I was going with just a smart ass post (hey we can say ass now), but if you insist. What I have observed with my power meter is on a flat to slightly rolling terrain with no significant wind it's about 600 Calories for 19-20mph average. As merlin said, to know for sure you need a power meter. Although even then it's a guestimation, although a more accurate one then with HRM. In the end ride a lot track calories if scale keeps going up cut back on calories.

UD