In this paper author demonstrates the relationship between spectral efficiency and energy efficiency for 10 Gbps, 40 Gbps and 100 Gbps transmission systems with NRZ-OOK, NRZ-DPSK and DP-QPSK modulation formats respectively. The focus of this research is the wavelength division multiplexed (WDM) transmission systems with Forward Error Correction (FEC) technology with required Bit Error Rate (BER) level that does not exceed 10-3. For this purpose the signal BER level versus cross-channel interval response is evaluated. Obtained data was used for the regeneration length calculations with purpose to define the number of regeneration steps for 200 km and 2600 km transmission systems. In order to observe relationship between spectral and energy efficiency, power consumption was calculated using different cross-channel intervals – 37.5 GHz, 50 GHz and 100 GHz, by means of what different spectral efficiencies were reached. The total power consumption of transmission system was calculated by defining the quantity and power consumption values for all WDM system elements that require electrical energy. In this paper author demonstrates how the amount of transmitted information influence the total system power consumption in each considered transmission system and by using obtained data and spectral efficiency data, the systems energy efficiencies were calculated.