Hard to tell exactly. SMSL claims 150W x 150W (2Ω), 90W x 90W (4Ω), and 50W x 50W (8Ω). Converting those figures using RapidTables we end up with:
6W, 7.2W, and 8W. I can't go past 40/70 on the volume knob before it gets way too loud so I'm not sure when audible distortion would kick in.