Modernization of road lighting is an activity that brings real savings in the costs incurred for the use of road lighting installations. It is particularly visible when luminaires with discharge light sources are replaced with LED luminaires. There are operational and investment factors that significantly influence the payback period of the modernization investment. One of the operational factors is the operating time of the luminaires during the day. Another investment factor is sometimes the requirement to separate the power supply circuit from the existing electrical network to power the luminaires. The article presents the scope of the impact of these factors on the payback period of modernization investments based on examples of real modernization investments.