The ability to measure free-living and activity-specific energy expenditure (EE) is useful for a variety of purposes. Heart rate variability (HRV) monitoring is emerging as a means for estimating EE and other physiological measures. The purpose of this study was to assess the accuracy of HRV-derived EE across a range of physical intensities and during free-living. Participants (n=30) completed two treadmill tests (walk and VO_2max) measuring EE via Indirect Calorimetry (IC) and with the FirstBeat Bodyguard HRV monitor. Participants also wore the HRV monitors continuously for four consecutive days under free-living conditions. During the walk test, HRV-EE estimates across analysis conditions correlated moderately with IC estimates of EE (r=0.60-0.75;​ p<0.05). During VO2max testing, HRV-EE estimates across analysis conditions correlated strongly with IC estimates of EE with (r=0.85-0.98;​ p<0.05). During free-living conditions, daily average and 4-day total HRV-EE estimates across all analysis conditions correlated strongly (r=0.75-0.98;​ p<0.05). HRV-EE estimation improves as activity-intensity increases. HRV-EE estimates improve further with the addition of IC-measured HR_max and VO_2max, particularly at low intensities;​ however, meaningful differences were not seen between values when considering group means. HRV-EE estimates are sufficiently accurate to indicate this method possesses practical utility and may be used for individual EE monitoring.