The exerspy said I only burned about 100 calories during 1.5 hours of yoga. Is that right?

Answer: Perceived exertion (subjective assessment) reflects the interaction of the body and mind. What you consider in your head (psychologically) to be very challenging may not be physiologically difficult to your body. Do not be concerned with any one single factor such as sweat rates, shortness of breath or work intensity. Instead, try to concentrate on the total inner feeling of exertion. Most yoga classes offered in health clubs across the United States incorporate the popular Hatha yoga. This involves different combinations of poses in the sitting, supine (on your back), standing and prone (on your stomach) positions. Poses can be as easy as lying on the floor to more complex, difficult poses that require years to master. Very few studies have looked at the energy cost (or calories burned) of yoga. One study found that Indian men burned approximately 3.79kcals/min during 44 minutes of different styles of yoga. Another more recent study (2005) found that Hatha yoga required 53% lower calories than walking on a treadmill at 3.5 miles/hour.