Demand response methodologies for primary frequency response (PFR) for under or over frequency events. Aspects of the present disclosure include methods for controlling a fleet of distributed energy resources equipped for PFR and quantifying in real time an amount of primary frequency control capacity available in the fleet. In some examples, the DERs may be configured to consume and discharge electrical energy in discrete energy packets and be equipped with a frequency response local control law that causes each DER to independently and instantaneously interrupt an energy packet in response to local frequency measurements indicating a grid disturbance event has occurred.