The most popular Box-Jenkins method, generally used for short-term forecasting, is modified to make it suitable for medium and long-range forecasting. The non-stationarity and seasonality have been identified and, after removing trends and/or seasonality, the series are tested for stationarity by various methods. The series have been fitted for different auto-regressive moving average (ARMA) models in the multiplicative modes. The parameter values have been determined from autocorrelation function (a.c.f.) and partial auto-correlation function (p.a.c.f.) cor-relograms and the whiteness of the residue has been checked. A forecast has been made for energy demand for one year with the help of this model and the result has been compared with actual demand.