My impression is that could be a pretty thorny calculation. Having to do with efficiency, it seems you would get different power draw/power dissipation at different operating temperatures. Plus the issues of how you calibrate power draw relative to duty cycle (CPU utilization). In my experience whenever you look at power dissipations you need to favor empirically measured dissipations of systems under "worst case" and "typical" operations.
I don't want to pretend I'm an expert - but trying to analytically predict power consumption with a reasonable degree of accuracy seems like a pretty daunting problem.
Then again, maybe someone has a tool based on interpolating/extrapolating empirically measured data?