Measuring Power and Energy
A wattmeter is a combination of a voltmeter and an ammeter. It measures the product of voltage and current. Its current coil consists of a small number of turns of large diameter wire, as it must cater for the load current. Its voltage coil consists of a large number of turns of small diameter wire, as the current flow through it is very low. It has three terminals. It is connected as shown in Figure 8.
The kWh meter has a disc, which rotates at a speed, which is directly proportional to the power being consumed. After a given number of revolutions the scale will advance by one more unit. One unit of electricity is equivalent to one kilowatt of energy consumed in a period of one hour.
A representative of the Supply Authority will read the meter every two months and the consumer will receive a bill based on the energy consumed.
Example
Exercise
Examine a domestic consumer’s electricity bill and determine how the account figures are calculated.