The above discussion brings up a point germane to the OP. If you use a Garmin with a wheel sensor and upload to Strava, even if you don't use a PM it is possible to get a reasonable estimate of calories burned. Strava provides an average watts number for your rides and total kJ. It uses the equations of motion to calculate these numbers: nothing to do with your HRM. I think it's usually close for loop rides, as it can't take wind into account, but the more wind, the less accurate.
So weigh your bike in ready-to-ride trim, water bottles half full. Weigh yourself in your average kit. Enter these into Strava. Enter a ride. Note the kJ figure.
As previous posts indicate, kJ is approximately equal to calories burned.
Last edited by Carbonfiberboy; 10-08-14 at 03:56 PM.
Reason: Error: I changed "Garmin" to "Strava"