Abstract The parameters characterizing biomass flow through the harvester units in laboratory conditions were determined for shoots of big bluestem, giant miscanthus, Spartina pectinata, giant knotweed, Virginia mallow, and Jerusalem artichoke harvested in two growth phases. The input energy for cutting and harvesting of plants was also compared with the outlet energy contained in methane from biomass harvested in the second growth phase. The power needed to cut the biomass was inversely proportional to the power needed to its deformation through the screw scrolls of the harvester and compaction by the feeding rolls. Based on a mathematical model, it was found that biomass with an optimum moisture of 65–70% wet basis (w.b.) required the smallest dry matter (DM) specific work for cutting. The amounts of energy used for cutting and harvesting were 1.22% and 4.87% of the energy contained in the methane produced from the big bluestem, and up to 2.05% and 8.22% for the miscanthus. The input energy values had a greater impact on these indicators than the energy contained in the methane, and this was probably related to the greater reaction of the cutting resistance to moisture.