Electrical energy is different from electrical power because it includes the element of time. The amount of electrical energy produced, or used, is determined by multiplying the electrical power (watts) by the amount of time electricity flows (hours). The result is expressed in the units for electrical energy, watt-hours (W h). For many uses of electricity, the watt-hours value is a large number; so, the electric industry has adopted a more convenient unit, the kilowatt-hour (kW h), as the basic unit of electrical energy. One kilowatt equals 1000 W. The amount of electrical energy used can be determined by:
1,000 Wh where EE = Electrical energy (W h); P = Power (W); T = Time (hr)
Problem: How much energy (W h) is required to operate a 150-W light bulb for 3.5 h?
Expressed in kW h:
Electrical energy also is used to determine the cost of using electricity. It is common practice for an electric utility company to sell electricity on a cents (0) per kilowatt-hour basis. When the electric rate (0/kW h) and the amount of electricity (kW h) are known, the cost of operating any electrical appliance or motor can be determined. In the following example the units cancellation method is used.
Problem: What will it cost to operate the light bulb in the previous problem, When the electricity costs 10 0/kW h?
Solution: Using units cancellation:
Cost (cents) = 0.525 kW h x -= 5.25 or 5.2 cents kW h
When the electricity costs 10 0/kW h, then it will cost 5.2 cents to operate the 150 W light bulb for 3.5 h.
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