Work = Power x Time
Practical Units of Power and Work. Starting with the watt, we can develop several other important units. The fundamental principle to remember is that power is the time rate of doing work, while work is power used during a period of time. The formulas are:
Power = work / time
and
Work = power x time
With the watt unit for power, one watt used during one second equals the work of one joule. To put it simply, one watt is one joule per second. Therefore, 1 W = 1 J/s. The joule is a basic practical unit of work or energy.
A unit of work that can be used with individual electrons is the electron volt. Note that the electron is charge, while the volt is potential difference. Now 1eV is the amount of work required to move an electron between two points having a potential difference of one volt.
Since 6.25 x 1018 electrons equal 1C and a joule is a volt-coulomb, there must be 6.25 x 1018 eV in 1J.
Kilowatt-hours
This is a unit commonly used for large amounts of electrical work or energy. The amount is calculated simply as the product of the power in kilowatts multiplied by the time in hours during which the power is used. This is the unit of energy you need to know.
Example: A light bulb uses 100 W or 0.1 kW for 4 hours (h), the amount of energy used is:
Kilowatt-hours = kilowatts x hours
= 0.1 x 4
= 0.4 kWh.
We pay for our household electricity in kilowatt-hours of energy.
