So are you talking about a PC of comparable specifications or one that is more powerful? You've said the former in your second to last post, and the latter in your last post.
The iMac will likely offer lower power consumption than the average tower desktop machine offering similar performance due to very efficient power supplies (that the majority of systems likely won't have as the increased efficiency of the superior power supply is outweighed by the lower cost of a "good" power supply), a few mobile components (namely graphics cards), and if you really want to argue, better cooling (quite a bit of that premium goes into design) with fewer, smaller fans that don't need to really spin up all that often (which will make a minimal difference).
However, quite literally all of this is nitpicking now. You pay less for the tower machine but you have higher power consumption, you pay more for this, more for that, etc...at this rate you might as well start talking about the overall carbon emissions for every component!
To be honest it only really needs to go as far as the initial purchase cost, selling cost, the cost of software updates, a little bit of electricity if the difference is massive. Not much else.
