Techie nerd tidbits relevant to Alternate
discussing energy issues, it helps to have a rough idea of a few basic
definitions and principles.
Definitions of common units of energy
A "Watt" is a unit most commonly used as an expression
of electrical power (although it is equally valid as an expression
of mechanical power). It is named after James Watt (1736-1819), a
Scottish inventor who perfected the steam engine. Common expressions
power(in Watts) =
voltage (Volts) x current (Amps)
(example: a 1200 watt hair dryer will draw 10 Amps at 120 Volts)
milliwatt = 1/1000 of a Watt
kilowatt (or KW) = 1000 Watts
megawatt (or MW) = 1,000,000 Watts
A Watt is a unit describing
the instantanious power that an appliance is drawing at any one moment.
The total amount of energy used is found by multiplying the power
by the length of time the appliance is on. This is most commonly expressed in
"Kilowatt Hours" (or kw-hrs) and is what the power company generally
uses to calculate your bill.
Having kilowatts of power available at our
fingertips is taken for granted in the modern age. We often don't
recognize what it would take to provide this power the old fashon
way (which in olden days meant slaves or animal power). When
electricity was being introduced to American homes there were ads
and articles calculating the number of human slaves a Roman nobleman
needed to get the same power the modern home can have "for just pennies
a day" (at least in 1930). A hard working human generates about 100
watts for a few hours at a time. Given an electrical
service of 10 KW it would require 100 humans to generate
an equivalent amount of power! A slightly flawed comparison perhaps,
but it is still worth keeping in mind.
In order to give you a
feel for these quantites, here are typical values for everyday household
|Solar Path Light
||.12 Watt Hrs
||60 Watt Hrs
|Cable TV box (off)
||240 Watt Hrs
|Cable TV box (on)
||400 Watt Hrs
|100 W Bulb
||400 Watt Hrs
||250 Watt Hrs
||375 Watt Hrs
||2.8 KW Hrs
||at 1/3 duty cycle |
||8 KW Hrs
||varies by season |
||3.5 KW Hrs
||all elements on|
||7.5 KW Hrs
||160 KW Hrs
by season |
A few observations worth noting:
average American home uses ~25 KW HRs per day. Over 24 hours
this works out to an average power of somewhere around 1
KW. This is often used a "rule of thumb" in press articles about new
power plants (a 550,000 KW plant can supply 550,000 homes, etc).
- however, if
mom is cooking a big meal on the electric stove while running
the washer and electric clothes dryer, daughter is using her
hair dryer and the central AC is on, the peak power
can be 25 KW or better. Therefore, many American homes are built with
an electrical service of 200 Amps at 240 volts, which works out to
48 KW. The difference between average power and peak power is important
whether you are planning an electrical system for your house or for
an entire state.
- Some items (hair dryers, toasters) use a lot of power
but are only used for minutes a day so the total energy used is low.
- Many modern electrical items (computers, TVs, cable boxes, almost
anything with a "power brick") consume some amount of power even when
they are "off". This is often because the device is actually in sleep
mode with some vital functions still active so it will "turn on" quickly.
Even though each device may draw modest power, it adds up to a lot
of energy because they are all drawing power 24 hours a day, 365 days
a year. Someone coined the term "energy vampires" for this effect.
- Similarly, after space heating/cooling and water heating, the biggest
single energy user in many homes is the refrigerator because it runs
(at least part time) 24/7.
Much of the rest of the world
uses International Standard (SI) units based on Metric units of measurement.
The common unit of energy is the joule, which equals 1 watt for 1
second. Since there are 3600 seconds in an hour, 1 watt Hr equals
3600 joules and 1 KW hr equals 3.6 million joules. One BTU equals
1055 joules and one kilowatt hour equals 3412 BTU.
In the US
we tend to use horsepower to measure mechanical power. One horsepower
is defined as 550 foot-pounds per second of mechanical work. This
is equal to 746 watts. In Europe it common to rate engines using kilowatts.
A European car might have an engine rated at 200 kw, which would be
advertised as 268 horsepower here in the US.
often use words like power, energy and work interchangably, but sometimes
it is important to distinguish between instantanious power (watts,
horsepower) and total energy or total work over time (BTU, joules,
- there are a number of units for power
and energy commonly used in particular industries or
parts of the world but they can all be translated from one to
the other with the appropriate conversion table.
A British Thermal Unit (BTU) is defined as the
amount of energy needed to raise the temperature of one pound of water
by one degree Farenheit. In the US, the BTU serves as the most common
unit of measurement in a couple of important areas:
cooling - The oil burner in my furnace is rated at 80,000 BTU per
hour. The window air conditioner in the den is rated at 8200 BTU per
Describing the energy contained in various fuels - a
a gallon of gasoline = 124,000 BTU
a gallon of diesel fuel or heating oil = 139,000 BTU
a gallon of propane = 91,000 BTU
a ton of coal =
It is a British
Unit because its part of the old English system of measurements along
with feet, pounds, rods, furlongs, etc. (There are some pretty odd
measurements, click here
Why is it a
Thermal Unit British?
Energy is never
created or destroyed (but it sometimes gets misplaced)
One of the
first things we learned in high school physics was that energy is
never created or destroyed, it simple gets converted from one form
to another. In our modern energy infrastructure there are a
lot of conversions going on all the time: heat to mechanical motion,
motion to electricity, electricity to light or back to heat, etc.
When discussing various energy schemes, it is often important to understand
that some of these conversions happen more easily and at greater efficiency
than others. Here is my simplified efficiency score card:
combustion to heat: - This can be close to 100% if you have the right
setup to capture the heat and put it where you want it. The best natural
gas furnaces claim 97% efficiency, while an open fireplace can be
very energy inefficient since most heat goes up the chimney and combustion
air comes from the room.
- heat to mechanical energy - 10% to 40% -
Heat engines (most often steam turbines, internal combustion engines)
are limited by thermodynamics to only convert a portion of available
heat to motion. Large engines (power plants, ships) that run at a
near constant speed have the highest efficiency. Smaller engines that
must vary their power (cars) have lower efficiency
- mechanical motion
to electricity, electricity to motion - both electric generators and
electric motors can have conversion efficiencies well into the 90%
- electricity to heat - 100% - simple resistance coils (as in
your toaster) work quite well.
- electricity to light - 2%-30% - your
basic incandescant bulb only converts about 2% of the electrical energy
into light. The reset is wasted as heat. Flourescent bulbs are about
4 times as efficient (~10%) while the new white LED based lamps can
be 4 times as efficient again .
Sometimes just studying the efficiencies
can indicate which path is likely superior. Lets take electric heat
as an example. If you use natural gas to generate electricity, perhaps
one third of the BTUs in the natural gas are converted to electricity.
If you are a significant distance from the power plant,
transmission losses may further reduce the power recieved at
your home. You may have to burn 4 units of energy at the power plant
to get 1 unit of heat at your home. You are far better off if you
can pipe the natural gas to a furnace in your home and capture almost
all of the heat directly. Efficiency matters. (Of course, you might
use the electricity to run a heat pump, which can be a great improvement
over direct electric heat in some circumstances.)