A trained cyclist is roughly 25% efficient, and 1 kCal = 4.184 kJ. The power meter precisely measures kJ. The end result is a near 1:1 ratio between kCal burned

A trained cyclist is roughly 25% efficient, and 1 kCal = 4.184 kJ. The power meter precisely measures kJ. The end result is a near 1:1 ratio between kCal burned and kJ measured. Sure, the power meter under reports total expenditure. For example, it doesn't exactly account for BMR, increased expenditure due to elevated heart and respiration rate, utilization of muscles in your arms and torso, etc. However, it is extremely accurate compared to the alternatives.