A method is provided for estimating the degree of saturation (S) of a reversible solid ammonia storage material (3) in a storage unit (1). The storage unit (1) is equipped with a heater (2) to release ammonia and a connected tube (4) for ammonia flow. The initial temperature (TINIT) is measured with a sensor (9) in or around the storage unit (1) before any heating is initiated. Heating is initiated while recording the active time of heating (t) or the amount of energy (Q) released by the heater. The desorption pressure created by solid storage material in the storage unit (1) is measured via a pressure sensor (8) in fluid communication with the storage unit (1). The time (tTARGET), or the heat (QTARGET) where the pressure reaches a certain target pressure (PTARGET) is recorded. The values of the target-pressure time (tTARGET), or the target-pressure heat (QTARGET), and the initial temperature (TINIT) are used to compute an approximate degree of saturation (S).