RAP Burruss, 12/05
topic is human energy usage. It is framed here in terms of the
body heat of individual people.
Imagine you're invited to a dinner party of about a dozen
people in a typical post-WWII sized house, i.e., not a huge
house. Imagine, too, that this dinner party takes place in the
spring or fall, on an evening that's too cool for the AC and too
warm for the furnace.
If you can imagine such a thing, then you've probably done
it, which means you might have noticed how warm a small room can
get when a dozen or so people are sitting in it. Body heat. It
adds up. The host of the party might have to turn on the AC or
open some windows to cool the place down.
Human body heat can be used as a basis for thinking about
energy in the large global context.
The number of human beings reached five-billion around 1987.
In those days (not even 20 years ago), the entire human
population could have fit within the area encircled by the
Beltway around Washington, D.C. It would have been a
standing-room-only situation, but it would have been possible.
Fact #1 -- The body heat of 5-billion people equals
the electric power output of 500 large nuclear power plants --
which is also roughly the amount of power that would come from
the detonation of one Hiroshima Bomb every two minutes.
Fact #2 -- The body heat of five-billion people, if
distributed over the ~300 square miles encircled by the D.C.
Beltway, would be about twice the average solar input to that
same area on a hot summer afternoon -- which is to say that while
there would have been enough standing room for all
humanity inside the D.C., beltway, the rising air temperature
would cause people to start dropping within an hour or so, while
a rising column of humid air would cause huge thunderstorms to
form downwind of Washington.
The amount of heat that one human body radiates can be
calculated on the basis of the energy in our daily bread -- i.e.,
dietary energy, nominally 2,000 calories per day. (A calorie is
a unit of energy, or of heat; it is not a unit of the weight that
you'll gain if you eat bacon and eggs for breakfast everyday and
never get exercise.)
Fact #3 -- A Krispy Kreme original glazed donut
contains about 170 calories of energy, as does a standard hand
grenade. A dozen donuts (or bananas, which also have about 170
calories each) could supply enough energy to move a typical human
body through one typical day. (A person like Lance Armstrong,
when he's practicing or racing, needs about three times as much
dietary energy as an average person.) If human beings could
digest high explosive, then a dozen hand grenades could power a
human body through one day.
Fact #4 -- The 170 calories in one donut (or banana or
hand grenade) is enough to keep your body moving for about two
hours, which works out to about one-eighth of a horsepower. By
contrast, the same amount of energy in the hand grenade can be
used up in a thousandths of a second, which works out to about a
million horsepower. Same amount of energy (170 calories) --
different rates of use: The hand grenade expends its 170 calories
fast, so it has high power, which makes things nearby move away
The 'horsepower' was defined by James Watt, the Englishman
who invented the steam engine that fired up the Industrial
Revolution in the late 1700s. Mr. Watt needed a way to compare
his engines with horses, which were the main sources of power in
those days other than firewood, peat, and a little bit of wind
and water power, and the slaves in the colonies.
James Watt died in 1819, and 70 years later, the Metric unit
of 'power' was defined and named after him. One 'watt' of power
is defined as one joule of energy per second, wherein said
'joule' is named after James Joule, another Englishman, who was
born the year before James Watt died. The 'joule' was defined as
the Metric unit of energy in 1889, the same year that Mr. Joule
died. One of Mr. Watt's horsepower units equals about 750 watts.
A kilowatt is a thousand watts. In most countries, the power of
automotive and other engines is usually stated in kilowatts
instead of horsepower.
Imagine that dinner party again, with the dozen
or so guests in that one traditional-sized dining room. The meal
ends, and the table is cleared. Desert is served, and then the
guests move to the living room for coffee and conversation.
It's nighttime, so the room is lit by two lamps, one at
each end of the sofa in the living room. The light bulbs in the
lamps are 100-watt units, which means they use 100 joules of
energy each second.
Fact #5 -- The average power of a human body is about
100 watts, the same as a living room lamp. (A hundred watts is
the same amount of power as 2,000 calories of food energy per
day. [A calorie of energy, by the way, equals about 4,200 joules
of energy; the 100-watt lightbulb radiates one calorie of energy
in the form of heat and light every 40 seconds, as does, on
average, a human body.])
If the living room is lit by two 100-watt lamps, then the
total heat load in the room is 100 watts for each actual person
(maybe a little more than that, actually, since the guests are
expending extra energy in digesting their excellent dinner), plus
the energy of 'virtual human beings' in the form of electric
A 'virtual person' can be defined in terms of each 100 watts
of power used by appliances such as toasters, cars, furnaces,
computers and space heaters. Thus the 1,000-watt electric heater
that is beside me on the floor as I write this is equivalent to
10 virtual people -- i.e., the heater is equivalent to 10 people
sitting in my office with me, keeping the room warm.
(Incidental Fact -- The process of generating
electricity is about 30 percent efficient, which means that about
33 'virtual people' worth of coal or oil has to be burned at the
power plant in order to push the 10 virtual people through a
copper wire to the heater in my office.)
When James Watt was born in 1736 (a few years after Newton
died), 100-watt virtual people were much rarer than now. They
were powered by firewood, horses, a little bit of coal, water
wheels and windmills. Slaves were effectively virtual people.
Back in those days, each actual human being had, on average, at
most two virtual people working full time for him or her, heating
the house, hauling goods, powering carriages, planting,
cultivating, harvesting and loading tobacco, cotton, wheat and
In the nearly two centuries since Mr. Watt's death, the per
capita number of virtual people for all humanity has risen from
about two to 20. I.e., for every living person these days, there
are 20 virtual people who do most of our work. They 'eat' coal
and oil and natural gas, and a little bit of uranium.
Fact #6 -- Humanity's average energy use rate (i.e.,
power), according to Science magazine, is 13-trillion
watts. Dividing that by 6.5 billion people gives about 2,000
watts per living person, which corresponds to 20 virtual people
for each actual human being. Here in the United States
(according to year-2000 Department of Commerce data), 281-million
Americans used 3.3-trillion watts, which works out to 118 virtual
people per actual-living American. It's as if each American has
about 120 virtual slaves taking care of such business as heating
houses and offices and carrying us to and from work or the mall.
The 10 virtual people which the heater beside me represents
are part of my share of the world-averaged 20 virtual people who
'work' for each of us human beings. They represent virtual
Since I use the heater only a few hours each cold day, it
actually represents only about one virtual slave. But, as an
American, where are the 110 other of my virtual slaves?
Because of the electric heater, I can keep the thermostat of
the house set at about 50 degrees -- which makes me feel
virtuous, if not warm. But when evening approaches, and my
partner, Joan, is coming home from work, I turn up the
The data plate on the furnace in the basement says it burns
gas at a rate that is equivalent to 425 virtual people. That is,
when the furnace is operating, it's as if I've invited 425 people
into the house to warm it up before Joan gets home.
But because the furnace operates only about two hours each
day during the cold months, its daily-averaged power is
equivalent to 35 virtual people whose body heat warms the house.
(The heating of hot water requires of seven additional 100-watt
full-time virtual slaves.)
Because they are virtual people/slaves -- ones who 'eat'
natural gas instead of bananas or Krispy Kreme donuts -- the
'food' bill for the 35 virtual slaves whose 'body heat' keeps my
house warm works out to $5.25 per day. If they were real people,
who would have to be fed things like bananas and donuts instead
of natural gas, then the food cost for my 35 virtual heat slaves
would be between $60 and $150 per day. If they ate potatoes, the
food cost for thirty-five 100-watt virtual people to heat my
house would be about $40. It's good that they eat natural gas
instead of steak.
The cost of 'food' for virtual people is rising. But it is
still a really good deal compared to real food. If my car ran on
energy from bananas rather than gasoline (which currently costs
$2.35 per gallon, according to the sign at the gas station that I
can see from my office window), I'd have to pay 25 times more to
fill my tank with banana energy.
Fact #7 -- Joan's car is a Honda del Sol that
burns a gallon carrying her 30 miles to work each morning. Her
driving time is one hour. Each gallon of gasoline contains
31,000 calories of energy (which happens to be enough energy to
sustain one typical human body for 15 days at a cost of about 15
cents per day). During her commute, Joan's car uses all that
energy in one hour, which works out to 360 virtual people moving
her car. (Actually, because car engines are about 25-percent
efficient, only about 90 virtual people keep the car moving while
all the rest of them fly uselessly out the tail pipe. [Joan's
160-horsepower-rated VTEC engine actually delivers an average of
only 12 horsepower during her commute.]) Over the course of a
year, she burns 600 gallons of gasoline, which is enough energy
to 'feed' 25 virtual people -- or real people, if they could
digest gasoline -- year round. That is, because of her
commuting, she has 25 virtual people working for her constantly
over the course of each year.
(Incidental Fact -- One additional passenger in Joan's
car would reduce by half the per-person energy cost of driving.
Vehicles that carry lots of people, such as buses, airplanes, and
trains get MUCH better miles per gallon of fuel -- per
passenger -- than cars. But when Joan flies to London to
visit her sister, the amount of fuel the airplane burns to get
her across the Atlantic Ocean works out to the equivalent of
2,700 virtual people moving her share of the airplane during the
8-hour flight. When you average out, over a year, the three
round-trip flights Joan makes to England, it's as if she has 15
virtual slaves whose full-time, year-round effort goes to moving
her body back and forth across the Atlantic Ocean.)
We haven't yet talked about the number of coal- and
oil-eating virtual people it takes dig rocks from the ground and
pull the iron and aluminum and other materials from them, which
are then shaped by yet other virtual people into cars, airplanes,
computers, cell phones, Ipod Nanos, houses, highways, etc., etc.,
-- and each American's indeterminate share of virtual people who
move the American army to the other side of the earth, and so on
. . .
Fact #8 -- Virtual people and real people both emit
carbon dioxide. Since each living human being has effectively 20
virtual slaves, it follows that 20 times the present world
population of living people -- i.e., 130-billion people -- would,
on the basis of exhaled carbon dioxide alone (i.e., without any
industrial CO2 emissions), cause an increase in the atmospheric
carbon dioxide. But, since energy from carbohydrates, such as
bananas and donuts, produces twice as much carbon dioxide per
calorie or joule of energy than does energy from fossil fuels,
then 65-billion people would possibly be enough to cause a
planetary-scale imbalance due to of exhaled breath alone. Thus
we have a possible Malthusian limiting number for the human
population of the earth.
Summary -- The energy that is used by human beings
comes mostly from coal, oil, natural gas and uranium. It is used
to mine metals and other materials that can be shaped into cars,
computers, airplanes, cities, highways and everything else.
Energy from oil, etc, can also be used to make the cars and
airplanes move, to transport food and manufactured goods, and to
heat or cool houses, offices and shopping centers. And it is
used to plant, cultivate, harvest, package, store and cook food.
For all of humanity, the per capita energy use rate of human
beings is 20 times the metabolic energy use rate of a human body.
On average, each actual human being has 20 virtual people/slaves
working for him or her.
For Americans, the per capita energy use rate is 120 times
the metabolic energy use rate of an American. Each American has
120 virtual people/slaves working for him or her.
In Europe and Japan, the per capita energy use rate is about
60 times the metabolic energy use rate of each European or
In China and India, the per capita energy use rate is less
than 20 times the metabolic energy use rate.
In large areas of Africa and South America, the per capita
energy use rate, mostly from the burning of wood, is close to the
metabolic energy use rate of people in those areas.
When averaged over a one-year period, Americans 'employ' the
following numbers of virtual people/slaves:
-- per capita number of virtual people used in industry:
-- per capita number of virtual people used to power the
national fleet of cars and SUVs (averaging 12,000 miles per year
at 20 mpg, 2002 basis): 25
-- per capita number of virtual people used to project
American military power (estimated): 15
-- annualized number of virtual people to move one person from
eastern North America to England: 15
-- per capita number of virtual people used in commerce:
-- per capita number of virtual people used in heating and
cooling residences and making hot water: 11
-- number of virtual people to keep computer operating all the
-- number of virtual people to keep front porch lit year
-- number of virtual people to power a modern electric