How Does An Electric Meter Read Power

Blue Power Meters
shaunl / Getty Images

Have you ever wondered how the utility company knows how much power you use each month? In short, it uses an electric meter. To understand what is happening with power measurement, it is helpful to understand where the electricity comes from that powers your home and how an electrical system works. But basically, the utility company's service lines connect to the weatherhead on your house. From there, service wires travel down through a pipe and connect to the utility's electric meter.

This meter measures all of the electrical energy that passes through it on its way to being used in your house.

Types of Electric Meters

Older types of electric meters are mechanical and use a system of gears and wheels to record electricity consumption. Newer types are digital and use sensors the detect electrical power and record it in a memory device. Older meters have a clear glass dome, sort of like an oversized mason jar. Usually, there are five dials and a large rotating wheel that sits below them. These types of electric meters must be read by the owner of the property or a utility worker; they cannot be read remotely. A digital electric meter has a digital readout on the meter itself and can be read remotely at the utility company's office.

Measuring Kilowatt-Hours

Electrical power is measured in watts. A watt is the product of the voltage and amperage (or current) in an electrical circuit; 1 volt x 1 amp = 1 watt.

But power is merely the measure of electrical potential. To measure electrical usage, or energy usage, you have to add an element of time. Therefore, electrical usage is a measurement of watts used over a period of time.

Electric meters record electricity usage in kilowatt-hours. To simply that, 1 kilowatt hour = 1,000 watt-hours.

For example, if you turn on a 100-watt light bulb for 10 hours, you will consume 1,000 watt-hours or 1 kilowatt-hour of power; 100 watts x 10 hours = 1,000 watt-hours or 1 kilowatt-hour.

How an Electric Meter Reads Electricity

To record how much electricity passes through from the utility lines to your home's electrical system, a meter must measure both the voltage and the amperage (current) of the circuit wiring at all 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.

A digital electric meter has AC sensors that detect voltage and amperage in the incoming electrical circuit. AC is short for alternating current, the type of electricity used by most household electrical systems and appliances and other electrical devices.

While mechanical meters are highly reliable, digital meters are better at picking up all of the power in a circuit, making them slightly more 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 in the two readings will tell you how many kilowatt-hours you used that month.

To take a reading, start with the number digit on the left and read towards the right. Look at each of the five dials and note the number the needle is pointing to. 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 clockwise like a clock. The second and fourth dials run counterclockwise.