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Electricity Calculations: Percent Efficiency Power Cost. 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. Energy is measured in units called Joules.
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Electricity Calculations: Percent Efficiency Power Cost
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
Energy is measured in units called Joules Energy is also measured in units called kilowatt-hour (kWh) kilowatt-hour (kWh) -1 kWh is the use of one killowatt of energy for one hour -3 600 000 times as much energy as 1 Joule
Electrical Efficiency Efficiency: comparison of the energy output of a device with the energy supplied e.g. Clothes dryers -older clothes dryer uses 800 kWh per year -new clothes dryer uses 300 kWh per year -they both do the same job but the new dryer is more than twice as efficient
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.
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
Electrical Efficiency Calculating efficiency of a Device: percent efficiency = energy out x 100% energy in short form:
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.
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
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
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%
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.
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
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.
Power – Watt meter Watt meter provide a convenient way to measure power consumption.
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
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
Example: a laptop computer uses a 75 W adapter when it is plugged in. Electricity costs 5.6¢/kWh. Calculate how much it would cost to operate the laptop for 1 year for 24 hours per day.
Example: a laptop computer uses a 75 W adapter when it is plugged in. Electricity costs 5.6¢/kWh. 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
Electrical Efficiency Example: a laptop computer uses a 75 W adapter when it is plugged in. Electricity costs 5.6¢/kWh. 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¢/kWh
Electrical Efficiency Example: a laptop computer uses a 75 W adapter when it is plugged in. Electricity costs 5.6¢/kWh. 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
Example: a laptop computer uses a 75 W adapter when it is plugged in. Electricity costs 5.6¢/kWh. 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)
Example: a laptop computer uses a 75 W adapter when it is plugged in. Electricity costs 5.6¢/kWh. 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)
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¢/kWh
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¢/kWh 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¢/kWh Unknown: cost to operate
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¢/kWh 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¢