Teflon(R) films, in low-concentrating solar collectors can reduce the heat losses substantially. The reflector emittance in the infrared has an impact on the heat losses, Use of highly emitting reflectors instead of low-emitting reflectors increases the overall heat losses by about 5-8%. The conversion of experimentally measured heat losses into heat losses for real collectors and practical material considerations is discussed.