While many flagship thermoelectrics perform best at high tem- perature, the majority of applications require good low-T performance. We predict π SnSe to have ZT = 1.1 and 1.3 at 300 K with n = 4 × 10 19 and 10 20 cm -3 , respectively, which are ∼2-3 × larger than p-type Pnma SnSe (Fig. ). The "device efficiency" η of a TE material is given by: 2