get a killawatt, here in the USA they cost $15-20 USD. you plug it into the wall, and then plug your power supply cord into it and it tells you what you're pulling. you can easily find out this way and it would be a lot cheaper than hiring a specialist. the only problem i see, is if you guys in australia use a different type of outlet then the one the killawatt is using.
my opinion is that it is not your computer. there is simply some variable that your family is not considering. ex/ your sister used X appliance a lot more (and knows it) and isn't saying anything. or your sister used X appliance more periodically in intervals, so that it's not in her conscience that she used it that much, when in reality she did. or, you used your computer a lot more without knowing it. my point is, there is some other variable that you guys are not factoring in--it is not your computer if it is run just 5 hours a day.
Edited by smash_mouth01 - 2/8/11 at 2:02pm