This study deals with a new approach to tackle the challenges encountered in the filters-based reactive power calculation techniques utilized in the control system of Static VAr compensators (SVCs). In this approach, step and time-varying responses are improved by a combination of differential and filter-based methods. Then, optimum values of the main parameters of the low-pass and band-stop filters are determined considering three criteria: accuracy, speed, and capability for following the time-varying loads. To assess the performance of proposed method, a new objective function is introduced, calculated for two sets of signals, captured from a real filed in Mobarakeh Steel Company (MSC) in Isfahan/Iran. Then, the proposed and the other reactive power calculation methods are assessed in practice and simulation. Also, the algorithm is implemented using a TMDSCNCD28335 as the processor to assess the real-time feasibility of the method. To do so, reactive power variation of an electrical arc furnace is emulated in a prototype test bench and the algorithm is run in real-time by the processor during the system operation.