With the Polar you can do a fit test and it essentially makes an estimate of your VO2 max. I think the higher your VO2 max, the more calories you burn.

I use the calories burned number as way to show how much effort I put out. I figure as long as I'm getting the data from the same place, it will be consistent in its flaws. It's interesting to do a Spinervals in the morning and then do the same one in the afternoon. My calories burned is usually higher in the afternoon. Not by a huge amount, but there is a difference.

V.