Thanks for the replies. I'm not looking to calculate an exact value, I'm just wondering what parameters to look for when choosing a mosfet. Assuming Vgs=Vds=12V (as it is in my application) and instantaneous switching, will this calculation at least put me in the ball park? P=1/2*(Input+output capacitance)*12V^2*f. As this is a work related project, there is a limit to how much time I can spend on this right now, so I can't get into the depths of all the variables that goes into this.