The matter of power consumption in the wired telecommunication networks has become a major challenge for researchers and designers due to the potential financial benefits and environmental issue. On the other hand the Information and Communication Technology (ICT) segment experiencing continuously increasing global Internet traffic due to the emergence and renewal of the bandwidth-intensive services and applications. The higher capacity of transmission system can be achieved by enchasing the spectral efficiency level, which in its turn, in the Wavelength Division Multiplexing (WDM) systems, is defined by frequency slots and data rates of transmission channels. However, on the grounds of cross-channel interference, decreasing of channel spacing adversely affects the transmission reach of signals, which leads to the higher power consumption due to increased number of signal regenerations. Therefore the following situation results into continuous assessment of tradeoffs between Power and Spectral efficiencies in the transmission systems. Following research aims to an examination of Single–Line–Rate (SLR) WDM systems based on 10 Gbps signals with nonreturn-to-zero on–off-keying (NRZ – OOK) modulation, which are commonly used at existing legacy networks. The focus of this study is transmission systems with the Forward Error Correction (FEC) and required Bit Error Rate (BER) level lower then 10-3. In order to observe relationship between system’s capacity and power consumption, authors estimate the power efficiency at different spectral efficiency stages, by means of modifications in the frequency slot, to wit from 12.5 GHz to 37.5 GHz. Calculations have been prepared for various signal transmission lengths. Additionally, the power ratio of 3R (reamplification, retiming, reshaping) is evaluated with the purpose to quantify the impact of signal regeneration procedure on the total power consumption of transmission system. The results are presented as functions of transmitted information amount, with the aim of providing flexible data for different system realizations.