Earlier today, I was in Washington with Bill Colton, ExxonMobil’s vice president of corporate strategic planning, to officially unveil our 2015 version of The Outlook for Energy: A Vision to 2040. The Outlook is our annual projection of the trends in energy and the economy that will shape the way the world looks over the next several decades. We use the assumptions in the Outlook to guide our business planning and investments.

Each year the Outlook contains some special features that are particularly worth exploring. This year’s section on energy density is no exception.

An important way to think about the differences among fuels is in terms of energy density – the amount of usable energy that’s packed into a certain amount of space.

As the Outlook explains, “Fuels high in energy content use less space and are often the easiest to transport for various uses. This helps explain why gasoline is prevalent as a transportation fuel and why people in high-rise buildings do not rely on wood for heating and cooking.”

5 logs = 1 gallon = 13,000 batteries

Wood served as mankind’s main energy source until the modern age was ushered in by the Industrial Revolution, so it makes sense to compare the energy density of today’s sources to wood. The energy that is contained in a gallon of gasoline, for instance, is roughly equivalent to the energy generated by burning five logs.

To put that further into perspective, consider that a gallon of gasoline contains the amount of energy that can be contained by 13,000 AA batteries.

Those of us in the United States and many other (though not all) nations rely almost exclusively on modern energy sources and technologies to meet our daily needs, so considering our energy use in terms of the energy density of other fuels might yield some surprises. For example, the daily energy per capita demand in OECD countries is about 21 logs’ worth. (In non-OECD countries, the average is 6 logs.)

Think about that: Each of us would have to burn the equivalent of 21 logs every day to generate the energy we currently use to run our households, our cars, and our businesses, in addition to the energy needed to support global travel, trade, and manufacturing.

Forests for flying?

Of course, we don’t use logs for our energy these days. And we couldn’t even if we had unlimited forests; just imagine trying to drive to work, fly a jet airliner, or power the Empire State Building by burning logs. Or consider sending a rocket to the moon with only the energy sources that were available up until the 19th century.

Thanks to energy-dense fuels and modern energy technologies, we don’t have to.

That’s the miracle of modern energy sources. They permit us to do the things that our ancestors—resigned to cutting down trees to light and heat their lives—could hardly dream of doing.

Consider that it used to take 25 days to travel 2,000 miles by stagecoach. In 1620, it took the Mayflower 66 days to cross the Atlantic Ocean.

Today those distances can be traversed in hours.

Phileas Fogg and Nellie Bly strove to race around the world in 80 days. In 2014, that could be accomplished in just two or three, with months to spare.

Keeping the concept of energy density in mind doesn’t just help explain the marvels of the modern age. It also illuminates the coming changes over the next few decades that are highlighted in the Energy Outlook. It gives context to exciting trends like growing GDP, mass urbanization, and the expanding global middle class.

Exciting trends make for an exciting story. I invite you to open the Outlook and take it in.