Home energy use

I did some calculations on the energy use in my house which I share with two other people. Our energy expenditure is a bit too high to my taste. Partly it has to do with the old house whose insulation may be lacking. The outdoor AC unit is probably operating inefficiently because of old age. The service person said its coils and fins are clogged with dust and dirt. The landlord does not seem too enthusiastic about cleaning or replacing it as it technically works. But in peak summer it kind of works all the time and still struggles to bring the inside temperature around 75F. I did my best to seal any leakage through the door and window edges.

Our utilities charges from 2009 Sep to 2010 Sep are respectively as follows:

$ 153, 153, 339, 333, 288, 381, 171, 150, 330, 375, 366, 450 .
That would make average cost $290. Per head $97 .

You see we are not the greenest people around. I really wish we didn't use this much energy.

I couldn't do much about the AC . The housemate upstairs is very sensitive about heat.
I am trying to employ a space heater for my room in the winter to see if I can save on the gas for heating. I am more sensitive to cold in the winter than heat in the summer.

I find that the highest gas bill in the winter was $239 and the lowest gas bill in the summer was $35. So the difference I assume is roughly from the centralised heating in the winter and that is roughly $200.

The highest electricity bill in the summer cost $339 and the lowest electricity bill in the winter cost $89 . I assume the difference roughly came from the AC in the summer and that is $250.

The electricity costs more than the gas in our house, though the costs are comparable. By using a space heater I am trying to trade electricity for gas, though hopefully more efficiently.

The electricity costs roughly 18 cents/KWh in a month in which the electricity bill is $178 which is kind of our average bill.
That mean if I can get by with the space heater which is rated 1.5KW on for 5hrs / day it would cost $40 . If it leads to only 50% usage of the centralised gas heating its cost will be down by 50% that is $100. The overall saving in that case will be $60. Not bad. Also, it may be warmer in my room this winter.

By the way the heater I purchased is Honeywell Electronic Ceramic Heater from target. There was only a store model left which I purchased for a 15% discount at a price of $23 .
http://www.amazon.com/Honeywell-Electronic-Ceramic-Heater/dp/B000TGX58E
I am happy with this heater. It is not the quietest thing. I wouldn't be making any audio recording with this thing on. It has a little more sound than the a fan. It doesn't bother me much.