I recently suffered a broken clavicle and have been relegated to riding on a trainer. To keep myself motivated, I've been tinkering with trying to figure out calories per workout. I've used the Kurt Kinetics power functions (www.kurtkinetic.com/calibration_chart.php) for my trainer and compared to the estimates given by my HR monitor. The results are suprisingly similar.

A sample ride on the trainer at a moderate pace (HR ~150) my results are as follows:

Time: 45 minutes
Distance: 9.3 miles
Ave: 12.4 mph
Cadence ave: 77 rpm
HR: approx. 150 (not a continuous meter, I take a reading every few minutes)

According to the KK formula for my trainer (Blackburn fluid), 12.4 mph corresponds to a 130w output, so 45 minutes equates to 353 kcals. The HR monitor calculated 360 kcals.

On the other hand, SportTraks and Garmin Connect both grossly overestimate calories. Both simply use time and distance to estimate calories and are about 30% optimistic.