We present a novel concept of a dual-purpose power converter for dc or ac grids. These universal dc-dc/ac power electronic interfaces are suitable for application in the dc or single-phase or three-phase ac grids using the same terminals. The idea is to utilize the same semiconductors and passive components in the dc-ac and in the dc-dc configurations with minimal additional components and to maximize their effectiveness in any mode. Possible realizations along with challenges of simultaneous application of dc and ac grids are discussed. The "Conclusion " section addresses different fields of application and the main benefits, challenges, and future research.