A Method of Deriving Overall Accuracy for Single Phase Watthour Meters

The ANSI Standard equation for overall accuracy places a small amount of emphasis on light load accuracy for watthour meters. Results from a recent load research study indicated a significant amount of energy is accumulated at lighter loads where watthour meter accuracy is poorer. Perhaps the standard equation for overall accuracy is a consensus of opinions prior to utilities performing load research studies. Although small inaccuracies of watthour meters at lighter loads may seem insignificant, they become large when applied to an entire meter population. ANSI C12.1-1982 may be interpreted to mean that 20% of all energy registration occurs at light load. The results of a load research study show that watthour meters spend a great deal of time at light loads. A method was developed to establish at which loads watthour meters register energy consumption. Watthour meters spent 85% of the time and registered more than 40% of the total energy for a one year period at light loads.