The objective of this study was to evaluate the validity of total energy expenditure (TEE) provided by Actiheart® and Armband®. Normal-weight adult volunteers wore both devices either for 17 hours in a calorimetric chamber (CC, n = 49) or for 10 days in free-living conditions (FLC) outside the laboratory (n = 41). The two devices and indirect calorimetry or doubly labelled water, respectively, were used to estimate TEE in the CC group and FLC group. In the CC, the relative value of TEE error was not significant (p > 0.05) for Actiheart® but significantly different from zero for Armband®, showing TEE underestimation (−4.9%, p < 0.0001). However, the mean absolute values of errors were significantly different between Actiheart® and Armband®: 8.6% and 6.7%, respectively (p = 0.05). Armband® was more accurate for estimating TEE during sleeping, rest, recovery periods and sitting–standing. Actiheart® provided better estimation during step and walking. In FLC, no significant error in relative value was detected. Nevertheless, Armband® produced smaller errors in absolute value than Actiheart® (8.6% vs. 12.8%). The distributions of differences were more scattered around the means, suggesting a higher inter-individual variability in TEE estimated by Actiheart® than by Armband®. Our results show that both monitors are appropriate for estimating TEE. Armband® is more effective than Actiheart® at the individual level for daily light-intensity activities.