DEMAND
D-01/1
Lifecycle Energy Analysis of a Car
This worksheet covers:

Life cycle energy analysis of a car and a lightweight car (manufacturing energy,
transport energy and energy associated with use)

Example of how to convert from Joules to kWh

Comparison of an average car and lightweight car
How much energy does a car use?
When considering how much energy a car uses, we must consider the energy used in the whole
lifecycle of the car, not just when it’s being driven. First, we need the average weight and
material composition of a car. Then we need to consider the energy required to get the
materials and the energy required to process them to make the car. Then, we will consider the
energy cost of transporting the car from the site of manufacture to the final user. Finally, we
will consider the amount of energy used when driving the car.
Why is the total energy used by the car
important?
A lot of energy goes into producing and processing
the materials used in a car. Some of this energy
can be reused if we recycle the components of the
car, but a lot of cars end up in landfill instead.
Some of the processing energy is not reusable – if a
piece of sheet metal has been shaped to become
the shell of the car, we cannot get back the energy
put into shaping it. Therefore it is important for us
to know the total amount of energy used by the car
over its lifetime.
What’s the average mass and material composition of a UK car?
The mass of a mid-size average car is about 1500kg. This is made up of approximately:
Material
Iron and Steel
Lead, copper, zinc
Aluminium
Plastics
Other (mainly glass and
rubber)
Percentage
Of Car
63%
3%
6.5%
13%
14.5%
How much energy do we need to produce and process these materials?
The table below shows the embodied energies and processing energies of the materials used in
manufacturing a car (Source: Ashby, 2009). The embodied energy of a material is the energy
per unit mass consumed in making the material from its ores and feedstock.
Material
Mass
Material Embodied Processing
Total Energy (MJ)
(kg)
Energy (MJ/kg)
Energy (MJ/kg)
Low Carbon Steel 945
22
2.4
23100
Lead alloys
45
55
0.59
2500
Aluminium alloys 98
220
3
21900
Acrylic PMMA
195
100
11
21600
Silica Glass
108
31
16
5080
Natural Rubber
109
66
7.6
8020
This gives a total energy of 82200MJ
DEMAND
Lifecycle Energy Analysis of a Car
D-01/2
How much energy is used in transporting the car from the manufacturers to the end user?
Let’s assume the car is manufactured in Italy, which is approximately 2000km away. Let’s say
750km of this is transport by sea, and 1250km is by land. The energy used can be calculated as
below (Source: Ashby, 2009):
Transport Type
Transport Energy
(MJ/tonne.km)
Sea freight
0.16
32 tonne truck
0.46
This gives a total energy of approximately 1040 MJ.
Distance
(km)
750
1250
Energy
(MJ)
180
863
How much energy does the car use during driving?
Assuming this is a gasoline powered car, the energy consumption is 2.1 MJ/tonne.km. So if the
car is driven 80km every work day, plus a little at the weekends we have a yearly usage of
25000km. This gives an energy consumption of:
2.1 MJ/tonne.km x 1.5 tonnes x 25000 km/year ≈ 78800 MJ/year
Assuming the life span of the car to be 10 years, this is a total energy use of 788000 MJ.
So what’s the total energy consumption?
Adding together all these different consumptions of energy, we find a total as follows:
Consumption
Energy Use (MJ)
Materials
82200
Transport
1040
Use
788000
Total
871000
What is this energy consumption in kWh?
A Watt (W) is a Joule (J) per second (s), so Wh are really Jh/s (Joule hours/second). This means
to convert from Joules into Wh, we need to multiply by the number of hours per second:
Wh = Jh/s = Joules x hours/second = Joules x 1/3600
= Joules
3600
Then, to make this kWh, we divide by 1000, so that:
kWh =
Joules
3600 x 1000
= Joules
3600000
Or 1 kWh = 3,600,000 J
This means the energy consumption can be written as
871000 MJ
3600000 J/kWh
= 242000kWh.
How does this compare to the total daily use of 195 kWh/person/day?
To convert this total to kWh/person/day we need to consider both the lifetime of the vehicle,
and it’s average occupancy.
Then,
consumption in kWh/person/day =
consumption in kWh
.
number of days x average number of people
So for example, if the lifetime of the car is assumed to be 10 years (or 3650 days) and the
average occupancy 1.5, the consumption is
242000 kWh
3650 days x 1.5 people
= 44 kWh/person/day
DEMAND
Lifecycle Energy Analysis of a Car
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What’s the total energy consumption of a lightweight car?
We can do the same analysis as done before for the average car, but considering the different
composition of the lightweight car (Sources: Ashby, 2009 and MacKay, 2009):
MATERIALS
Material
Percentage Of Car
%
Embodied Energy
Material Processing Total
MJ/kg
MJ/kg
MJ
Iron and Steel
Lead, copper, zinc
Aluminium
Plastics
Glass
Rubber
CFRP
35
3
20
12
7
7
16
22
55
220
100
31
66
273
2.4
0.59
3
11
16
7.6
-
7173.6
1389.75
37464
11211
2773
4342.4
36582
Total Car
100
-
-
100936
TRANSPORT
Transport Type
Transport Energy (MJ/tonne.km)
Energy (MJ)
Sea freight
32 tonne truck
0.16
0.46
100.8
483
USE
Yearly usage
Energy Consumption
Energy Consumption in 1 year
Energy Consumption in 10 years
25000
2.1
44100
441000
km
MJ/tonne.km
MJ
MJ
TOTAL
Consumption
Energy Use (MJ)
Materials
Transport
Use
100936
583.8
441000
Total
542520
How does this energy consumption compare to the average car?
Energy Use in Lightweight
Car By Type of Consumption
(MJ)
9x105
9x105
8x105
8x105
7x105
7x105
6x105
6x105
EnergyConsumption(MJ)
Energy Consumption(MJ)
Energy Use in Car By Type of
Consumption (MJ)
5x105
4x105
3x105
4x105
3x105
2x105
1x105
1x105
0
Doing the same calculation as for the
average car, the energy use of the car is
151000 kWh. Assuming an average
capacity of 1.5 people and a lifetime of
10 years we can also calculate the energy
use to be 28 kWh/person/day
5x105
2x105
0
Materials
Transport
Type of Consumption
Use
We can see that while the materials
energy cost has increased slightly with
the lightweight car, the energy
associated with actually using the car has
been dramatically reduced.
Materials
Transport
Type of Consumption
Use
So having a lightweight car instead of an
average one, has reduced the energy
consumption by 25kWh/person/day.
Sources


(MacKay, 2009) David J C MacKay. Sustainable Energy – without the hot air, UIT Cambridge, 2009. Also
available free online from www.withouthotair.com
(Ashby, 2009) Michael Ashby. Materials and the Environment, Eco-Informed Material Choice, ButterworthHeinemann, 2009