Weight　loss　has　been　suggested　as　an　important　indicator　of　torrefaction　severity　for　describing　the　performance　of　this　thermal　pretreatment　process.　The　present　study　establishes　a　correlation　of　torrefaction　performance,　severity,　and　energy　usage　for　the　applications　of　this　pretreatment　for　solid　fuel　(biochar)　production.　Three　different　biomass　wastes,　namely,　spent　coffee　grounds,　Chinese　medicine　residue,　and　microalga　residue,　have　been　torrefied　in　nitrogen　at　a　temperature　range　of　200–300　°C　for　15–60　min　to　produce　biochars.　Their　properties　such　as　the　enhancement　factor　of　higher　heating　value,　energy　yield,　decarbonization,　dehydrogenation,　deoxygenation,　and　atomic　O/C　and　H/C　ratios　have　been　investigated.　The　obtained　results　indicate　that　the　introduced　dimensionless　parameter　(the　torrefaction　severity　index　in　terms　of　the　weight　loss　of　biomass)　correlates　sufficiently　with　the　biochar　properties;　the　exception　is　the　O/C　ratio　as　a　consequence　of　the　intrinsic　difference　in　components　between　the　utilized　wastes.　A　new　parameter　(the　upgrading　energy　index)　was　also　defined　to　account　for　the　energy　efficiency　of　the　torrefaction　system.　This　index　follows　a　decreasing　trend　with　increasing　torrefaction　severity　index.　This　shows　that　biochar　quality　or　heating　value　is　intensified　at　a　higher　torrefaction　severity;　however,　the　energy　efficiency　is　lowered.　©　2018　Elsevier　Ltd