I play a fair amount of chess online, on a chess site that updates my chess rating based on my performance using, I believe, the ELO system.

I do not play in rated, "real" tournaments that often, and so I do not have an official ELO rating.

Is my online chess rating a good estimate of my "real" rating? I've heard that one's online rating is systematically biased upwards compared with one's "real" rating, is there any truth to this?