This paper presents an optimization method for the power contract under the High Voltage tariff structure in Morocco. The optimization is based on minimization of the billing cost, formulated as an objective function that takes into account the power charges, the excess of the power contract charges and the bi-seasonality of the tariff structure. This method is applied to the real case study of the Bouregreg pumping station in Morocco, which feeds potable water to Rabat and Casablanca and their respective surroundings. The financial gains are then evaluated for the reference year (2013) and validated for the year 2014. This method can be used as a first step of an energy efficiency project, and will allow substantial savings without important investment.