This may seem like a strange question to ask, considering iPhones obviously are charged with electricity, not gasoline.

But the answer speaks to why gasoline and other liquid fuels will remain an important part of the energy mix in the future.

In ExxonMobil’s recently released Outlook for Energy, we predict that by 2040, about 90 percent of the global transportation fleet will still be powered by liquid petroleum fuels – that is, gasoline, diesel, and jet fuel.

When asked why that’s the case, Bill Colton, ExxonMobil’s vice president for Corporate Strategic Planning, often starts the discussion using this fact to put it in perspective:

All of the energy concentrated in one gallon of gasoline is enough to charge an iPhone once a day for almost 20 years.

Clearly, there’s a lot of energy in a gallon of gasoline. And energy density is one of the key factors behind the reliability, affordability, versatility and convenience of any fuel. These are key elements that drive consumer choices today and will continue to drive consumer choices in the future.

So, let’s take a look at the role energy density plays in fuel choices, and how it affects consumer convenience and choice.

Energy Density

Consumers typically want to pay the least amount of money for the most amount of any product, energy included. So, the energy content of any fuel is a critical component of consumer choice. When it comes to transportation, though, another factor comes into play – namely that the energy to power a vehicle must be carried on the vehicle.

One of our top scientists uses the analogy of backpacking when talking about the importance of energy density: You want to buy the lightest, most easily carried food for backpacking, but it also needs to contain a lot of energy to keep you going. Likewise, gasoline and diesel are the lightest and most energy-dense fuels to carry for transportation. A typical car’s gasoline tank contains less than 100 pounds of gasoline but can power a 3,000 pound car for 400 miles at 60 miles per hour. This performance sets a high standard, and there are few transportation fuels currently on the market that are as light, energy dense and portable as gasoline or diesel.

Convenience

The energy density of a fuel also contributes to its convenience.

For example, contrast the 300 to 400 miles that a gasoline vehicle can take you with what it would take to do the same in an electric vehicle. Electric vehicle batteries have just a fraction of the energy density of gasoline, meaning they would have to be charged multiple times during a 400-mile trip. There’s currently no major infrastructure for charging electric vehicles on the road, and it can take hours for an electric vehicle battery to charge.

Consumers at times may take for granted the convenience and time-savings offered by the existing fuel station network. The technological processes that recover crude oil from the earth, transport it to refineries, refine it into gasoline and diesel, transport it to fuel stations and store it over time are so incredibly advanced that consumers can fill up with gasoline 24 hours a day, seven days a week, in as many quantities as necessary. That’s a convenience that does not currently exist with other transportation fuels.

Affordability

The energy density of a fuel also contributes to its affordability.

When gasoline prices are high, you may hear more discussion about introducing more biofuels, especially ethanol, as a means to reduce price. After all, some consumers see the lower E85 price at the station and (understandably) think it’s more affordable than regular gasoline. But a gallon of E85 contains roughly 25 percent less energy than a gallon of gasoline, meaning you end up paying more because you fill up more often – even though the E85 price per gallon at the pump is lower.

A few years ago, Edmunds.com did an interesting test on a flex-fuel SUV, driving it from San Diego to Las Vegas and back first on gasoline and then on E85 to find out the difference in fuel economy and cost. They found that fuel economy was 26.5 percent worse when using E85. That means having to buy more fuel to go the same distance, meaning this sort of trip using E85 could cost a consumer about $20 more than using gasoline.

Looking to the Future

Even though I talked about the current challenges of fueling vehicles with electricity and biofuels here, that doesn’t mean we don’t expect further technology advances in this area that will greatly expand the use of such vehicles. In fact, you might be interested to know that in our Outlook for Energy, we actually predict that hybrids and other advanced vehicles will account for nearly 50 percent of the vehicles on the road in 2040, compared to just 1 percent today.

More efficient vehicles will mean that global demand for fuels to power the light-duty vehicle fleet is likely to actually plateau and gradually start to decline about 10 to 15 years from now, while still meeting the needs of consumers.

There’s a lot more information about the future demand for transportation fuels, advances in vehicle efficiency and more in our 2012 Outlook for Energy – download a copy at exxonmobil.com or visit our interactive website for more data.