A system for storage and dosing of ammonia that comprises a solid ammonia storage material capable of binding and releasing ammonia reversibly by adsorption/absorption, said system being able to release ammonia gradually according to a demand that can vary over time with intermediate periods of no ammonia demand, said system comprising: - a main storage unit and a start-up storage unit, the storage units holding ammonia storage material, - at least one one-way valve via which the main storage unit is in communication with the start-up storage unit, the one-way valve preventing any back-flow of ammonia from the start-up storage unit to the main storage unit; - heating devices (8, 9) arranged to heat the main storage unit and the start-up storage unit separately to generate gaseous ammonia by thermal desorption from the solid storage material; - a controller arranged to control the heating power of the main storage unit and the start-up storage unit, thereby enabling ammonia release from the start-up and/or the main storage unit; - a dosing valve arranged to control ammonia flow from the storage units according to a demand; - a pressure sensor arranged downstream of the at least one one-way valve to measure the pressure in connecting tubes between the at least one one-way valve and the dosing valve; characterised in that the controller is a controller arranged to determine the pressure in the main storage unit by carrying out a method during a start-up phase in which heating of the start-up storage unit and the main storage unit has already been started comprising: a) interrupting ammonia release from the start-up storage unit; b) relieving pressure in the system; c) measuring the pressure, which is caused to decrease by the steps a and b, with the pressure sensor downstream the at least one one-way valve, and inferring from this measurement the pressure of the main storage unit; d) if the pressure inferred of the main storage unit has not yet reached a desired ammonia supply pressure for normal operation, resuming ammonia release from the start-up storage unit and repeating the start-up phase method, or - if the pressure of the main storage unit has reached the desired ammonia supply pressure - ceasing heating of the start-up storage unit. - a main storage unit and a start-up storage unit, the storage units holding ammonia storage material, - at least one one-way valve via which the main storage unit is in communication with the start-up storage unit, the one-way valve preventing any back-flow of ammonia from the start-up storage unit to the main storage unit; - heating devices (8, 9) arranged to heat the main storage unit and the start-up storage unit separately to generate gaseous ammonia by thermal desorption from the solid storage material; - a controller arranged to control the heating power of the main storage unit and the start-up storage unit, thereby enabling ammonia release from the start-up and/or the main storage unit; - a dosing valve arranged to control ammonia flow from the storage units according to a demand; - a pressure sensor arranged downstream of the at least one one-way valve to measure the pressure in connecting tubes between the at least one one-way valve and the dosing valve; a) interrupting ammonia release from the start-up storage unit; b) relieving pressure in the system; c) measuring the pressure, which is caused to decrease by the steps a and b, with the pressure sensor downstream the at least one one-way valve, and inferring from this measurement the pressure of the main storage unit; d) if the pressure inferred of the main storage unit has not yet reached a desired ammonia supply pressure for normal operation, resuming ammonia release from the start-up storage unit and repeating the start-up phase method, or - if the pressure of the main storage unit has reached the desired ammonia supply pressure - ceasing heating of the start-up storage unit.