The measurement of the integral, with respect to time, of the power in an electric circuit. The absolute unit of measurement of electrical energy is the joule, or the charge in coulombs times the potential difference in volts. The joule, however, is too small (1 watt-second) for use in commercial practice, and the more commonly used unit is the watt-hour (3.6 × 103 joules). The most common measurement application is in the utility field.
Electrical energy is one of the most accurately measured commodities sold to the general public. Many methods of measurement, with different degrees of accuracy, are possible. Basically, measurements of electric energy may be classified into two categories, direct-current power and alternating-current power. The fundamental concepts of measurement are, however, the same for both.
There are two types of methods of measuring electrical energy: electric instruments and timing means, and electricity meters.
Electric instruments and timing means make use of conventional procedures for measuring electric power and time. Typical methods are listed below. See also Electric power measurement.
1. Measurement of energy on a direct-current circuit by reading the line voltage and load current at regular intervals over a measured period of time. See also Current measurement; Voltage measurement.
2. Measurement of energy on a direct-current circuit by controlling the voltage and current at constant predetermined values for a predetermined time interval.
3. Measurement of energy on an alternating-current circuit by reading the watts input to the load at regular intervals over a measured period of time.
4. Measurement of energy on an alternating-current circuit by controlling the voltage, current, and watts input to the load at constant predetermined values.
5. Measurement of energy by recording the watts input to the load on a linear chart progressed uniformly with time.
Electricity meters are the most common devices for measuring the vast quantities of electrical energy used by industry and the general public. The same fundamentals of measurement apply as for electric power measurement, but in addition the electricity meter provides the time-integrating means necessary for electric energy measurement. A single meter is sometimes used to measure the energy consumed in two or more circuits. However, multistator meters are generally required for this purpose.
Watt-hour meters are generally connected to measure the losses of their respective current circuits. These losses are extremely small compared to the total energy being measured and are present only under load conditions. Watt-hour meters used for the billing of residential, commercial, and industrial loads are highly developed devices. See also Electrical measurements.
