Charge and heat transport through three-terminal nano-structures is a "hot" topic in view of numerous applications. In an Augsburg-Lublin cooperation [B. Szukiewicz, U. Eckern, and K. I. Wysokiński, New J. Phys. 18, 023050 (2015)] we have investigated, in particular, non-equilibrium properties of such devices with the help of Green function techniques. The maximum output power as function of the control parameters is determined.