Calculating Energy Costs
The watt (W) is a unit of electrical power, which is the rate at which we use
energy. We pay the electric company for the use of energy. A kilowatt (kW) is equal
to 1000 watts: kilo is a prefix that means 1000.
The simplest definitions of kW and kWh are as follows:
1
kW = one thousand watts (and a watt is one joule of energy per second)
2
kWh = using a thousand watts for an hour (3,600,000 joules).
!
Kilowatts are a unit of power, while kWh is a unit of energy. Think of it this
way: kW defines how much energy a device uses or generates in a given amount of
time. One kilowatt is the power of one thousand joules (J) per second (s).
Meanwhile kWh defines how much energy that device actually used or generates.
So, a 100-watt light bulb that is on for 10 hours needs 1 kWh (1,000 watt-hours). This
is the same as ten 100-watt bulbs burning for one hour.
Equation: kW x
(power) x
P x
hours = kWh
hours
t
= (energy)
=
E
*(P = E/t!)
Example:
If a water heater had a 4.5 kW heating element and ran for 1 hour, what amount of
kWh would it consume?
kW x hr = kWh
4.5 kW x 1 hr = 4.5 kWh
In order to determine the COST of energy use you must multiply the kWh by
the cost per kWh.
The Environmental Cost of Using Electricity
Using energy results in pollution of our environment. Energy is used by power
plants to make electricity. It is also used when we drive cars, take airplanes, mow the
lawn and cook our food.
Many types of pollutants are produced by power plants: CO2, particulates
(particulates are small particles that can contribute to smog) and oxides of sulfur
(Oxides of sulfur contribute to acid rain pollution.)
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Sample Questions:
1. Complete the chart. Assume 8.5 cents per kWh.
Appliance
Average Use (hr/
day)
Annual Energy
Consumption (kW)
Vacuum cleaner
0.1
38
Hair dryer
0.25
Computer
4.0
520
Central air
conditioning
12 (60 days/year)
1500
IGNORE
Annual Cost ($)
100
2. Mini Christmas lights use about 1/2 watt per bulb. Say you have a string of 100.
a. How many watts is this total?
0.5 W x 100 bulbs = 50W = 0.050kW
b. If you use these lights for 10 hours a day for 8 weeks, how many kWh is this?
kWh = h x kW = (10 x 8 x 7) x 0.05W = 28 kWh
c. How much will these lights cost you if each kWh costs 7¢?
28 kWh x $0.07 = $1.96
3. A 100-watt light bulb that is on 10-hours a day for 30 days. How much energy will it use?
· 10 hours per day x 30 days = 300 hours (energy is all about time of operation)
· 300 Hours x 100 Watts = 30,000 watt hours (energy is also about connected load or Watts)
· A kilowatt (kW) is 1000 watts
· 30,000 watt hours ÷ 1000 watts = 30 kilowatt hours (kWh as on the electric bill )
4. If the cost is $0.09053 per kWh, how much did it cost to operate that lamp?
· 30 kWh x $.09053 per kWh = $2.72 cost of operation
5. How much would I save if I replaced the 100-Watt light bulb with a 60-Watt light bulb that would
produce the same amount of light energy if the lights were on 10-hours a day for 30 days?
· 100-Watt original light - 60-Watt Replacement light = 40-Watt reduction
· 300 Hours of operation (Problem 1) x 40-Watt Reduction = 12000-Watts saved
· 12000-Watts hours saved ÷ 1000 Watts = 12 kWh
· 12 kWh x $.09053 (Problem 1) = $1.09 savings
· $1.09 Savings ÷ $2.72 Original Cost of operation = .40 x 100 = 40%
6. he 100-Watt bulb has a life expectancy of 1,000 hours. How much will it cost to burn the bulb
for 1,000 hours?
· 100 Watts x 1000 Hours = 100,000 watt hours
· 100,000 Watt hours ÷ 1000 Watts = 100 kWh
· 100 kWh x $.09053 per kWh = $9.53
7. The 60-Watt bulb has a life expectancy of 4,000 hours. How much will it cost to operate during
its life?
· 60 Watts x 4,000 Hours = 240,000 watt hours
· 240,000 watt hours ÷ 1000 watts = 240 kWh
· 240 kWh x $.09053 per kWh = $21.72
Delivery Services: this portion of a bill includes charges from your local distribution company (electric
utility). These services continue to be regulated by State Government. These charges are measured per
kilowatt hour (kWh), which equals 1,000 watts of power used over a one-hour period.
Rate: this lists your delivery service rate. Most customers have a “residential rate,” which provides for
basic power delivery services.
Customer Charge: this is a fixed charge that covers a part of the electric company’s fixed cost to deliver
electricity
Distribution Charge: this covers the local, lower-voltage power lines (wires) and associated facilities that
transport the electricity from distribution substations to homes and businesses. Substations, owned by
local distribution companies (the electric companies), receive electricity from power-generating facilities
and distribute it to customers.
Transition Charge: this fixed charge allows your distribution company to recover costs incurred before
the 1997 Electric Restructuring Act for its: 1) investments in building or operating power generation plants
and 2) other activities mandated by that 1997 law. This charge will decrease over time as these costs are
recovered.
Transmission Charge: this is the cost of delivering electricity via high-voltage power lines (wires) and
associated facilities that transmit the electricity from power plants to distribution substations.
Energy Efficiency (also called “Conservation” and “Demand Side Management”): this is a fixed charge
required by the Commonwealth for all electric ratepayers to help fund programs to improve energy
efficiency and/or reduce energy use.
Renewable Energy: this is another fixed charge required by the Commonwealth to help fund the
Massachusetts Renewable Energy Trust, which finances the development of facilities that produce power
from solar, wind and other renewable resources.
Supplier Services: this portion of your bill includes charges from a power supplier or aggregator for
generating the electricity. This is the only part of the bill that is subject to competition among power
supply companies.
Basic Service - Fixed: customers may choose a competitive power supplier, the company that generates
their electricity. Most consumers have not selected a supplier and are listed as a “Basic Service – Fixed”
customer. This rate provides the most stable electric bills – rather than a variable rate, which fluctuates
monthly based on the current market price for electricity.
Generation Charge: this covers the amount of electricity you use – the energy that comes from power
plants and other facilities that produce the electricity transported to homes and businesses via
“distribution substations.” The generation charge is set through contracts between power generators and
distribution companies, as well as the wholesale electricity market, which is under Federal Energy
Regulatory Commission (FERC) oversight.
Calculating Energy Costs
The ______ (W) is a unit of electrical power, which is the _____ at which we use energy.
We pay the electric company for the use of energy. A ______ (kW) is equal to 1000 watts: kilo is a
prefix that means 1000.
The simplest definitions of kW and kWh are as follows:
1
kW = ___________ watts (and a watt is one joule of energy per second)
2
kWh = using a thousand watts for an _________ (3,600,000 joules).
!
Kilowatts are a unit of __________, while kWh is a unit of __________. Think of it this
way: kW defines _______ _________ energy a device uses or generates in a given amount of
time. One kilowatt is the power of one thousand joules (J) per second (s). Meanwhile kWh defines
how much energy that device actually ___________ or ___________. So, a 100-watt light bulb
that is on for 10 hours needs 1 kWh (1,000 watt-hours). This is the same as ten 100-watt bulbs
burning for one hour.
Equation: kW x
(power) x
P x
hours = kWh
hours
t
= (energy)
=
E
*(P = E/t!)
Example:
If a water heater had a 4.5 kW heating element and ran for 1 hour, what amount of kWh would it
consume?
In order to determine the COST of energy use you must multiply the kWh by the
__________ per kWh.
The Environmental Cost of Using Electricity
Using energy results in _____________ of our environment. Energy is used by power
plants to make ______________. It is also used when we drive cars, take airplanes, mow the lawn
and cook our food.
Many types of pollutants are produced by power plants. In San Diego County, both
________ and _______ are burned during the production of electricity. On average, the following
pollutants are produced: ______, _______________ (particulates are small particles that can
contribute to ______) and _________ of sulfur (Oxides of sulfur contribute to _____________
pollution.)
Sample Questions:
1. Complete the chart. Assume 8.5 cents per kWh.
Appliance
Average Use (hr/
day)
Vacuum cleaner
0.1
38
Hair dryer
0.25
100
Computer
4.0
520
Central air
conditioning
12 (60 days/year)
1500
IGNORE
Annual Energy
Consumption
(kW)
Annual Cost ($)
2. Mini Christmas lights use about 1/2 watt per bulb. Say you have a string of 100.
a. How many watts is this total?
b. If you use these lights for 10 hours a day for 8 weeks, how many kWh is this?
c. How much will these lights cost you if each kWh costs 7¢?
3. A 100-watt light bulb that is on 10-hours a day for 30 days. How much energy will it
use?
4. If the cost is $0.09053 per kWh, how much did it cost to operate that lamp?
5. How much would I save if I replaced the 100-Watt light bulb with a 60-Watt light bulb
that would produce the same amount of light energy if the lights were on 10-hours a
day for 30 days?
6. The 100-Watt bulb has a life expectancy of 1,000 hours. How much will it cost to
burn the bulb for 1,000 hours?
7. The 60-Watt bulb has a life expectancy of 4,000 hours. How much will it cost to
operate during its life?
8. What could you do at home to decrease your electricity bill?