Many control and monitoring applications including thermostats, doorbells, and security systems utilize 60 Hz transformers with grid-voltage inputs and 16 V to 24 V ac outputs. The new DOE Level VI efficiency standard places stringent requirements on the no-load quiescent loss of grid-interface power supplies, including these low-voltage transformers. This paper explores a low-cost but highly effective approach to reduce the quiescent loss and improve the power density of these transformers while meeting the Level VI efficiency standard. By reconfiguring windings based on the operating power level, one can achieve higher average efficiency, lower quiescent loss, and higher power density. It is experimentally demonstrated that compared to a conventional transformer design that meets the new Level VI standard, the proposed switched-winding transformer approach can more than double the achievable transformer power rating at the same size, weight and core, while requiring only a modest increase in complexity.