my experience is that the calculations of my Sigma HRM are dramatically higher than from my Powertap. Typical flat century ride might be something like 3000KJ(essentially equivalent to dietary calories, due the inefficiency in converting dietary calories to energy) , while the HRM will tell me I burnt 5000-6000 calories.

While you can argue about the percentage of dietary calories converted to KJ, the powertap is at least measuring actual energy produced. I tend to trust the Powertap numbers a lot more than calcualtions from a HRM.