How to Read a Wattmeter

Analog Electric Meter

A wattmeter measures the electrical usage rate in watts, and describes the amount of power used over a given period of time. Your electrical meter is one familiar wattmeter. Another common wattmeter plugs in between your wall-socket and a household device, such as a toaster or television set. Recently available larger devices effectively duplicate your electrical meter, but provide better information in an easy-to-read form. Many power companies now supply customers with these "smart" devices; some even record usage over a period of time. These new logging wattmeters will help you to understand your electrical usage and then to reduce it.

Things You'll Need

• Small digital wattmeter.
• Larger analytical devices that include wattmeters in a package with other functions that will help you to monitor electrical usage for an entire residence.

Instructions

Using and Understanding Wattmeters

1. Read your electrical meter to find the rate of electrical usage in kilowatts for your entire house at the moment you read it, Assume that rate is constant for one hour, which gives you the number of kilowatt hours for one hour of use. Look at your bill to see how much one kilowatt hour costs. Multiply the number of kilowatt hours by the cost per kilowatt to find your use costs per hour.

2. Determine whether your electric meter is analog or digital. An analog meter will have some kind of spinning disk visible under the glass. Somewhere on the meter there will be a number preceded by "Kh". That number is your Kilowatt factor. Write it down. A digital electric meter will give you a digital readout in kilowatts and, often, other useful information.

3. Use a stopwatch (many smart phones have this function built-in) to determine how long it takes the spinning disk in an analog meter to make a single revolution. Now use your Kilowatt factor number in this simple equation: 3600 times your Kilowatt factor divided by the number of seconds for the disk to make one revolution. The number you get is your usage rate in watts.

4. Use a digital wattmeter that does the math for you. The readout is your usage in watts. Plug an inexpensive wattmeter into an outlet, then plug an appliance into the wattmeter and the readout will be the wattage of that appliance. Many of these small wattmeters allow you to input your cost per kilowatt and will also give you a readout of the cost of that particular appliance per hour.

5. Get a more expensive and higher-capacity wattmeter that gives you detailed information about your electrical usage and costs for your entire house. These must be installed by an electrician. Increasingly, power-companies will supply and install these for you, either for a modest monthly fee or even without cost (it is in their interest to reduce electrical consumption and these meters make it much easier for you to monitor and reduce usage).

Tips & Warnings

• Note that many power companies now have variable rates; a kilowatt on a hot summer day costs more than a kilowatt in the middle of the night. If your power company uses variable rates, find out what those rates are for each time period, then determine your usage in kilowatt hours for each to better understand your electrical usage and costs.

• If you turn on a 1000 watt light-bulb (which would be pretty bright), and left it on for an hour you would have consumed 1 Kilowatt/hour (a unit of energy). Look up the cost of a kilowatt on your electrical bill and that's how much burning the bulb for one hour costs.

• We are used to measuring rates in terms of so many X's per Y: miles per hour, feet per second, points per game. But other terms describe rates with a single word; a "watt" is one of them. One watt is "a unit of power equal to 1 joule per second." (http://wordnetweb.princeton.edu/perl/webwn?s=watt). One joule is a unit of energy equivalent to about four calories. Think about how much exercise you would do to burn four calories and you'll have a good understanding of the energy in one joule.

• Smaller inexpensive digital wattmeters have limited capacities. Note the capacity of your wattmeter (it will be indicated on the instrument) and and don't plug large appliances into it that will likely exceed its rated limit (notably dishwashers, washers and driers).

References

• Saving Electricity:How Do I Measure the Amount of Electricity Something Uses?
• SmartHome: Kill A Watt, Kill A Watt EZ and Kill a Watt Power Strip and Energy Meter
• Smart Works: High-Resolution Inline Energy Metering

Resources

• The Inspired Economist: A Smart Meter for Your Green Future; 2009