An active electrical energy meter is a device which measures the active electrical energy consumed in a circuit.
| I | = | the electrical current flowing through the meter; |
| In | = | the specified reference current for which the transformer operated meter has been designed; |
| Ist | = | the lowest declared value of I at which the meter registers active electrical energy at unity power factor (polyphase meters with balanced load); |
| Imin | = | the value of I above which the error lies within maximum permissible errors (MPEs) (polyphase meters with balanced load); |
| Itr | = | the value of I above which the error lies within the smallest MPE corresponding to the class index of the meter; |
| Imax | = | the maximum value of I for which the error lies within the MPEs; |
| U | = | the voltage of the electricity supplied to the meter; |
| Un | = | the specified reference voltage; |
| f | = | the frequency of the voltage supplied to the meter; |
| fn | = | the specified reference frequency; |
| PF | = | power factor = cosφ = the cosine of the phase difference φ between I and U. |