Energy Expenditure (EE) (kcal/day) is a key parameter used to guide obesity treatment, and it is often measured from CO2 production, VCO2 (mL/min), and/or O2 consumption, VO2 (mL/min) through the principles of indirect calorimetry. Current EE measurement technologies are limited due to the requirement of wearable facial accessories, which can introduce errors as measurements are not taken under free-living conditions. A novel contactless system, the SmartPad, which measures EE via VCO2 from a room’s ambient CO2 concentration transients was evaluated. First, SmartPad accuracy was validated by comparing the SmartPad’s EE and VCO2 measurements with the measurements of a reference instrument, the MGC Ultima CPXTM, in a cross-sectional study consisting of 20 subjects. A high correlation between the SmartPad’s EE and VCO2 measurements and the MGC Ultima CPX’s EE and VCO2 measurements was found, and the Bland-Altman plots contained a low mean bias for EE and VCO2 measurements. Thus, the SmartPad was validated as being accurate for VCO2 and EE measurements. Next, resting EE (REE) and exercise VCO2 measurements were recorded using the SmartPad and the MGC Ultima CPXTM at different operating CO2 threshold ranges to investigate the influence of measurement duration on system accuracy in an effort to optimize the SmartPad system. The SmartPad displayed 90% accuracy (±1 SD) for 14–19 min of REE measurement and for 4.8–7.0 min of exercise, using a known room’s air exchange rate. Additionally, the SmartPad was validated by accurately measuring subjects’ REE across a wide range of body mass indexes (BMI = 18.8 to 31.4 kg/m^2) with REEs ranging from ~1200 to ~3000 kcal/day. Lastly, the SmartPad has been used to assess the physical fitness of subjects via the “Contactless Thermodynamic Efficiency Test” (CTET).