you are right - those things do play into the amount of energy consumed and the money used to pay for that energy - but it is a start and gives you an idea...
i know of tenants who were all up in arms because their heating bill this winter was over $350!!! But they like to run around in shorts & t-shirts & the babies are running around in diapers only...And the thought of lowering the thermostat to 68 or 70 was met w/ a "NO WAY!!!"...Well - then, pay the $350 gas bill..
but if i am a family of one and i am buying my home from a family of 4, then when i get that level bill info, my guess is that i am going to have smaller bills...
I have to agree about the size of the family and what they like (temperature etc) we cut out electric bill by 40% by unplugging anything that wasn't in use. We started doing this the end of last summer and were shocked when we compared out bills to the year before, we also switched all our light bulbs to the new energy efficient ones. Leaving things plugged in even when they are "turned off" still draws out electricity.
On another note my neighbor beside me, same size house, they have 2 kids their electric bill is more then three times compared to ours (there are only two of us in our house no kids). Our water bill obviously is lower then theirs because there are only two of us using it compared to their 4 etc. The only thing I think can be somewhat comparable is the gas bill but then again they may cook more or less then we do or use their gas dryer more often then we do, we only use the dryer once a week and usually it's two loads a week.