| Intermediate Technology Education | Energy and Power Technology | Pre-Design | Topic 3 | Optional Activity 4 |

Electrical Energy Measurement

The purpose of this activity is to investigate how electrical energy is measured and sold and to survey common household appliances as to their energy consumption 
  • Use a watt-hour meter
  • Read a typical electrical tag on an appliance
  • Survey typical household appliances as to their energy consumption using an energy meter

Is it Power or Energy?

Figure. Digital Watt-Hour Meter

In many parts of the country, the utility company that generates and sells electricity to the general public, government, and industry advertises itself as the power company. If you check your "power" bill you will find that the unit used to measure the amount of electricity used is the kilowatt-hour.

Recall that in activity Work, Energy, and Power you learned that

  • Potential energy is stored energy
  • Kinetic energy is energy of motion
  • The unit of measure for energy is the joule
  • By definition, 1 joule of energy will lift 1 newton of weight a height of 1 meter
  • Work is the expenditure of energy to create motion (including movement of electrons)
  • The unit of work is the joule.
  • By definition, 1 joule of work is done when 1 newton of weigh it lifted 1 meter.
  • Hence 1 joule of work is equal to 1 joule of energy expended
  • Power is a measure of how long it takes to do work
  • The unit of measure for power is the watt
  • 1 watt of power is defined as 1 joule of work completed in 1 second

Or stated another way

1 watt of power = 1 joule of work / 1 second

which we know consumes 1 joule of energy

Electrical energy consumption is generally measured in kilowatt-hours.  The term kilowatt-hour means that power is multiplied by time. We know that

1 kW (kilowatt) = 1000 W (watts).

And from the above review, we know that

Power = Work / Time

Multiplying both sides of the equation by Time, we have

Power × Time = (Work / Time) × Time.

or

Power × Time = Work × Time / Time

If you know how equations and fractions work, you can see Time cancels out on the right side of the equation and we are left with Work (really energy), resulting in 

Power × Time = Work

So the power company is selling us energy (Joules). Here is the same argument done using the equations:

Since the kilowatt-hour is a unit of energy it can also be expressed in Joules. See if you can follow the logic below!

So, it turns out that a kilowatt-hour is a lot of Joules!

To put it in concrete terms, a kilowatt-hour is enough energy to keep the average toaster running for about an hour. Currently, depending on where you live, a kilowatt-hour costs about $0.09 (9 cents)

Each electrical service has a meter which keeps track of how much energy you use and is usually read once a month by the company to calculate your bill.

Figure. Utility Company kilowatt-hour Meter

Although $0.09 per kilowatt-hour doesn't seem like much, the number of devices that are connected in the typical household add up. Large appliances such as dryers, stoves, heaters, and hot water tanks use large quantities of electricity. All appliances, when new, must have a sticker attached that indicates some of the electrical properties of the unit.

Look at the sample appliance sticker below. Circled in red is the most important electrical information. The unit runs on 120 volts at 60 Hz (Hertz). That tells us that the unit is designed to run on Alternating Current (AC), the type usually found in wall receptacles. More importantly it tells us that the device draws 9.9A (Amperes) of current. That is a fairly high current, roughly two thirds of what a typical household circuit can supply. If will be an expensive unit to run if left on continuously. The power "draw" on this unit is roughly 1200 watts or 1.2 kilowatts

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