We studied the heating efficiency of an electrically heated vest (EHV), its relationship to the microclimate temperature distribution in a three-layer clothing ensemble, and the effect of an EHV on the clothing's total thermal insulation by both theoretical analysis and thermal manikin measurements. The heat losses at different ambient conditions and heating states were recorded and the heating efficiency of the EHV was calculated. It was found that the EHV can alter the microclimatic temperature distribution of the three-layer clothing ensemble. The EHV can provide an air temperature of 34 degrees C around the manikin's torso skin. The highest temperature on the outside surface of the EHV was around 38 degrees C, which indicates that it is safe for the consumer. The higher the heating temperature, the lower the heating efficiency obtained. This was due to much more heat being lost to the environment, and hence, the heat gain from the EHV was smaller. The heating efficiency decreased from 55.3% at 0 degrees C to 27.4% at -10 degrees C when the heating power was set at 13 W. We suggest adjusting the heating power to 5 W (step 1) at an ambient temperature of 0 degrees C, while at -10 degrees C using 13 W (step 3) to provide the consumer a thermal comfort condition.