Ahstract-A phase locked loop (PLL) is commonly used to measure the electrical frequency deviations in PV grid tied inverters. However, this control is affected by sudden changes of power that usually happens with this type of power generation as the solar irradiance is intermittent. Thus, the aim of this paper is to estimate the frequency at the point of common coupling by using a state observer. The evaluation of the observer is developed in DIgSILENT PowerFactory under variable solar irradiance. The results show that the observer gives a more accurate frequency value than a PLL when quick changes of solar irradiance occur.