Figuring energy consumptions and costs

This page show you how to read the meter supplied by your utility company for basic appliances.

As people become more conscience about the amount of power they consume I think it is time to share what I know about how to read your total household energy consumption.
Some people buy these fancy meters like the Kill-O-Watt that read the consumption of each item in their house, then add it all up.  That works and it is extremely accurate for a particular item, but it is a pain in the neck, and you will forget items like clock radios, cell phone charges, and other phantom draws in your house, so in the end you really don’t have an accurate total reading.  Don't get me wrong, meters like the Kill-O-Watt meter is cool to check a particular item and I own one myself, but you can do basically the same thing with the meter in which you are billed from.
The only real advantage of an independent meter is it will tell how many hours per day something runs and figure usage that way. Instead of you having to know the amounts of time.  But for the most part you have an idea of how long certain things in your house run so you can do a lot of the figuring right then and there.

This page will cover breaking down the readings from the meter that you already have on your pole that the electric company was kind enough to provide you with but never shared how you can use it to your advantage.

A lot of people can read their meter to tell how many Kilowatts they have used in a given time, but I would bet that not too many know how to tell what kind of power is being consumed at any particular moment in time.  So I am going to cover how to read the fine scale of your meter that you probably didn't know you had. 

To do this you are going to need a pencil, paper, stopwatch, your electric bill to find your Kwh cost, and a calculator to do your figuring. 

First go out to you meter and look on the face of it.  You will see the letters Kh and a number.
Most I have seen read 7.2.  Just make sure yours is Kh 7.2 as well.
See the Picture of the meter below.

click for large view

So you know the 7.2 means 7.2 Watt-hours for each rotation of the metallic disc in the meter.
Or a Kilowatt charged to you for every 138.8 revolutions of the disc (assuming the 7.2 Kh number).

Now the math:

We are going to use the average 7.2 like the meter shown above as our example. 

There are 60 seconds in a minute, and 60 minutes in a hour. So 60 times 60 equals 3600
60 X 60 = 3600 which is the amount of seconds there are in an hour. 

Meters Wh per revolution is 7.2.
7.2 times 3600 equal 25920.    7.2 X 3600 = 25920.

The 25920 (or whatever number is right based on the Wh number of your meter is the base number that will be used from now on.  So write the correct number down for all other calculations. 

Now turn off all unnecessary things in your house.  TV’s things that would not be on if you are not home.

Go out to you meter with your stopwatch.

Wait for the disc to line up the reference mark with the front reference line on the meter.
Start the stopwatch; and as soon as the disc makes one turn back to the starting mark stop the watch.

Lets say for example that it took 38 seconds to make a full turn. 

Take you reference number that matches your meter.  Mine being 25920 and divide it by the 38 seconds it took to make a revolution.

25920 divided by 38 seconds equals 682 watts per hour of consumption. 
25920 / 38 = 682

So how much is your house costing you per hour and per day with nothing but the basic stuff running?

You can look on your electric bill and you will see what the cost of a Kilowatt-hour is.  Lets say for example that it is 9.5 cents per Kilowatt-hour. sometimes shown as(.095)
you take the 682 watts per hours times the 9.5 cents per Kilowatt-hour which equals 6.479, which means your house is costing you about six and a half cents per hour running the basic stuff.

682 X 9.5 = 6.479 by the way that is 6 point 479 "cents" of a dollar so add a “0” in front .06479
Without the .06479 your calculator will read it as basically 64 cents.  huge difference!

Now take the .06479 cents per hour cost and times it by 24 hours in the day.

.06479 times 24 equals 1.55496 which tells you that your house running the basic stuff costs you 1.55 per day.

Now lets fire up something big like the AC or electric stove and go time a revolution of the disc again.

Lets say that your AC causes your disc to spin at a rate of 1 revolution per every 8 seconds.

Take you base number 25920 divided by 8 seconds per revolution equals 3240 watts per hour.  Or 3.2 Kw per hour.  If you know that your ac runs on average of 14 hours per day.  You can figure your cost per day like this.

3240 watts per hour times 14 hours of run time equals 45360 Kilowatts-hours for its total run time.
3240 X 14 = 45360

Now take the total run time consumption of 45360 Kilowatt-hours, which we need to divide by 1000, to get figure the actual Kilowatts consumed since a Kilowatt is 1000 watts.

45360 divided by 1000 equals 45.36 kilowatts in total consumption
45360 / 1000 = 45.36 

Then multiply 45.36 by the 9.5 cents per Kilowatt-hour you pay and you will know what it costs to run it on an average 14 hour day.

NOTE: to figure your cost of the total Kilowatt hours used you will need to use .095 instead of 9.5.
Example:

45.36 X .095 = $4.3092 total cost per day, or 4 dollars and 30 cents per day to run your AC.

That doesn’t sound too bad until you times that by the 30 day billing cycle.

$4.30 times 30 days per month equals 129.00 dollar per month added to your basic consumption we figured earlier.

Now you know why your bill goes up so much in the summer time. 

You can use this system to figure the cost of any appliance one your side of the meter.  once you know your basic draw you can subtract that from any other appliance you turn on and read to have an exact usage of that appliance.

Lets break that down since we have enough info to do that.

We know the AC along with the basic other stuff drew 45360 Kilowatt-hours.  Subtract what we know is our basic draw we figured first which was 682 from that number.
45360 - 682 = 44672 for just the AC part of the reading.

Now you know how to test each appliance without having to unplug and place it on a separate meter.

Summary of math equations used: 

60 seconds in a minute, times 60 minutes in an hour equals the amount of seconds in an hour. 3600

The meters Kh number (commonly 7.2) times the 3600 seconds in an hour for your base number of 25920 to figure your Kilowatt-hours used at any given time. 

Number of seconds of a  revolution of disc, divided by the above number (which commonly would be 25920) to give you watts per hour consumption of an added appliance or house.

Anything less than 1000 is just watts per hour, anything over 1000 is Kilowatts per hour which would need to be divided by 1000 to figure cost.  10000 / 1000 = 10 Kw.

Closing note:  This is not 100 percent accurate for some loads such as your Air Conditioner.  The reason why is your AC kicks on and draws more amperage/wattage at startup, then it levels out to a steady constant draw.  Use this information as a way to figure the steady constant draw and the average costs of items, not as a way to dispute a Kwh or two on your electric bill.
The meter on the pole should be accurate and unless they read your meter wrong you owe the amount shown on your bill. ;)

 Return to Tips and Tricks

Home