Voltage instability usually occurs when the power systems are heavily stressed. In addition, the voltage instability is associated with not meeting the reactive power demands because of limitations on the production and transmission of reactive power. It may or may not be initiated by a disruption, but is usually characterized by shortage of fast-acting reactive power reserves. Reactive power reserves can be classified according to the load's and the generation's perspective. In terms of loads, the focus is on the margins to collapse while from the generation point of view, the focus is on the amount and value of reserves. This paper defines a improved reactive power reserve from generation's perspective called as ' The Effective Var (Q) Reserve'. Conventional reactive power reserve is measured by the difference between the maximum reactive power considering the generator's capability curve and the current reactive power of generator. However, all generators could not affect on the whole power system. Thus, the effective reactive power reserve is computed by sensitivity based on generator reactive power outputs and reactive power loads. The effective reactive power reserve is verified using static and dynamic simulation in the test system and the Korean power system.