Peak-load pricing of natural monopolies was analyzed until now as a one-step production process with constant average rate costs. However, electric current underlies a multi-step process with decreasing average costs for power transmission. A private, vertically separated network opertor will charge profit maximizing peak-load prices and will cause a high welfare loss due to its monopolisic position. The paper examines in a two-stage model with decreasing average cost on the transmission stage, which consequences there are of prohibiting the monopolist to take different prices. Here, this interdiction enhances welfare, if the monopolist continues to serve customers in both markets (peak and off-peak). The analyzed regulation rule "only one price" is easy to apply and to supervise, as a consequence it is practicable and cost-effecitve.