Watt-Hour Meters

Electrical energy, as you now know, is measured in watt-hours or kilowatt-hours (kWh). Not surprisingly, a metering device that indicates energy in these units is called a watt-hour meter or a kilowatt-hour meter.

The most often used means of measuring electrical energy is by using a small electric motor, the speed of which depends on the current, and thereby on the power at a constant voltage. The number of turns of the motor shaft, in a given length of time, is directly proportional to the number of kilowatt-hours consumed. The motor is placed at the point where the utility wires enter the building.

This is usually at a point where the voltage is 234 V. At this point the circuit is split into some circuits with 234 V (for heavy-duty appliances such as the oven, washer, and dryer) and general household circuits at 117 V (for smaller appliances such as lamps, clock radios, and television sets). If you’ve observed a kilowatt-hour meter, you have seen a disk spinning, sometimes fast, other times slowly. Its speed depends on the power being used at any given time. The total number of turns of this little disk, every month, determines the size of the bill you will get, as a function also, of course, of the cost per kilowatt-hour.

Kilowatt-hour meters count the number of disk turns by means of geared rotary drums or pointers. The drum-type meter gives a direct digital readout. The pointer type has several scales calibrated from 0 to 9 in circles, some going clockwise and others going counterclockwise. Reading a
pointer-type utility meter is a little tricky, because you must think in whatever direction (clockwise or counterclockwise) the scale goes. An example of a pointer-type utility meter is illustrated in following figure. Read from left to right. For each meter scale, take down the number that the pointer has most recently passed. Write down the rest as you go. The meter shown in the figure reads a little more than 3875 kWh.


A utility meter with four rotary analog dials. In this example, the reading is a little more than 3875 kWh.