How many burritos does it take to run your car?
Strange question, right? We know that cars don't run on burritos. But, even if they did, we usually associate burritos with calories, not mileage. It's entirely possible to convert calories into some other unit of energy metric and figure out mileage, but...why?
Just as strange as it would be to convert burritos to miles per gallon, it's equally strange to talk about BTU's and exajoules in the 21st Century, but that's how much of the global energy industry measures various fuel types. BTU's, or British Thermal Units, were developed in the late 1800s as a means to measure heat. In Britain (British) in the 1800s, locomotives used heat and steam (Thermal) and engineers needed a way to measure that power (Units); hence, BTU's were born. Useful at the time, but not universally useful anymore.
Why does this matter? Well, as the energy world diversified away from steam locomotives all the way to electric vehicles, a lot of the ways we talk about energy remain stuck in the past. That's bad because fossil fuel talking points are deceiving us into believing switching to a renewable and electric future is harder than reality. Akshat Rathi, self-described climate and energy nerd at Bloomberg News (who also happens to have a Chemistry PhD from Oxford) pointed out in a recent article, "Much of the energy from burning coal, oil and gas is simply lost as waste heat, meaning a switch to renewables isn’t that big a leap."
Here's an example:
An internal combustion engine (ICE) gasoline vehicle may drive 30 miles on one gallon of gasoline (30 MPG), while an electric vehicle (EV) would use just 7.5 kilowatt hours (kWh). How do you compare a liquid fuel (gasoline) to something that is weightless (electricity)? Folks in the fossil fuel industry use BTU's as the common metric. A gallon of gasoline contains a massive 116,090 BTU's. Each kilowatt hour (kWh) of electricity contains a puny 3,412 BTU's. This is where a fossil fuel advocate will say, "SEE! You need 34 kilowatt hours for each gallon of gasoline! Energy density!" The trick here is BTU's are designed to measure heat, not work. Remember, our electric vehicle required just 7.5 kilowatt hours to travel 30 miles, or a total of 25,590 BTU's, to do the same amount of work as a 116,090 BTU gallon of gasoline.
On a BTU basis, an electric vehicle is 80% more energy efficient than gasoline (25,590 BTU/116,090 BTU).
With the two vehicles in our example, nearly 80% of the BTU's come from gasoline, and just 20% of the BTU's come from kWh's. This drastically overstates the importance of fossil fuels while understating the importance of renewable energy and electricity. Rathi says, "Even though fossil fuels meet roughly 80% of the world’s primary energy demand, they are responsible for only 60% of its useful energy..." The thermal waste from fossil fuels means replacing those energy sources with renewables and electricity is a lot easier than represented.
As EV's replace ICE vehicles, BTU math becomes...strange3. In a 50/50 ICE/EV future (where half of all fleet vehicles are gasoline and the other half are EV's, not unlike our two vehicle example earlier), the total amount of BTU's used are about 60% of a 100% gasoline scenario (e.g., two gasoline vehicles versus one gasoline vehicle and one electric vehicle). If you measure the energy ratio in BTU's, even in a 50/50 ICE/EV future, still 82%% of the BTU's are provided by gasoline. Fossil fuel advocates will wave these numbers around shouting "Energy density!", even while losing half of the original market share. EV's don't provide more than a majority of BTU's until they hit 82% of the total vehicle fleet - but by then, gasoline stations will be as ubiquitous as watering troughs for your horse and buggy. Measuring energy in total BTU's gives the fossil fuel industry a false sense of importance, and security.
This isn't just a problem for the transportation sector, it's also a problem for the electricity sector. You see, it takes about 10,000 BTU's to generate one kilowatt hour of electricity in a coal-fired power plant, or some natural gas power plants, or nuclear reactors. Those power plants use heat to boil water in a boiler, to create steam and pressure, which then turns a crankshaft and ultimately an electric generator. Sounds like a old-timey steam engine locomotive, right? Steam engine locomotives and steam turbine power plants are basically the same. Thermal fuels for electricity production waste about 66% of the BTU's. But solar panels and wind turbines generate kilowatt hours, directly. There is no thermal waste. As we move towards 50% renewable energy (and higher), the total BTU numbers will look remarkably similar to our transportation chart provided earlier - because when you burn stuff, you're being inefficient. Fossil fuel advocates can claim BTU victory for a very long time, even while representing an even smaller portion of the transportation and electric sectors.
When it comes to replacing outdated energy measurement units, we've been here before. Edison described his lightbulb power against a common metric everyone understood - candles. Lightbulbs were measured in candlepower until society gradually accepted watts as the replacement metric. Even today, we use 60 watt and 100 watt bulb nomenclature, even though our LED technology has advanced far beyond incandescent bulbs, using a fraction of the power. As we become more familiar with LED's, our common metric will not be based on how much energy we consume (watts), but by how much work is provided (lumens). For lighting, lumens will replace watts, like how watts replaced candlepower. For EV's, mileage and kWh's will replace gallons and BTU's.
As a quick note, the earlier chart looks pretty linear - a nice steady, slow decline in ICE vehicles over time and a nice, simple, easy to understand increase in electric vehicles. That's not how technology advances. When the iPhone was introduced in 2007, Nokia and Blackberry scoffed at the keyboard-less screen. Thirteen years later, you can't even buy a Blackberry or Nokia. As auto manufacturers move to electric vehicles, they'll switch over entire manufacturing lines, not 10, 20 or 30% of a factory. Entire factories, all at once. So don't let the linear chart fool you. Technology adoption happens slowly, and then very rapidly.
Back to burritos.
A good burrito contains about 1,000 calories - that's actually kilocalories, because here in the USA, we don't want any mention of the metric system, so we drop the kilo. One kilocalorie contains almost 4 BTU's, meaning our burrito contains 4,000 BTU's. Remember, it takes 116,090 BTU's of gasoline to travel 30 miles, so you'd need about 29 burritos to drive 30 miles in your gasoline vehicle. That's almost 1 burrito per mile in your gasoline vehicle, or 1 BPM. Alternatively, it takes about 100 calories to walk a mile; so our one burrito could sustain us for about 10 miles, or 10 BPM. In walking versus gasoline driving, three burritos is equivalent to one gallon of gasoline in actual walking work, but you'd need 30 burritos if you're driving.
Sure, BTU's are useful at measuring how to burn things, but when our economy moves away from burning stuff, it ceases to be useful as the only (or even, predominant) metric.
In Back to the Future, Marty McFly needed 1.21 gigawatts in his souped-up DeLorean to head back to 1985. That's 1,210,000 kilowatt hours, or about 4 billion BTU's. Marty would need about 1 million burritos to power the DeLorean.
Is this a useful way to measure energy? Nope.
But it sure is entertaining.