The world's future energy supply will include intermittent renewable sources, such as solar and wind power. To guarantee reliability of supply, fast-reacting, dispatchable and renewable back-up power plants are required. One promising alternative is parked and grid-connected hydrogen-powered fuel cell electric vehicles (FCEVs) in "Vehicle-to-Grid" systems. We modified a commercial FCEV and installed an external 9.5 kW three-phase alternating current (AC) grid connection. Our experimental verification of this set-up shows that FCEVs can be used for mobility as well as generating power when parked. Our experimental results demonstrate that present-day grid-connected FCEVs can respond to high load gradients in the range of -760 % s-1 to + 730 % s-1, due to the parallel connection of the high voltage battery and the fuel cell stack. Virtual power plants composed of multiple grid-connected FCEVs could perform higher power gradients than existing fast-reacting thermal power plants with typical power gradients of 1.67 % s-1. Hydrogen consumption in 9.5 kW AC grid-connected mode was 0.55 kg h-1, resulting in a Tank-To-Grid-AC efficiency of 43% on a higher heating value basis (51 % on a lower heating value basis). Direct current to alternating current efficiency was 95 %.@en