An accurate estimate for energy requirement of water production is of great importance in the cogeneration with low-temperature multi-effect distillation plants since the cost of energy requirement accounts for the foremost part of the water production cost. This paper presents an economic analysis of the energy cost of water production by the equivalent enthalpy drop theory that is proven to be an effect method for partial quantitative analysis. The theory can be used to accurately calculate both the energy requirement of water production and the influence of extraction on the turbine's efficiency. The electricity loss for water production (ELWP) and the specific fuel cost for water production are analysed as the function of the pressure of the extraction and the heating steam temperature. The results indicate that ELWP and the specific fuel cost are more accurate to evaluate the performance of the desalination process than gained output ratio (GOR) that is widely used to evaluate the performance of MED. It is also shown that the energy requirement of water production is favoured at lower heating steam temperature and extraction pressure. It is concluded that the cogeneration with LT-MED plant not only adequately utilizes the capacity of power plant, but also economically and efficiently resolves the problem of supply water for coastal power plants.