How can we analyse fuel cell stack voltage degradation with transient phenomena and are existing durability indicators as distance driven or operating hours still relevant in commercial FCEVs, used for driving and vehicle-to-grid (V2G) purposes? The mean stack voltage drop is measured over fuel cell operating time and produced electricity in 4 commercial Hyundai ix35 FCEVs. The experiments show that a durability indicator expressed solely in operating hours, distance or produced energy is not relevant for combined driving, V2G and idling load profiles. An indicator consisting of several usage parameters is recommended.