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Understanding Electricity Calculations & Efficiency

Learn about energy, power, efficiency & cost in electrical systems. Calculate efficiency, power usage, and operating costs of devices. Enhance your knowledge with practical examples and formulas.

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Understanding Electricity Calculations & Efficiency

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  1. Electricity Calculations: Percent Efficiency Power Cost

  2. Electrical Energy Energy: a measure of the ability to do work • The five most common types of energy are: • Electrical energy • Heat • Light • Sound • Mechanical

  3. Energy is measured in units called Joules Energy is also measured in units called kilowatt-hour (kWh) kilowatt-hour (kWh) -1 kWh is the use of one killowatt of energy for one hour -3 600 000 times as much energy as 1 Joule

  4. Energy Guide Sticker

  5. Electrical Efficiency Efficiency: comparison of the energy output of a device with the energy supplied e.g. Clothes dryers -older clothes dryer uses 800 kWh per year -new clothes dryer uses 300 kWh per year -they both do the same job but the new dryer is more than twice as efficient

  6. Electrical Efficiency Not all of the energy put in can be converted into useful energy. Sometimes energy is lost (usually into heat energy) while the device is running. A lightbulb converts electrical energy to light energy. However bulbs get hot after they have been on for a period of time. The useful energy islightand the heat is wasted energy.

  7. Electrical Efficiency Efficiency is a measure of how much the energy that goes into a device is converted into useful energy. Calculating efficiency of a Device: percent efficiency = useful energy out x 100% total energy in No device is 100% efficient

  8. Electrical Efficiency Calculating efficiency of a Device: percent efficiency = energy out x 100% energy in short form:

  9. Electrical Efficiency Example: A light bulb uses 100 J of energy and produces 35 J of light energy. Calculate the energy efficiency of the light bulb.

  10. Electrical Efficiency Example: A light bulb uses 100 J of energy and produces 35 J of light energy. Calculate the energy efficiency of the light bulb. Given: Eout = 35 J Ein = 100 J Unknown: % efficiency

  11. Electrical Efficiency Example: A light bulb uses 100 J of energy and produces 35 J of light energy. Calculate the energy efficiency of the light bulb. Given: Eout = 35 J Ein = 100 J Unknown: % efficiency Eout Select: % efficiency = _____ x 100% Ein

  12. Electrical Efficiency Example: A light bulb uses 100 J of energy and produces 35 J of light energy. Calculate the energy efficiency of the light bulb. Eout Select: % efficiency = _____ x 100% Ein Solve: % efficiency = 35 J_ x 100% 100 J =35%

  13. Electrical Efficiency Try this: A toaster oven uses 1200 J of energy to produce 850 J of thermal energy. Calculate the efficiency of the toaster oven.

  14. Electrical Efficiency Try this: A toaster oven uses 1200 J of energy to produce 850 J of thermal energy. Calculate the efficiency of the toaster oven. Solve: % efficiency = 850 J x 100% 1200 J = 71% therefore the toaster oven is 71% efficient Now you try some calculations on your sheet

  15. Electrical Power The rate at which electrical energy is produced or used. That is, the amount of energy transferred per second (Joules per second) Units: measured in Watts (W) or kilowatts (kW) -one Watt is equal to one Joule of energy used per second e.g. 100 W light bulb uses four times as much energy per second as a 25 W bulb.

  16. Power – Watt meter Watt meter provide a convenient way to measure power consumption.

  17. Power Watts to kilowatts (divide by 1000) 500 joules/sec = 500 watts = 0.5 kilowatts 1500 joules/sec = 1500 W = 1.5 kW Try the conversions on your sheet

  18. Power Calculate the power of a kettle that uses 55 000 Joules of energy in 60 seconds. P = E (joules) / time (sec) P = 55 000J / 60 sec = 917 J/sec = 917 W = 0.917 kW Now you try some

  19. Cost of Operating Electrical Devices:

  20. Example: a laptop computer uses a 75 W adapter when it is plugged in. Electricity costs 5.6¢/kWh. Calculate how much it would cost to operate the laptop for 1 year for 24 hours per day.

  21. Example: a laptop computer uses a 75 W adapter when it is plugged in. Electricity costs 5.6¢/kWh. Calculate how much it would cost to operate the laptop for 1 year for 24 hours per day. Given: power = 75 W = 0.075 kW

  22. Electrical Efficiency Example: a laptop computer uses a 75 W adapter when it is plugged in. Electricity costs 5.6¢/kWh. Calculate how much it would cost to operate the laptop for 1 year for 24 hours per day. Given: power = 75 W = 0.075 kW time = 24 hours per day for 365 days = 8760 hours cost = 5.6¢/kWh

  23. Electrical Efficiency Example: a laptop computer uses a 75 W adapter when it is plugged in. Electricity costs 5.6¢/kWh. Calculate how much it would cost to operate the laptop for 1 year for 24 hours per day. Unknown: cost to operate Select: cost to operate = power x time x cost of used electricity

  24. Example: a laptop computer uses a 75 W adapter when it is plugged in. Electricity costs 5.6¢/kWh. Calculate how much it would cost to operate the laptop for 1 year for 24 hours per day. Unknown: cost to operate Formula: cost to operate = power x time x cost of used electricity Solve: cost to operate = 0.075 x 8760 x 5.6 = 3679¢ ($36.79)

  25. Example: a laptop computer uses a 75 W adapter when it is plugged in. Electricity costs 5.6¢/kWh. Calculate how much it would cost to operate the laptop for 1 year for 24 hours per day. Unknown: cost to operate Select: cost to operate = power x time x cost of used electricity Solve: cost to operate = 0.075 x 8760 x 5.6 = 3679¢ ($36.79)

  26. Electrical Efficiency Try this: Calculate the cost of operating a 1500W hair dryer to dry your hair for 6 minutes per day for 3 days. The cost of electricity is 5.6¢/kWh

  27. Try this: Calculate the cost of operating a 1500W hair dryer to dry your hair for 6 minutes per day for 3 days. The cost of electricity is 5.6¢/kWh Given: power = 1500 W = 1.500 kW time = 6 minutes per day for 3 days = 6 min x 3 = 18 min = 0.30 hours cost = 5.6¢/kWh Unknown: cost to operate

  28. Electrical Efficiency Try this: Calculate the cost of operating a 1500W hair dryer to dry your hair for 6 minutes per day for 3 days. The cost of electricity is 5.6¢/kWh Select: cost to operate = power x time x cost of used electricity Solve: cost to operate = 1.500 x 0.30 x 5.6 = 2.52¢

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