Flexible power-to-heat systems that incorporate thermal energy storage are key technologies to decarbonize industry. This study examines the thermal and dynamic performance of a power-to-heat MW-scale pilot plant that integrates molten salt electric heating and thermal energy storage to generate steam on demand. Operational data collected under nearly design and off-design conditions are discussed. The dynamic performance of the electric heater unit is also assessed and presented in detail, including time constants and ramping up efficiencies. The results confirm the ability of the system to deliver reliable heat on demand whilst simultaneously being able to provide grid-flexibility. Round-trip thermal efficiencies above 94 % were obtained when operating near design conditions. Thermal losses of approximately 0.5 and 2.3 °C/h were recorded for the cold and hot tanks respectively during standstill periods. Thermal losses are negatively affected by the small scale of the investigated facility and should be expected to decrease inversely proportional to the anticipated increase in size of tanks in full scale commercial installations. The ability of the electric heater to ramp up its electric power consumption from 0 to 5 MWe in 5 s is demonstrated, showing the capacity of the system of participating in frequency regulation markets. The electric heater is characterized by thermal time constants below 500 s and ramping up thermal efficiency of 80 %. This work provides the ground for performance validation of molten salt based flexible power-to-heat systems, reaffirming their potential as industry heat decarbonization assets, and highlighting opportunities for improvement.