A method for estimating a parameter relating to solids recovered from a wellbore includes drilling a wellbore using a bottomhole assembly; conveying solids in the wellbore to the surface using drilling fluid; separating the drilling fluid from the solids; and continuously dropping the solids into a solids evaluator positioned above a dryer. The solids evaluator includes a chute having a vertically aligned bore in which the dropped solids reach a constant velocity, a sensor assembly generating a microwave field, and a control unit. The control unit estimates a mass flow rate of the solids based on the generated signals. The method may further include generating the signals representative of a mass flow rate of the dropped solids using the sensor assembly and using the control unit to estimate the mass flow rate of the solids based on the generated signals.