This paper aims to determine the optimal design and operating conditions of an air-to-air thermoelectric heat pump (THP) composed of thermoelectric modules (TEMs) and two heat sinks, coupled to an energy-efficient building. We focus on the optimal number of TEMs and the optimal electrical current for three standard cases. The performance is quantified using two different coefficients of performance (COP) definitions: performance of THP only (COPHP) and performance of THP coupled to an energy-efficient building (COPu). These COP are also studied for both instantaneous and seasonal conditions. We show that for each coupling, a range of modules exists, with which the maximum performance of the THP can be approached, by simply adapting the electrical current for each condition. Finally, the seasonal performance COPHPs/COPus for the French city of Macon is estimated and compared for the three cases considered. The original sizing approach proposed here leads to a slight increase of the COPHPs/COPus (up to +4%), compared to the classical design based on instantaneous nominal conditions (norm). Moreover, we show that the optimal number of modules can be largely reduced (up to -39% for the cases considered) compare with nominal condition method. (C) 2013 Elsevier B.V. All rights reserved.