Interpolation ?

[Barkster]

Hello.

I am trying to establish my electricity consumption for any period throughout the year, obviously summer months or quarters will be less than winter months or quarters.

I have meter readings from odd points in the year as follows:

28/05/2010 9069
12/06/2010 9339
17/06/2010 9455
23/08/2010 10891
04/11/2010 12735
03/02/2011 15547
10/05/2011 18331
20/07/2011 20056

In order to compare our various electricity companies' disparate offerings, I need to be able to pick any 3 month period and have a stab at what my consumption will be for that period.
I figured I if I could come up with the approximate meter readings for every day of the year, then I would be home and dry - I just cannot figure out how to interpolate all the days between the above-mentioned known points.
I would greatly appreciate someone's help on this.
Regards
Stephen Barker

 

[jasonb75]

To get a daily average for any peroid

=(end reading - start reading)/(end date - start date)

does that help?

 

[Taul]

If your dates are in A1:A8 and your meter readings are in B1:B8
Then in cell C1 you can type the date of your choice, and in cell D1 use this formula:-

=INDEX($A1:B8,MATCH($C1,$A1:$A8,1),2)+($C1-INDEX($A1:B8,MATCH($C1,$A1:$A8,1),1))*(INDEX($A1:B8, MATCH($C1,$A1:$A8,1)+1,2)-INDEX($A1:B8, MATCH($C1,$A1:$A8,1),2))/(INDEX($A1:B8, MATCH($C1,$A1:$A8,1)+1,1)-INDEX($A1:B8,MATCH($C1,$A1:$A8,1),1))

There are some limitations with this formula but it will give you the basic interpolation you require. Hopefully, someone else will post a better solution for you.

I have a feeling you will probably want to extrapolate outside the date range, in that case you will need something a little more elaborate.

cheers
Paul