Abstract
Reliable efficiency evaluations are recognized as one of the major challenges in thermoelectric technology. Existing heat-flow and guarded-heater methods suffer from uncertainties due to heat losses from the lateral surfaces of the thermoelectric module. Here, we introduce an apparatus that encloses the module with a thermally matched guard ring, separated by a vacuum gap, establishing near-isothermal sidewalls that suppress radiative losses from the lateral surfaces. Measurements with the hot side up to 703 K and an input heat flow up to 124 W showed input-output agreement within 0.5 W. Conversely, the unguarded configurations exhibited discrepancies exceeding 5 W. Thermal simulations qualitatively supported these trends, and a comparison with a reference instrument confirmed the electrical consistency while revealing the differences in thermal boundary conditions. This measurement configuration connects the two approaches, enabling reproducible efficiency evaluations with an expanded uncertainty of 1.4% and providing a technical basis that may inform future international standardization.