The invention relates to a device for indirectly determining a solar flux incident on a structure, comprising means for calculating solar flux according to a theoretical solar flux model taking into account a current value of cloudiness, The device being characterized in that it comprises a first external temperature sensor situated at a location exposed to solar radiation, and a second external temperature sensor situated in the shadow, the calculation means receiving as input a temperature bias value, Defined as the difference between the temperatures measured by the first sensor and the second sensor, and the calculation means being configured to perform an estimation of the nebulosity value as a function of said temperature bias value.