  The average thermodynamic power of a time-dependent external potential in the
white-noise Langevin model is derived using a Green's function solution. The
power appears as a driving term in the differential equation for the average
energy and determines whether the solution is stationary or non-stationary.
Different dynamics are illustrated with explicit models: a linear potential
with a static magnetic field, a linear potential perturbed with an oscillating
component and a magnetic field switch modeled using a $\tanh$ protocol.
