Engineering Tutorials

Calculating Power
When discussing electricity, we need to address the issue of power. Power is the rate at which work is performed, or the rate at which energy is expended. Work is often expressed in joules. In electrical terms, one joule of work is accomplished when a voltage of one volt causes one coulomb of electrons to pass through a circuit. When this amount of work is accomplished in one second, it is equal to one Watt. A watt is the basic unit of power. One watt is also defined as the amount of work that is accomplished when a voltage of one volt causes one ampere of current to pass through a circuit. This relationship between power, voltage, and current is expressed by the following formula:

The calculations show us that the 100-watt bulb has less resistance and, therefore, more current flow. This shows that the 100-watt bulb converts electric energy faster, performs more work, gives off more light/heat, and uses more energy.

Kilowatt
For your electric company to determine how much to charge each customer each month, they simply read from the meter the amount of power that was consumed over that period of time. Because electricity is consumed at a rather high rate, it is impractical to talk or calculate in terms of watts. You probably are familiar with the terms kilowatt and kilowatt-hour from looking at an electric bill. A kilowatt, abbreviated kW, is equal to 1,000 watts. A kilowatt-hour, abbreviated kWh, is equivalent to 1,000 watts consumed in one hour.

One kilowatt = 1kW = 1000 watts
One megawatt = 1MW = 1,000,000 watts

Charges for electricity used in your home are calculated by multiplying the kilowatt-hours used by the rate per kilowatt-hour charged by your electric utility.