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 %.