Lesson 14

Electrical Energy Use in the HomeLast week we calculated how much energy

appliances use and how this correlates to the length of time that they are used.

Energy = power x time

Now we are going to look at how much it costs to use the electricity.

Just like meat at the Northern has a price per pound, electricity has a cost per kilowatt hour (kWh).

 If the cost of a kilowatt is 0.08$, all we need

to know is how much energy is used to find out how much it will cost.

Cost of energy used = amount energy used (kWh) x price per kWh

Example 1:If a 500 W refrigerator/ freezer uses 75 kWh of energy,

and it costs 0.08$/kWh. How much does it cost to operate the refrigerator for one month?

Cost of energy used = amount energy used x price per kWh

Cost of energy used = 75 kWh x 0.08$ / kWh = 6.00 $ Therefore, it costs 6.00 dollars a month to run the

fridge.

Example 2: A 200 watt TV is used 12 hours a day for 30 days. How

much will it cost to run if each kWh costs 0.08$? E = P x tE = (200W ÷ 1000W) x (12 h x 30 days)E = 0.2 kW x 360 hE = 72 kWh Cost of energy used = amount energy used x price

per kWh Cost of energy used = 72 kWh x 0.08$/kWhCost of energy used = 5.76 $

Practice questions Try the 4 questions, if you need help just ask

A microwave oven uses 15kWh a month. How much does it cost to run for one month?

Cost of energy used = amount energy used x price per kWh

Cost of energy used = 15 kWh x 0.08 $kWhCost of energy used = 1.20 $

A toaster uses 5kWh hours of electricity per month, what would it cost to use?

Cost of energy used = amount energy used x price per kWh

Cost of energy used = 5kWh x 0.08 $kWhCost of energy used = 0.40 $

A 60 W incandescent bulb uses 30 kWh a month. How much does it cost to run for one month?

Cost of energy used = amount energy used x price per kWh

Cost of energy used = 30 kWh x 0.08 $kWhCost of energy used = 2.40 $

A 500 W clothes dryer uses 150 kWh of electricity a month. How much does it cost to run for one month?

Cost of energy used = amount energy used x price per kWh

Cost of energy used = 150 kWh x 0.08 $kWhCost of energy used = 12.00 $

Although the cost of one incandescent light bulb is relatively inexpensive, it is important to remember that there are multiple light bulbs in every house and many are left on for multiple hours a day. The cost of lighting can quickly add up when you calculate all of the light bulbs in a house.

Fluorescent bulbs cost much more to use than incandescent light bulbs do but they use less energy for the same amount of light. Which one is more cost effective in the end? A few factors must be looked at: cost of the bulb, life of the bulb and power usage. We will compare the bulbs over the life of one fluorescent.

Bulb 1: 60 watt incandescent Cost: 0.50 $Life: 1 000 hPower: 60 WSince incandescent bulbs burn out

faster we will need to calculate the cost of all of the bulbs. 10 000 h ÷ 1000 h = 10 bulbs.

10 bulbs x 0.50 $ = 5.00 $

Energy used: E = P x tE = (60W ÷ 1000kW) x (10 000 h)E = 0.06 kW x 10000 hE = 600 kWh

Cost of energy used = amount energy used x price per kWh

Cost of energy used = 600 kWh x 0.08 $ kWhCost of energy used = 48.00 $

Total cost for 10 00 hours of light =Cost of bulbs + cost of energy 5.00 $ + 48.00 $ = 53.00 $

Bulb 2: 13W fluorescent Cost: 10.00$ Life: 10 000 hPower: 13 W

One bulb provides 10 000 hours of light so we only need to buy one bulb.

Total bulb cost = 10.00$

Energy used: E = P x tE = (13W ÷ 1000kW) x (10 000h)E = 0.013 kW x 10000 hE = 130 kWh

Cost of energy used = amount energy used x price per kWh

Cost of energy used = 130 kWh x 0.08 $ kWhCost of energy used = 10.40 $

Total cost for 10 00 hours of light =Cost of bulbs + cost of energy 10.00 $ + 10.40 $ = 20.40 $

Now we can calculate how much the fluorescent bulb would save us after 10 000 hours.

Savings = total cost of low efficiency appliance – total cost of high efficiency appliance

= 53.00 $ - 20.40$ = 32.60 $.

Methods of conserving energyThawing frozen food in the refrigeratorUsing a microwave instead of a conventional

oven Cleaning dust from light fixtures Replacing light fixtures and appliances with

more efficient ones when they wear out. Drying laundry on a clothes line

Methods of conserving energyBlocking sunlight with shades and blinds to

reduce air conditioning useUsing timer switches and programmed

thermostats Turning down the heat in the winter by 1°and

turning up the air conditioner by 1°. Turing off lights when leaving a room or

reducing the time lights are on. Sealing up drafts around the house.

Calculating the Cost of Power Worksheet