Get a good program for multiple linear regression analysis. Excell has one that will give you coeficients of the regression terms but not any statistical data to analyize it. The last time ( to long ago to be to much help to you )I did it I used a program called SAS or SASS.
Once you get it you can try correlating power usage with all kinds of things - Like; population, school enrollment, car registration, degree days in a year, square footage of retail space in the area, employment, unemployment, average income, the dow jones average, runoff from the local river etc. Agood program will let you put the data in very quickly and test it for degree of correlation.
It's interesting to see what works. You may find things that correlate with power growth but don't have any causual link. Get a good statistics book or better take a good stat course.