There is provided an electrically operated aerosol generating system (100) for receiving an aerosol-forming substrate (115). The system comprises a liquid storage portion (113) for storing liquid aerosol-forming substrate, an electric heater (119) comprising at least one heating element for heating the liquid aerosol-forming substrate, and electric circuitry (109) for measuring an electrical resistance of the heating element and for determining depletion of liquid aerosol-forming substrate based on a relationship between a power applied to the heating element and a resulting change of electrical resistance of the heating element. There is also provided a method comprising providing an electrically operated aerosol generating system comprising a liquid storage portion for storing liquid aerosol-forming substrate and an electric heater comprising at least one heating element for heating the liquid aerosol-forming substrate and determining an electrical resistance of the heating element and a power applied to the heating element, and determining depletion of liquid aerosol-forming substrate heated by the heater based on a relationship between a power applied to the heating element and a resulting change of electrical resistance of the heating element.
