  In this paper we consider thermal power of a heat flow through a qubit
between two baths. The baths are modeled as set of harmonic oscillators
initially at equilibrium, at two temperatures. Heat is defined as the change of
energy of the cold bath, and thermal power is defined as expected heat per unit
time, in the long-time limit. The qubit and the baths interact as in the
spin-boson model, i.e. through qubit operator $\sigma_z$. We compute thermal
power in an approximation analogous to `non-interacting blip' (NIBA) and
express it in the polaron picture as products of correlation functions of the
two baths, and a time derivative of a correlation function of the cold bath. In
the limit of weak interaction we recover known results in terms of a sum of
correlation functions of the two baths, a correlation functions of the cold
bath only, and the energy split.
