An energy expenditure estimating apparatus is disclosed with an accelerometer input configured to receive accelerometer data from an accelerometer and an ECG input configured to receive ECG data from an ECG sensor. A parameter estimation module is configured to calculate a time-domain value for heart rate variability and/or other parameters including ECG-derived respiration rate, heart rate above sleep, frequency-domain heart rate variability and accelerometer activity count, using the ECG and accelerometer data. An energy expenditure estimating apparatus estimates energy expenditure using the parameters received from the parameter estimation module.
