Energy and Human Ambitions on a Finite Planet, 2021a

5 <strong>Energy</strong> <strong>and</strong> Power Units 82
kWh? At a typical electricity cost of $0.15 per kWh, about how
much does it cost per year to run the refrigerator?
20. The chapter banner image (page 68) shows data from the author’s
utility bill, indicating 230 kWh of electrical usage for a 30-day
period in 2020. What does this rate of energy usage translate to, in
Watts?
21. Heating a typical house might require something like 200 W of
power for every degree Celsius difference between inside <strong>and</strong>
outside temperatures. If the inside temperature is kept at 20 ◦ C
<strong>and</strong> the outside temperature holds steady all day <strong>and</strong> night at 0 ◦ C,
how much power is required to maintain the temperature?
Hint: either treating it as if it is only on for
12 hours, or operating at half-power for 24
hours will yield equivalent answers.
i For instance, if the temperature difference
is 10 ◦ C, the house will require 2,000 W
of steady input to maintain temperature.
22. If Problem 21 had resulted in 5,000 W, 45 how much energy is used 45: it does not, exactly
in a 24-hour day, in Joules? Express in the most natural/convenient
multiplier (i.e., J, kJ, MJ, GJ, etc.) depending on the scale.
23. If Problem 21 had resulted in 5,000 W, 46 how many kilowatt-hours 46: it does not, exactly
(kWh) are expended in a 24-hour period? At an electricity cost of
around $0.15 per kWh, 47 about how much will it cost, per day, to 47: . . . regionally variable
maintain heat?
24. If Problem 21 had resulted in 5,000 W, 48 how many Btu are required 48: it does not, exactly
in a day to maintain temperature? How many Therms is this? At a
typical cost of around $1.25 per Therm, about how much does it
cost per day to heat the home?
25. If Problem 21 had resulted in 5,000 W, 49 how many gallons of 49: it does not, exactly
liquid propane 50 would be consumed in heating the home for a
day? At a cost of around $2.50 per gallon, about how much does it
cost per day to heat the home?
50: Hint: related to Problem 24 <strong>and</strong> the
fact that a gallon of propane contains 0.915
Therms of energy.
26. The chapter banner image (page 68) shows data from the author’s
utility bill, reflecting 230 kWh of electricity <strong>and</strong> 4 Therms of gas
usage. Annoyingly, the units are different. How do the actual
energies compare, if expressed in the same units? 51 How would
you capture in a simple sentence the approximate comparison of
energy use for each?
51: . . . recommend kWh as common basis
27. The chapter banner image (page 68) shows part of the hot water
heater label in the author’s home, showing a rating of 40,000 Btu/hr.
How much power is it capable of putting out, in Watts?
28. The chapter banner image (page 68) shows the energy label associated
with the author’s hot water heater, estimating that it will use
242 Therms per year. If the estimated energy cost is distributed
evenly across 12 months, what would the utility bill be expected
to report? Based on an actual utility bill in the same image, the
usage for one billing period was 4 Therms. How does actual usage
compare to estimated usage, as an approximate percentage?
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.