At my utility company our current method for estimating utility meters is based on previous history.  For example, if your meter was estimated today the system would retrieve previous 6 months of user consumption to calculate current estimation (average usage between the 6 months).  Consumption would be retrieved from Aug 2010, Aug 2009, Oct 2009, Oct 2008,Sep 2009, and Sep 2008.

Example:
Aug 2010 KWH 1800
Aug 2009 KWH 1450
Oct 2009 KWH 520
Oct 2008 KWH 610
Sep 2009 KWH 990
Sep 2008 KWH 850

6,220 / 6 = 1037 KWH (Your Sep estimated consumption)

My Question:  How could I apply HDD/CDD to the estimation process to make estimations closer to accurate?  I have been scrambling different percentages pertaining to HDD/CDD to add/subract KWH per month estimated, but still am not comfortable.

Thanks