We present a method to calibrate the light to heat conversion in an aqueous fluid containing nanoparticles. Accurate control of light and heat is of dramatic importance in many fields of science and metal nanoparticles have acquired an increased importance as means to address heat in very small areas when irradiated with an intense light. The proposed method enables to measure the temperature in the environment surrounding nanoparticles, as a function of the exposure time to laser radiation, exploiting the properties of thermochromic cholesteric liquid crystals. This method overcomes the problems of miscibility of nanoparticles in liquid crystals, provides temperature reading at the microscale, since the cholesteric liquid crystal is confined in microdroplets, and it is sensitive to a temperature variation, 28°C-49°C, suitable for biological applications.