Boltzmann transport theory with constant relaxation time approximation is applied to calculate electron transport coefficients in disordered multi-atom alloys. It allows to determine physical quantities, such as electrical conductivity and Seebeck coefficient, when different models of the relaxation time are employed. In order to study the electronic structure and Fermi surface properties in a system containing complex chemical disorder, the Korringa–Kohn–Rostoker method with the coherent potential approximation is used. This methodology is applied to TiFe1−xNixSb half-Heusler alloys, which exhibit metal-semiconductor-metal crossover with a change in type of carriers (from holes to electrons), upon increasing x. Electron transport quantities as well as the electron lifetime related to impurity scattering are calculated. Thermopower is presented on two dimensional maps as a function of temperature and carrier concentration. A beneficial role of Co in enhancement of thermopower of both p- and n-type Ti(Fe,Co,Ni)Sb alloys is predicted, which is mainly due to the valence or conduction bands convergence.