This method for determining a person's activity in surroundings equipped with electrical appliances that are switchable, in response to a command from the person, between an ‘on’ mode and an ‘off’ mode, wherein the method comprises: the measuring of the magnetic field produced by one of the electrical appliances by means of a magnetometer carried by the person, the identifying of the electrical appliance in ‘on’ mode in proximity to the magnetometer by comparing the measured magnetic field with the pre-recorded magnetic signatures of these electrical appliances in ‘on’ mode, and the determining of the activity of the person on the basis of a pre-recorded activity identifier associated with the electrical appliance identified as being in ‘on’ mode on the basis of its magnetic signature.