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Homework Help: Units of electricity used

  1. Oct 6, 2009 #1
    How can I work out how many units of electricty are consumed in problems such as the following?

    During one day, a 2.5 kW appliance was used for 8 hours. How many Units of electricity were used?

    I know that the answer is 20 but I have no idea how that figure is arrived at. Also, in what unit is the electricity measured. What exactly does "20 units of electricy" mean?

    Thanks a lot
  2. jcsd
  3. Oct 6, 2009 #2


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    kW is a measure of power = the rate of energy used (kW = 1000J/s)
    Electricity is sold in units of kWh = one kilowatt for one hour ( = 1000J/s * 3600s = 3,600,000J )
    So a 2.5 kW appliance used for 8 hours = 2.5kW * 8h = 20kWh
  4. Oct 6, 2009 #3
    Thanks a lot for your help.
    So a 2.5 kW appliance means an appliance that consumes 2.5kW of energy if it runs for 1hour, is that right? Sorry, I'm a real beginner!
  5. Oct 6, 2009 #4


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    Not quite, 2.5kW is a unit of power, it is using 2.5 kJ (2500J) of energy per second.
    So after one hour it has used = 2500 J/s * 3600s = 9MJ = 2.5kWh of electricity.

    It's a bit strange using power (which is energy/time) multiplied by time to get back to energy - but Joules gives large numbers and kW and hours are more convenient for everday use it's easier.

    It's a bit like if we measured distance by having a unit of car speed (mph) and instead of saying it was 30miles to town we said it was 30mph-hours. So after one hour at 30mph you have gone 30 miles.
  6. Oct 16, 2009 #5
    Thanks a lot for that! It's a lot clearer now.
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