A monitoring method, device, and electric installation for monitoring ageing of at least one electric equipment unit. The monitoring method includes entering and storing ageing computation data by type of equipment, measuring physical quantities representative of environmental conditions of the electric equipment unit, the measuring of the physical quantities including measuring humidity content with a humidity sensor and measuring salinity content with a salinity sensor, computing ageing according to the measured physical quantities including the humidity content and the salinity content, and according to the stored ageing computation data, and generating electric equipment ageing data that indicates the ageing of the electric equipment unit.