Depends on the model, and the conditions. The only way to know the error in your HRM would be for you to replicate the exact workout under laboratory conditions.

The more factors your software takes into account, the more accurate it's likely to be. Calculations based on speed and distance alone are the least accurate - adding in your weight, equipment weight, elevation change, HR, and power output make it progressively more accurate.

But even so, it's only a software estimation of something that can only be directly measured by analyzing your breath.

As I said in the other thread... it's useless to get so hung up about calories out, because there's no way in the world to measure calories in. (what your body actually processes)