System inertia has long been used by system operators to keep the frequency of the system from straying too far from normal. Historically, inertia from traditional sources (i.e., the spinning mass of generators and motor loads) were abundant and it was not necessary to monitor the amount of online inertia at any one time. But as the role of inverter-based resources (i.e., wind, solar, and battery storage) increases, inertia from traditional sources is decreasing. System operators globally have found it to be valuable to estimate and monitor the online inertia to ensure the system will respond adequately to sudden chang