The force excerted on an electret in a slot-effect transducer, which has been calculated by several authors using different methods and thereby obtaining somewhat different results, is evaluated by a method not yet employed in this problem, namely by direct application of the energy principle. This method permits to extend the calculation of the force so far performed for neutral surface charge electrets symmetrically located in the capacitors of the slot effect transducer to the case of non-neutral space charge electrets asymmetrically located between the plates of the capacitors. After a short description of the electret slot-effect transducer the method of calculation is explained and the calculation summarized. The result is given in its general form and discussed for some special cases. The calculation shows that in the same way as in an electret capacitor transducer, the force excerted on a space charge electret in a slot-effect transducer can be evaluated by substituting the space charge electret by a surface charge electret, whose surface charge densities equal the equivalent surface charge densities of the space charge electret.