The utility company charges you for the power you use based on the monthly readings of an electric meter that measures the current passing through the service entrance into your electrical service panel. The meter can either be a mechanical analog meter that is read monthly by a utility service person who visits your home, or a newer digital meter that may send information via internet or radio signals.
Electrical Usage Is Measured in Kilowatt Hours
Whatever form of meter you have, it measures the amount of electricity you use in watts, or more specifically, kilowatt hours. A watt is the product of the voltage and amperage (or current) in an electrical circuit: 1 volt x 1 amp = 1 watt. But this formula represents merely the measure of the electrical potential. To measure actual energy usage, you have to add an element of time. Therefore, electrical usage is a measurement of watts used over a period of time. Your electric meter records electricity usage in kilowatt-hours. In simple terms, 1 kilowatt hour = 1,000 watt-hours. For example, if you turn on a 100-watt light bulb for 10 hours, the energy usage is calculated as 100 watts x 10 = 1,000 watts (or 1 kilowatt hour).
How a Mechanical Analog Electric Meter Works
The traditional analog meter is a mechanical device found near the service entrance where the utility's service wires enter a building—either from overhead wires that enter the weatherhead and drop down through the conduit to the meter, or from underground service wires. The meter is usually encased in a glass housing and has a metal disc inside that rotates as the circuits inside the building draws current from the service wires. If you observe the disc, you can see that it moves slower at times of low electrical consumption, such as at night, and faster during peak usage times.
Mechanical meters use two conductor coils that create magnetic fields. One coil is affected by current going across the conductor; the other coil is affected by the voltage going across the conductor. Together, the magnetic fields created by the coils turn a thin aluminum disc at a controlled rate. (Aluminum is not magnetic but is moved in this case through a principle known as an eddy current). The disc turns a series of gears that move the five dials that record the electricity in kilowatt-hours. This mechanism is called an indicator.
Your electrical consumption is read manually by a utility service person who visits the home to read the numbers on the dials. A mechanical electric meter cannot be read remotely. Your building's electrical consumption is calculated by subtracting last month's numbers from this month's reading.
A savvy consumer can learn to read these dials to gauge their own electric usage and verify that utility charges are accurate.
How to Read a Mechanical Electric Meter
To record your electricity usage you must have a starting point and an end point. If you want to know how much electricity you use over a month, take an initial reading on the first day of the month, then take a second reading at the end of the last day of the month. The difference between the two readings will tell you how many kilowatt-hours you used that month.
To take a reading, start with the number on the far left dial inside the meter and read towards the right. Write down each number in order from left to right. Let's say your initial reading (at the beginning of the month) shows 01050. The second reading (at the end of the month) shows 02050. Subtract the lower reading from the higher reading: 02050 - 01050 = 1000. This means you used 1,000 kilowatt-hours over the course of the month.
One important thing to remember is that if the dial is between numbers, the reading should be recorded as the smaller number. For instance, if the dial is between the 1 and 2, the reading would be recorded as a 1 until it passes the 2 on the dial. Also note that when you look at the five dials, the first, third, and fifth dial numbers run in a clockwise direction, while the second and fourth dials run counterclockwise. This is normal and not a cause for concern.
How a Digital Electric Meter Works
Digital electric meters come in several types. An older style contains a similar mechanical system that measures electrical flow from the service wires into the building, but it also has an analog-to-digital converter (ADC) that converts the measurement to a digital signal.
A newer type of digital electric meter has AC (alternating current) sensors that detect voltage and amperage in the incoming wires. This type of digital meter is better at picking up all of the power in a circuit, making it slightly more accurate than mechanical or ADC types.
Rather than dials, a digital meter has an electronic display that you or a service person can read manually on the face of the meter, or the signal can be sent to the utility company via a high-frequency signal that is transmitted back to the utility company on the electric service wires. In some styles of a digital meter, a passing service vehicle with the proper equipment can "read" the meter by a radio frequency signal emitted by the meter.
Where a home or business is equipped with solar panels or wind-generator turbines that are sending power back into the energy grid, the digital meter will record the net usage. The meter keeps track of the direction of energy flow, which can be read on the face of the meter as "delivered" or "received."