Five Chapter 5

The Cost of Electricity

• Appliances can do work because of the electrical energy supplied to them.
• Remember

• The last equation tells us that energy can be measured in units of power times units of time
• The standard unit used by energy companies is the kilowatt hour (kwh)
  • kw = the unit of power
  • h = the unit of time
• One kilowatt hour (1 kwh) - the energy dissipated in one hour by a load with a power of one kilowatt.
• The conversion from kilowatt hours to Joules is
  • 1 kwh = 3.6 X 10⁶ J = 3.6 MJ

Proof

E = P T

= (1 kw)(1 h)

= (1000 w) (3600 s)

= 3.6 X 10⁶ J

• Energy consumption is the power rating of appliance times the time it was used.
• Metres are installed on buildings to make the calculation and record the energy used.

e.g. A 520 kw appliance is used for 10 h. The cost is 6.70 cents / 100kwh. What is the cost of running this appliance?

Solution

E = ? E = P T

P = 520 kw E = (520 kw)(10h)

T = 10 h E = 5200 kwh

100 kwh = 6.70 cents

5200 kwh X

X = 348.4 cents = $3.48

January 6, 2014