if its a slot loading iMac G3 it will emit less noise with quiet harddrive fitted , in one of mine a i have a ssd inside and it is absolute silent .
A tray loader of course because those have a fan inside will be about as loud as a current iMac 21" or 27".
For draining electricity ...around the figures of a normal lightbulb 60-100watt and the iMac G3 is somewhere in between around 85-90 watt average a bit more if you rip a dvd and a bit less if its just looking up some website with flash content disabled and near no graphical content (e.g.pictures), for example looking up the MacRumors forum
I think mine just has a really f'd up hard drive, since its a slotloader