What's a Watt?

When discussing energy issues, it helps to have a rough idea of a few basic definitions and principles.

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 to remember:

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 items...

	Power	Hrs/Day	Energy	Notes
Solar Path Light	20 Milliwatts	6	.12 Watt Hrs
Xmas light Bulb	5 watts	12	60 Watt Hrs
Cable TV box (off)	10 watts	24	240 Watt Hrs
Cable TV box (on)	100 watts	4	400 Watt Hrs
100 W Bulb	100 watts	4	400 Watt Hrs
PC (active)	200 watts	4	800 Watt Hrs
Toaster	1000 watts	.25	250 Watt Hrs
Hair dryer	1500 watts	.25	375 Watt Hrs
Refrigerator	350 watts	8	2.8 KW Hrs	at 1/3 duty cycle
Oil Furnace	1000 watts	8	8 KW Hrs	varies by season
Electric stove+oven	7,000 watts	.5	3.5 KW Hrs	all elements on
Electric clothes dryer	7,500 watts	1	7.5 KW Hrs
Electric space heating	20,000 watts	8	160 KW Hrs	varies by season

A few observations worth noting:

an 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.

To summarize:

we 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, kilowatt hour).
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:

Heating and 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 hour.

Describing the energy contained in various fuels - a few examples:

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 = 20,000,000 BTU

a full cord of wood = 12,500,000 to 25,000,000 BTU (more on wood values)

It is a British Thermal 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) .

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:

hydrocarbon 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% range.
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.)