  In this Letter, we show thermoelectric conversion efficiency is
\emph{exactly} determined by \emph{three} independent material parameters
$Z_{\rm gen}$, $\tau$, and $\beta$. Each parameter is a figure of merit hence
improving $\tau$ or $\beta$ is an additional way to increase the efficiency.
The $Z_{\rm gen}$ generalizes the traditional figure of merit \emph{zT}. Two
degrees of freedom $\tau $ and $\beta$ reflect the temperature gradients of the
material properties and are crucial to evaluate the heat current altered by
non-zero Thomson heat and asymmetric Joule heat escape. Physical insights on
high $\tau$ or $\beta$ materials explain why the single parameter approaches
can be inaccurate for efficiency prediction.
