= Thermoelectric power factor
{title2=$S^2\sigma$}
The <thermoelectric power factor> is the product $S^2\sigma$ of the squared <Seebeck coefficient> and <electrical conductivity>, with units $\mathrm{W\,m^{-1}\,K^{-2}}$. It combines a material's voltage response to a <temperature gradient> with its ability to carry <electric current>. For example, $S=200\,\mathrm{\mu V\,K^{-1}}$ and $\sigma=10^5\,\mathrm{S\,m^{-1}}$ give $S^2\sigma=0.004\,\mathrm{W\,m^{-1}\,K^{-2}}$.
At fixed geometry and imposed <temperature> difference, a constant-property <thermoelectric generator> behaves as a voltage source with internal <electrical resistance>. Matching the external <electrical resistance> to it maximizes output <power>; the material dependence of that maximum is proportional to the <thermoelectric power factor>. Maximizing <thermal efficiency> also requires controlling <thermal conductivity>, which is why the <thermoelectric figure of merit> includes the thermal denominator.<Complex thermoelectric materials by G. Jeffrey Snyder and Eric S. Toberer>[pp. 105–107, power factor and zT trade-offs]{ref}
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