A kilowatt-hour (kWh) meter measures the amount of electrical energy being used by a device. Another unit of energy used by scientists is the joule. Electrical devices, such as light bulbs or appliances, have a power rating in watts, which corresponds to the number of joules of energy used every second. For example, a 60 watt light bulb uses 60 joules of energy each second of operation. A kWh meter measures accurately to a given tolerance, which is expressed as an amount the meter may be in error, but still functioning correctly.

- Skill level:
- Moderately Challenging

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### Things you need

- Light bulb
- Lamp
- Watt meter
- Stopwatch
- Calculator

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## Instructions

- 1
Screw a new light bulb into the lamp. For example, a 60 watt bulb.

- 2
Turn on the kWh meter. Plug the lamp into the kWh meter then switch it on. Time the passage of 10 minutes (600 seconds) on the stopwatch. Record the energy reading given on the kWh meter. For example the reading might be 0.0093 kWh. Turn off and unplug the lamp from the kWh meter. Turn off the kWh meter.

- 3
Plug the lamp into watt meter. Switch on the watt meter then the lamp. Record the wattage shown, for example 58.0 watts.

- 4
Multiply the watt meter reading by the time, in seconds, the lamp was plugged into the kWh meter to obtain the energy used by the light bulb in joules. Completing this step leads to 58.0 times 600 seconds, or 34,800 joules.

- 5
Convert the energy measured from joules to kWh by dividing by 3,600,000, since a kWh contains 3,600,000 joules on energy. Now you have 34,800 joules divided by 3,600,000 joules per kWh, which equals 0.0097 kWh.

- 6
Add and subtract the kWh meter's tolerance to the kWh meter reading. If the calculated number kilowatt-hours lies in this range, the meter is working correctly. For example, a tolerance of 0.0005 kWh gives a range of 0.0087 kWh to 0.0098 kWh. Given this information you would conclude the kWh meter is functioning properly.

#### Tips and warnings

- Consult the owners manual of the kWh meter to determine its tolerance.