The invention relates to a device and also a method for determining the degree of disinfection, and for determining the time point when a defined degree of disinfection is reached by a liquid, in particular drinking water, that is situated in a container which is light-permeable at least in a UV range. The device comprises in this case a UV measuring appliance for measuring the intensity of the UV radiation in the region of the container, a data analysis unit to which the values of the UV intensity which are measured by the UV measuring device are fed, and wherein the data analysis unit converts the UV intensity values measured at the start of a measurement at defined time points to form a characteristic, and wherein the data analysis unit compares the characteristics determined at defined time points with a predetermined characteristic, which predetermined characteristic corresponds to a defined degree of disinfection of the liquid, and wherein the device comprises at least one signal output appliance which, in the event that the determined characteristics reaches or exceeds the value of the predetermined characteristic, provides the signal output.