In the study of non equilibrium stationary states of dissipative systems (for instance, turbulence), the fluctuating injected power is a natural observable to look at, insofar as (i) it is usually experimentally easily accessible (ii) the average of this quantity is proportional to the mean entropy production. However, for the theoretician, this quantity is quite difficult to analyse, because of the lack of precise knowledge about stationary states in dissipative systems. As a result, dissipative models where the stationary state can be exactly computed can be a valuable tool to fill the gap between theory and experiments, as examples where the both the inner structuration of the stationary state and the statistical properties of the energy injection are known. Using fermionic techniques, I will show that the large deviation function of the time-integrated injected power of some 1D dissipative systems of classical spins can be computed exactly. The results allow to analyse the influence of the injection mechanism on the statistical properties of the stationary state, and it will be shown that some interesting non trivial temporal features of the injection are closely related to the spatial stationary profile of the energy field.