How much energy is there? How much is the world using? You can spend hours and hours investigating the finer points of that topic—and plenty of people spend their whole lives on it. Does that sound like fun, or what? Before you jump in, a few basics are in order.

A barrel is 42 gallons of oil. Although the actual familiar steel drum is used less and less these days, the somewhat arbitrary 42 gallons remains the worldwide definition of a barrel.

Natural gas is normally measured in cubic feet. Because the volume of a gas changes based on its pressure and temperature, different groups have different standard conditions, resulting in slightly different amounts of natural gas per cubic foot.

To get some kind of grand total that makes any sense, we need a standardized unit of measurement for gases, liquids and all other forms of energy. Since all forms of energy can be converted to heat, one approach is to use heat as the basis for measuring energy. We can measure that heat in British Thermal Units, or BTU. One BTU is the amount of heat energy needed to raise the temperature of one pound of water one degree Fahrenheit, which happens to be just about equal to the energy of a burning match.

When we are trying to measure huge quantities of energy on a global level, we can convert everything (barrels of oil, cubic feet of natural gas) to BTU, resulting in amounts that are in the thousands of trillions of BTU—or quadrillions—10 raised to the 15th exponential power, which is a 1 with 15 zeros after it (1,000,000,000,000,000). To simplify things, we refer to one quadrillion BTU as a “quad.”

But sometimes people measure energy in kilowatt-hours (as on your electric bill), or maybe in joules. The conversions for those are as follows:

1 kilowatt-hour = 3,412 BTU = 3,600,000 joules

(Thus, as you can see, a BTU is roughly about 1,000 joules.)

There are plenty of statistical mavens who prefer to use the “oil-equivalent” approach, which converts everything to the equivalent of almighty oil, either in “barrels oil equivalent” (BOE), or tonnes (the metric ones) oil equivalent (TOE). As in the previous examples, the numbers being compared are generally quite large, so we usually see MBOE or MTOE, the “M” signifying millions.

1 MTOE = 0.0397 quad = 6.75 MBOE

Practically all your quantitative conversionary questions can be answered at this handy site, which can calculate between just about any units you can imagine:

In 2005, the world consumed somewhere in the neighborhood of 5 x 1018 joules, or about 474 quads, or 139 x 1012 kilowatt-hours of energy from all sources. About 80% of that amount came from fossil fuels, with the rest from nuclear energy, alternative fuels and renewables. BP releases a nifty little study on the state of global energy every year.

The remaining recoverable fossil fuel energy worldwide is estimated by “Wikipedia sources” at about 379,000 quads (huge amounts of energy are locked in “unconventional” sources such as gas hydrates).

Another 2.37 million quads from uranium (nuclear power) remain. Both these sums are dwarfed by just one year of the total amount of solar energy that hits the earth, a whopping 3.6 million quads annually. However you want to measure it, there’s a LOT of energy out there…

…or then again, maybe the whole universe could have a sum total of no energy at all .

Chew on that for a little bit and get back to me.