#1W# is #1# watt. #1# watt of power means that #1# joule of energy is used in #1# second. therefore, the number of watts shows how many joules of energy are used in a second.
#1 kW# is #1000# watts. this means that #1000# joules of energy are used in #1# second.
#1 kWh# shows the amount of energy used at the same rate of #1kW# in an hour. #1kW# means that #1000# joules are used in a second.
#1# hour is #3600# seconds. therefore, #1000# joules per second is the same rate as #3600 * 1000# joules per hour.
#3600 * 1000 = 3,600,000#
therefore, #1 kWh# is equal to #3,600,000# joules.

in a #60W# light bulb, #60# joules are used every second.
if you count #1# year as #365.25# days, then #1# year has #31,557,600# seconds.
if #60# joules are used per second, then #1,893,456,000# joules are used in #31,557,600# seconds.
#1kWh# is #3,600,000# joules.
#1,893,456,000# joules is #(1,893,456,000)/(3,600,000) kWh#. this is #525.96 kWh#.
#$0.08 * 525.96# is #$42.08# to #2# decimal places.
if the #60W# light bulb is left on for #365.25# days, then it will cost #$42.08#.
if you count #1# year as #365# days, then #1# year has #31,536,000# seconds.
if #60# joules are used per second, then #1,892,160,000# joules are used in #31,536,000# seconds.
#1kWh# is #3,600,000# joules.
#1,892,160,000# joules is #(1,892,160,000)/(3,600,000) kWh#. this is #525.6 kWh#.
#$0.08 * 525.6# is #$42.05# to #2# decimal places.
if the #60W# light bulb is left on for #365.25# days, then it will cost #$42.05#.
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