  A macroscopic description of thermoelectric phenomena involves several
tensorial transport coefficients. Textbook microscopic Kubo formulas for them
are plagued with ambiguities in the definitions of the current operators and
the magnetization. We derive a version of these formulas for lattice systems
which is free from ambiguities but contains additional terms compared to the
textbook results. For symmetric components of thermoelectric tensors, we
identify a large class of lattice systems for which the additional terms vanish
with a natural choice of the energy current. To eliminate ambiguities in the
skew-symmetric components, one needs to interpret them as relative quantities:
only their differences for pairs of materials are well-defined.
