Piezoelectric actuators are a very attractive technology for active optics mechanisms in space applications due to their very high precision and reliability. However, self-heating of these actuators may be limit their use in space or under high loads, due to the limited ability to evacuate heat. Test procedures that reproduce the operational conditions of these actuators are important to check these operational limits. Here an effort to characterise the thermal emission of pre-stressed piezoelectric actuators in operation is presented. The technique allows direct measurement of the power dissipated by the test item via the control of the different heat transfer mechanisms, using the fall in power provided as measure of power dissipated by the actuators, instead of relying on direct temperature sensors. This allows the construction of a thermal model with a dissipation term readily integrated in system-level modelling to account for the dissipated heat of the piezo. The technique may also be applied to other piezo low power applications in the order of 1 W of thermal emission, and is adaptable to emulate the boundary conditions encountered in operations.