An apparatus for energy expenditure estimation comprises a heart rate sensor (101) for producing a heart rate value indicative of a heart rate of an individual, a heat-flux sensor (103) for producing a heat-flux value indicative of a heat-flux flowing through a measurement area on the skin of the individual, and a processing system (102) communicatively connected to the heart rate sensor and the heat-flux sensor.The processing system is configured to produce an estimate of the energy expenditure based on the heart rate value and the heat-flux value. The use of the heat-flux value improves the accuracy of the estimation especially during low-intensity exercise and rest, when both heart rate and acceleration value soften fail to provide information meaningful enough for energy expenditure estimation.