How to calculate power consumption every hour /day /month?

I have accumulated energy reading..usually i get the reading every hour then subtract the reading of the required hour and the reading of the previous hour..eventually if the data has no change i get 0 means no consumption..how i can apply this way in PI server or if there better way Plz advise but i should get a reading every hour even if no change 0 should be there

I have used range function to get the result in another created tag but its not accurate and  if no change in data i'm not getting a result