For Pressure washing sizing the Brushes 2 tips (#3.5) are the same as a #7 tip in your Pressure washer Pressure Washer Ratings could be: 1500pst at 4.0 to 5.0 gpm 2000psi at 3.5 to 4.0 gpm 2500psi at 3.0 to 4.0 gpm 3500psi at 2.5 to 3.25gpm 4000psi at 2.5 to 3.0 gpm The user manual is availale at this link: Counter Rotating Brush User Manual Counter Rotating Brush for Solar Panels