Area Control Error (ACE) is the difference between scheduled and actual electrical generation within a control area on the power grid, taking frequency bias into account. To elaborate, generating an amount of electricity in exact equilibrium with consumption (load), is extremely difficult and also quite impractical. Instead, generation controllers strive to continually alternate between over- and under-generating. For instance, in the PJM control area, the generation dispatcher's official target is to cross the break-even point (i.e. a zero ACE value) fifteen times per hour.

The formula for calculation of ACE follows:

ACE = (NIA - NIS) - 10b (FA - FS) Tob + IME


  • NIA represents actual net interchange (MWs)
  • NIS represents scheduled net interchange (MWs)
  • b represents the control area's frequency bias setting (MW/0.1 Hz)
  • FA represents actual system frequency (Hz)
  • FS represents scheduled system frequency (60.0 Hz in America)
  • Tob represents scheduled interchange energy used to bilaterally correct inadvertent accumulations (MWs)
  • IME represents a manually entered amount to compensate for known equipment error (MWs)

Details for the Area Control Error equation come from WECC's website--now defunct, but it can still be viewed off of Google's cached snapshot at: