This paper presents the operation and control architecture of a three-phase standalone inverter for uninterruptible power supply application. An inverter is connected to an output LC filter for a standalone application to have a better attenuation of the harmonics. During the operation of the inverter, the parameters of the output filter change from their nominal values. Such a condition makes the overall control architecture have a poor steady state and dynamic performance. Therefore, to enhance the performance of the inverter, a methodology is devised based on the estimation of the instantaneous charge transferred from the inverter to the grid. The proposed methodology is independent of the type of load connected to the inverter and can estimate the filter parameters without any noise or attenuation from their actual instantaneous values. Next, a control architecture is chosen and the proposed estimation architecture is verified when operated in unison with the controller. The overall architecture is verified via computer simulation based on MATLAB®/Simulink® and PLECS® and various case study results are obtained to prove the efficacy of the architecture.