There are lots of claims made with qualitative statements like "There's plenty of oil in the Arctic." What does that mean, exactly? Something plentiful to an ant might be invisible to an elephant. A relative term like "plenty" is meaningless here until we establish whether our demand for oil is ant-like or elephantine in comparison to the potential Arctic supply to that demand. Discussions of an energy technology or resource without mention of actual numbers or meaningful relative comparisons by which we can gauge its suitability are often not very useful and can even be downright misleading. This applies not only to fossil fuels like oil but to their potential renewable replacements as well.

How Do We Measure Energy?

Energy supplies and consumption are measured in numerical quantities all the way from planetary demand down to your own monthly electric bill. Exactly what do these numbers signify? First, we need to clarify a bit of jargon. What most people just call energy is really measured and reported as two different but closely related things, namely (i) power and (ii) energy, where 'energy' is much more strictly defined in a scientific way than is our more common usage of the word.

Rigorous definitions of power and energy can be somewhat difficult to present and understand, but a very simple automotive metaphor actually describes these concepts quite clearly in a way that is sufficient for our purposes here: Power is the size of your engine, and energy is the size of your gas tank.

The total amount of energy present in your car and available to make it move is essentially just the amount of gasoline that is in your tank. There are many different ways in which your car can move, however, and some of these require that we burn more gasoline over the same period of time. We could, for instance, probably drift very slowly downhill without using any gas at all; on the other hand, we would use a lot of gas to pull a boat up a steep hill at 60 mph, and in fact if our engine was unable to burn the necessary amount of gas quickly enough we would not be able to do this regardless of the size of our tank. Power, then, is essentially the rate at which we consume energy, e.g., one gallon of gas every half-hour. A high-power engine allows us to accomplish more difficult tasks (such as pulling a heavy load uphill) at the expense of consuming more gas over the same period of time. This automotive example intuitively yields a very important principle which we can state as a simple equation:

power = energy / time 

The inverse equation

energy = power * time

is also true, and we can illustrate this with an equally simple electrical example. Electrical energy traveling along a wire is physically different than the chemical energy of gasoline which we release through combustion, but the same underlying concepts of power and energy hold true.

The basic unit of electrical power is the watt and virtually all electrical products such as light bulbs, computers, etc., are clearly labeled to indicate how much electrical power they require for operation. An old-school, incandescent light bulb, for example, might be stamped with a 60W label to indicate that whenever it is turned on it will continuously draw 60 watts of power. Use of this light bulb for a long period as opposed to a few moments does not require more power in the form of a higher-capacity circuit (the electrical analogue to a bigger automotive engine), it just means that more energy is consumed over the longer time span to continuously provide a constant 60W of power. If, on the other hand, you want a brighter 100W light bulb, a louder home theater system or a bigger computer with more disk drives, you might need to install a higher-capacity circuit that is capable of safely delivering more power without overheating.

Just as a gas station sells you automotive energy in the form of gallons of gas, the electric utility sells you electrical energy in the form of kilowatt-hours, where one kilowatt-hour is the energy consumed by the continuous delivery of one kilowatt (or 1000 watts) of power over a period of one hour. It is important to understand that a kilowatt-hour is not one kilowatt per (or divided by) one hour but rather one kilowatt delivered continuously over (or multiplied by) one hour.

Electrical power generating stations are rated by the maximum amount of power they can continuously provide, e.g, 100 megawatts (MW). Such a station would deliver a total 100 megawatt-hours of energy if it were to operate at peak output for one hour. This distinction between watts and watt-hours is a very important one to make, particularly in the case of renewable facilities such as a wind farm which could theoretically be capable of generating the same 100 megawatts at peak capacity as a 100 megawatt coal-fired generator but might supply far fewer total megawatt-hours over the same time span due to wind variability.

Here are some tables which list the various units commonly employed, along with a few examples showing how much energy and/or power is required for various tasks. We have already stated that electrical power is measured in watts (W). The most common unit of measurement of energy currently employed is the joule (J), which is defined as the energy required to continuously produce one watt of power for one second. Single watts and joules are relatively miniscule quantities on the scale of human usage, so energy and power, like computer resources, are commonly reported in greater orders of magnitude such as kilowatts, gigajoules, etc.:

1 Kilo (K) = 1,000 (one thousand)

1 Mega (M) = 1,000,000 (one million)

1 Giga (G) = 1,000,000,000 (one billion)

1 Tera (T) = 1,000,000,000,000 (one trillion)

Over time many basic units other than watts and joules have been used to measure power and energy, and some are still seen quite commonly. Here are some useful conversions:

Energy Unit Conversions

1 calorie (Int Tbl) = 4.1868 J

1 BTU (ISO) = 1.0545 KJ

1 horsepower-hour = 2.684 MJ

1 kilowatt-hour = 3.6 MJ

1 bboe (barrel of oil equivalent) = 6.12 GJ

1 TCE (ton of coal equivalent) = 29.3076 GJ

1 TOE (ton of oil equivalent) = 41.868 GJ

Power Unit Conversions

1 horsepower = 745.7 W

Following below, in conclusion, are two short lists (in increasing quantity) of the energy and power required or produced by a few hopefully familiar tasks or facilities:

Energy Consumption

1 J = energy consumed by lifting a small apple 1 meter straight up

1 J = energy required to heat 1 gram of cool dry air by 1 degree C

300 to 800 KW-H = monthly electrical consumption by Oahu home
 

Power Consumption/Generation

5 to 30 W = power required by one CFL

25 to 100 W = power required by one incandescent bulb

200 W = human climbing stairs

2.5 KW = cruising automobile engine (about 33 hp)

5 to 10 MW = high-power electric locomotive

650 MW = capacity of HECO's Kahe oil-fired power plant

1.7 GW = total electrical generating capacity on Oahu

15 TW = power currently required by the entire human race   
 

Watch and LEarn

energyNOW! presents an animated explanation about where our electricity comes from.

Energy 101: Electricity Generation