Energy and Human Ambitions on a Finite Planet, 2021a

20 Adaptation Strategies 348
a) laptop computer
b) phone charging
c) toaster oven
d) television/display
e) lighting for a room
8. A microwave oven <strong>and</strong> a space heater might each draw 1,500 W
of electrical power. What determines which one uses more energy,
<strong>and</strong> describe a realistic scenario in which one uses a lot more
energy than the other.
9. If the temperature outside is steady at 10 ◦ C, how much more
energy must you expend over some period of time 69 in order to
keep the inside at a shorts-<strong>and</strong>-short-sleeves temperature of 22 ◦ C
vs. a dress-more-warmly 13 ◦ C inside?
69: ...effectively power, then
10. A utility bill for April indicates that your household used 480 kWh
of electricity <strong>and</strong> 20 Therms of natural gas. In addition, your
household has two cars, each using an average of one gallon of
gasoline per day. Convert everything to kWh per day for direct
comparison, <strong>and</strong> also express in W or kW of average power to put
in context against the 10 kW American overall average. 70 70: i Most is outside the home.
11. Following the setup for Problem 10, if the household consists of
two people, what is the per-person footprint in terms of kWh per
day for electricity, natural gas, <strong>and</strong> gasoline, <strong>and</strong> how does this
compare to U.S. national averages per person?
12. In looking at the utility bill referenced in Problem 10, <strong>and</strong> converting
Therms to kWh, it may seem that natural gas is a larger energy
expenditure. But if your local electricity is primarily sourced from
natural gas, converting the combustion energy into electricity at
40% efficiency (via a heat engine), what is the effective amount of
kWh used in the form of natural gas for the electricity, <strong>and</strong> now
how does this compare to the gas used directly in the household?
13. Use the two rule-of-thumb approaches (by mass <strong>and</strong> by cost) to
estimate the energy investment in a farm tractor, whose mass
is 2,400 kg <strong>and</strong> cost is $25,000. If the results are even within a
factor-of-two of each other, we can conclude that the estimate is
probably pretty decent as a rough guide.
14. Use the two rule-of-thumb approaches (by mass <strong>and</strong> by cost) to
estimate the energy investment in a laptop computer, whose mass
is 1.4 kg <strong>and</strong> cost is $1,300. If the estimates are strikingly different,
which do you suspect is more representative of the truth?
15. You <strong>and</strong> three friends want to take a trip together <strong>and</strong> are debating
whether to fly or drive in a gasoline-powered car. In the context
of fossil fuel energy use (<strong>and</strong> thus CO 2 emission), how good does
the car’s fuel efficiency need to be (in miles per gallon) in order for
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.