In this paper a detailed measurements-based analysis of the energy consumption of commercial broadband PLC modems is reported. Energy consumption measurements are carried out on the basis of pairs of many commercial PLC modems. Ethernet frames with variable sizes and bit/frame rates are sent on a modem, and received on the other modem. An analytic linear model that quantifies the energy consumption associated to Ethernet frames and PLC Physical Blocks (PBs) processing is proposed for a pair of PLC modems.