Frustrated lattices like the kagome network, where destructive interference creates flat electronic bands, have attracted great interest for realizing exotic quantum phases in solids. Here, we show that interband scattering between topologically flat and dispersive bands gives rise to a large thermoelectric effect. Thermoelectrics are materials that convert heat directly into electricity and are key for energy harvesting and cooling applications. Using first-principles calculations, we show that extreme interband scattering in the kagome system Ni3In1-xSnx may result in exceptional thermoelectric performance on par with Bi2Te3 –the only commercially available thermoelectric material around room temperature. Our discovery highlights a novel route to design thermoelectrics from metals and sheds new light on charge transport in topological materials. PRX 15, 021054 (2025)