Fossil fuels are associated with problems such as resource depletion and pollution, necessitating the exploration of alternatives. Giant miscanthus (Miscanthus × giganteus Greef et Deu), a perennial that can be harvested yearly, requires a low production energy input. It has less ash content and high heat efficiency and has attracted attention as an energy source. An on-site processing equipment, powered via a tractor and equipped with a chipper and a two-stage compression roller, was developed that can harvest 1000 kg of giant miscanthus per hour and simultaneously produce compressed pellets eliminating unnecessary processes such as transportation and processing. With its use, 33–74.5 kWh/t of electrical energy can be saved by producing pellets. The changes in moisture content between the produced compressed pellets and two samples of the ground product were measured immediately before compression for 24 h at relative humidity ranging from 65% to 80%. The moisture content was 6% initially; it ranged from 6.71% to 7.81% in compressed pellets, depending on the conditions, and from 7.44% to 9.82% in the ground sample immediately before compression, indicating the effect of the physical form of the biomass and humidity in the environment. The possible storage period (while maintaining the moisture content at 8–10% for optimal biofuel efficiency based on the measured data) was predicted. The optimal relative humidity of the storage environment for maintaining biomass quality for more than 6 months was predicted to be ≤77% and ≤70% for the compressed pellet and ground sample, respectively. Moreover, at a relative humidity ≥77%, giant miscanthus biomass, immediately before compression, had >10% moisture content in 2 days, warranting caution in storage.