Efficiency of energy conversion processes can be improved if waste heat is converted to electricity. A thermoelectric generator (TEG) can directly convert waste heat to electricity. The TEG typically suffers from low efficiency due to various reasons, such as ohmic heating, surface-to-surrounding convection losses, and unfavorable material properties. In this work, the effect of surface-to-surrounding convection heat transfer losses on the performance of TEG is studied analytically and numerically. A one-dimensional (1-D) analytical model is developed that includes surface convection, conduction, ohmic heating, and Peltier, Seebeck, and Thomson effects with top and bottom surfaces of TEG exposed to convective boundary conditions. Using the analytical solutions, different performance parameters (e.g., heat input, power output, and efficiency) are calculated and expressed graphically as functions of thermal source and sink temperatures and convection heat transfer coefficient. Finally, a two-dimensional (2-D) mathematical model is solved numerically to observe qualitative results of thermal and electric fields inside the TEG. For all calculations, temperature-dependent thermal/electric properties are considered. Increase in thermal source temperature results in an increase in the power output with adiabatic side wall conditions. A change in boundary condition to convection heat transfer from adiabatic boundary has a large impact on thermal efficiency.