Energy Cheat Sheet

Building an intuition about energy

Because I write about infrastructure, and there’s an enormous amount of energy infrastructure that needs to be built for the US to decarbonize, I’m spending an increasingly large amount of time writing about energy and energy-related topics. One challenge I have with this is that thinking about energy doesn’t come especially naturally to me. I have an engineering background, but it’s in structural engineering, which only requires analyzing things that are sitting perfectly still. To improve my thinking around energy and to try and build some better intuitions around it, I put together a little “cheat sheet” of various energy infrastructure facts.

Energy basics

If you’re not a physicist, it’s actually not amazingly straightforward to precisely define energy. Wikipedia describes energy as “the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat and light.”¹ But a reasonable working definition is “the capacity to get stuff done:” move things around, change their state, and so on. Everything we want to do — whether driving to the grocery store, turning iron ore into steel, or boiling a pot of water — requires energy; the more stuff we want to do, the more energy we require.

Per the first law of thermodynamics, we can’t create or destroy energy, but we can change its form. “Energy generation” is something of a misnomer — it really means changing energy from one form to another. A gas turbine, for instance, transforms the chemical energy of natural gas into the kinetic energy of rotating turbine blades. When we’re talking about energy or energy infrastructure, we’re usually talking about either a) moving energy around, b) changing the form it takes, or c) storing it while we wait to do A or B.

For a clearer picture of energy transformation, let’s dive a few miles below the Earth’s surface to a natural gas reservoir beneath the Permian Basin.² Natural gas stores energy in chemical bonds between hydrogen and carbon. The gas travels to the surface through a natural gas well and then moves to a processing facility, where contaminants are removed. After processing, the gas moves to temporary storage in a large underground cavern, then travels by pipeline to a natural gas export terminal. In the terminal, the gas is cooled, transformed into liquified natural gas, and loaded onto a liquified natural gas carrier for transport across the Atlantic. When it arrives at its destination terminal, it’s turned back into gas and sent by pipeline to a gas turbine power plant.

In the power plant, the gas is burned in a gas turbine. The burning converts the energy in the chemical bonds into heat and then turns that heat into kinetic energy of the rotating turbine blades. A generator connected to the turbine turns this kinetic energy into electrical energy, which takes the form of electrons moving back and forth in an alternating current. This electrical energy then moves through the transmission and distribution system, its voltage and current being modulated along the way by transformers to minimize distribution losses, until it eventually reaches someone’s home.

Inside the home, the electrical energy flows through the house’s wiring and into a phone charger, where it’s used to force lithium ions from a nickel-magnesium-cobalt cathode through an electrolyte and into a graphite anode, converting the electrical energy into chemical energy. When the phone then gets used, the ions will flow back from the anode to the cathode, creating an electrical current and converting the chemical energy back into electrical energy. This electrical energy then flows through various electrical components and is ultimately discharged as heat.

We’ve just followed a small amount of energy thousands of miles, from deep within a natural gas reservoir to a slightly warmed-up cell phone, and over a dozen transformations.

Of course, there are innumerable paths for a given quantity of energy to take. Natural gas alone could get piped directly to consumers to burn in stoves or water heaters get piped to a chemical factory that uses it to generate heat, or get flared directly at the well, where it turns straight into heat.

The purpose of our energy infrastructure is to create paths for the mass movement of energy and to change its form depending on what makes it easiest to move and what sort of work needs to be done at its destination.

Energy units

The range of different units we use to quantify energy can make it hard to build intuitions about energy. Different industries and sectors use their measurements of choice: joules, kilowatt-hours, British thermal units, and so on. Sometimes even the same type of device will use different units in different places. In electric vehicles, lithium-ion batteries give their capacities in kilowatt-hours but in smartphones, they use milli-ampere hours. Other times, the energy content will only be implicit: a barrel of oil, a cubic foot of natural gas, and a gallon of gasoline are all volumes of a particular substance, but they’re also acceptable measures of energy content.

One way to help our energy intuition is to do some simple unit conversions and review some reference values of what various amounts of energy represent

The basic metric unit of energy is the joule, which is the amount of energy needed to accelerate a 1-kilogram mass at 1 meter/second^2 over a distance of 1 meter. Wikipedia gives some nice examples of how much energy a joule is:

• The typical energy released as heat by a person at rest every 17 milliseconds

• The kinetic energy of a 2-kilogram mass traveling at 1 meter per second

• The energy required to lift an apple 1 meter

• The heat required to raise the temperature of 0.239 grams of water from 0 °C to 1 °C

• The kinetic energy of a 50 kg human moving very slowly (0.2 m/s or 0.72 km/h)

• The kinetic energy of a 56 g tennis ball moving at 6 m/s (22 km/h)

• The food energy (kcal) is slightly more than half of an ordinary-sized sugar crystal (0.102 mg/crystal)

Related to energy is power, which is the rate at which energy gets transferred. The metric unit of power is the watt, where 1 watt is energy transferred at a rate of 1 joule/second.

One problem with using joules is that a joule is a tiny amount of energy, and using it to describe quantities of energy used in everyday life results in huge figures: burning a gallon of gas releases about 121 million joules. For intuition building, it's useful to use a unit that doesn’t have so many trailing digits.

One common unit is the kilowatt-hour (kWh), the quantity of energy moved by a 1000-watt power source over an hour (ie: 1000 joules a second, or 3.6 million joules). A kilowatt-hour is an aesthetically unpleasing unit since it defines energy in terms of power, which is itself defined in terms of energy — but it turns out to be a nice unit for working with sort of “everyday” quantities of energy. The numbers aren’t too big, so kWh gets used in a lot of common energy cases, like your energy bill and EV battery sizes. kWh are also easily convertible to megawatts, the most common measure of power plant capacity.

For larger quantities of energy, we can use additional prefixes:

• 1 megawatt-hour = 1000 kilowatt-hours

• 1 gigawatt-hour = 1000 megawatt-hours (1 million kilowatt-hours)

• 1 terawatt-hour = 1000 gigawatt-hours (1 billion kilowatt-hours)

Here’s a table with various activities and quantities converted to kilowatt-hours, megawatt-hours, and gigawatt-hours:

So a gallon of gasoline is about 33.7 kilowatt-hours, a barrel of oil is about 1700 kilowatt-hours, a tanker truck is about 300,000 kilowatt-hours, and an LNG carrier is about 1 billion kilowatt-hours (1000 gigawatt-hours).

When I look at this table, what stands out to me is how much energy hydrocarbons (gasoline, oil, coal, natural gas) can store. 1 gallon of gas stores more energy than 2 weeks worth of food. The energy in a barrel of oil is only slightly less than what’s required to move a multi-ton shipping container thousands of miles across the ocean and more than what’s in a ton of TNT. A tanker truck contains the equivalent of almost 30 YEARS of average US household electricity use. An LNG carrier has roughly as much energy as a hydrogen bomb explosion. An oil supertanker has roughly the equivalent energy needed to heat and cool almost half a million California homes for a year. But as we’ll see, a major limitation of hydrocarbons is that much of this energy will be lost as heat in the transformation into other types of energy or to useful work.

For truly enormous quantities, like the amounts of energy used by entire countries, it’s useful to introduce one more unit: the quad. A quad is short for “a quadrillion British Thermal Units,” and it's how country-level energy consumption is often measured. 1 quad is about 293 terawatt-hours or 293 billion kilowatt-hours. Here are the top 20 countries by energy consumption in terms of both terawatt-hours and quads:

Power units

For power, we can likewise build some intuition by converting various rates of energy consumption into a single set of units. We used kilowatt-hours, megawatt-hours, and gigawatt-hours to quantify energy, but for power, we’ll use kilowatts, megawatts, and gigawatts. The table below gives some common activities and pieces of energy infrastructure and what their power capacity is:

Here again we see the huge energy capacity of hydrocarbons. Moving oil, gasoline, or natural gas around results in very high effective power output because you’re moving a lot of energy in a relatively short period. A typical US gas pump operates at 10 gallons per minute (600 gallons an hour). At 33.7 kilowatt-hours per gallon of gas, that’s a power output of over 20 megawatts, greater than the power output of an 800-foot tall offshore wind turbine. The Trans-Alaska pipeline, a 4-foot diameter pipe, can move as much energy as 1,000 medium-sized transmission lines, and 8 such pipelines would move more energy than provided by every US electrical power plant combined.

US energy production and consumption

Let’s zoom out from individual pieces of energy infrastructure and their capacities to see where our energy comes from, and how it flows to various uses. A good place to start is this Sankey diagram by Lawrence Livermore Lab, which shows overall US energy flows in quads (click to embiggen).

Via energy-2023-united-states.png (3249×1887)

There are a few obvious facts here. One is that most energy we consume gets wasted. Of the 93.6 quads (~27,400 TWh) the US consumed in 2023, only around 1/3rd of that went towards producing useful work. The rest was lost due to various inefficiencies, such as heat engine and transmission losses (we’ll look at losses and inefficiencies more deeply below).

Another obvious fact is that despite the burgeoning construction of renewable energy infrastructure, the majority of our energy still comes from burning hydrocarbons. Petroleum, coal, and natural gas combined are responsible for roughly 82% of total energy consumption in the US.

Related to this fact is that electricity generation is a relatively small fraction of our energy system: roughly ⅓ of energy inputs go towards generating electricity. For residential and commercial consumption, only around half of energy use comes from electricity. For industrial and transportation energy (the two largest sources of consumption), electricity is around 13% and less than 0.1%.³

What this chart makes clear, but also sort of abstracts away, is the enormous amount of infrastructure we’ve built for moving around hydrocarbons. The US has close to 1 million oil and natural gas wells, 3 million miles of natural gas pipeline, 145,000 gas stations, and capacity to refine 18.4 million barrels of oil a day.

This is why environmental advocates often focus on electrifying everything: decarbonizing energy infrastructure requires much more than just building low-carbon sources of energy like solar panels and wind turbines — it requires fundamentally reworking how our society moves energy around. It’s also why eliminating roadblocks and bottlenecks to energy infrastructure construction is so important.

We can also dive deeper and look at a sector-by-sector breakdown of energy use. The residential sector uses around 11.5 quads (3370 TWh) of energy, a little over 12% of total US energy consumption. This table breaks it down:

One major takeaway here is that most residential energy consumption goes into heating things up: Space heating (5.74 quads), water heating (1.69 quads), and clothes dryers (0.26 quads) together account for ⅔rds of residential energy consumption.⁴ You sometimes see air conditioners decried as wasteful by energy-minded environmentalists, but air conditioning is a much smaller share of energy consumption than heating.

For transportation, we can break down energy consumption by the type of fuel used.

Most transportation energy in the US is consumed in the form of gasoline and diesel fuel, with a relatively small amount of jet fuel. If we look at it by transportation mode, most energy (~78%) is consumed by cars, trucks, and motorcycles.

The huge amount of energy used by transportation also means that households are using a lot of energy that isn’t captured by the residential energy consumption statistics above. In fact, in a year, the average US household consumes more energy from burning gasoline (~24,000 kilowatt-hours) than what’s used by the entire rest of the house (~22,500 kilowatt-hours).

The commercial sector is not that different from the residential sector, with heating air and water using the largest fraction, with cooling and ventilation (ie: moving air around) also using large fractions.⁵ As with residential, its energy consumption is roughly split between electricity and natural gas.

And last, here’s industrial energy use.

With industrial energy use, we see a lot of the same patterns that we see in other sectors. One is that utility electricity is a relatively small amount of industrial energy consumption (less than 20%). Most industrial energy comes from burning fuel (mostly natural gas) directly. Once again, we see that heating things up accounts for a huge fraction of energy consumption: roughly half of all manufacturing energy goes into process heating: If we add process heat to residential and commercial air and water heating, we find that roughly 20% of total US energy consumption goes towards heating things up.

Conversions and efficiency

Going back to the overall US energy flow Sankey diagram, it's clear that most energy used in the US is ultimately wasted, with only a small fraction being used to perform useful work (moving cars, heating homes, operating electronics, and so on). Moving energy around and changing its form can’t be done perfectly efficiently (thanks in part to the 2nd law of thermodynamics), and all those conversions we require to get energy where it needs to be and in the form we need it whittle away the energy available to get things done.

(In some cases, however, you can cleverly move energy around in ways that avoid these losses. Heat pumps, for instance, which move heat from place to place, can operate at greater than 100% efficiency, in the sense that they can provide 3 or 4 kilowatts of heat energy for every kilowatt of energy they consume. And superconductors allow transmission of electrical energy with zero losses. But in most cases moving energy around and transforming it means losing some in the process.)

The biggest source of losses is probably heat engine inefficiencies. In our hydrocarbon-based energy economy, we often need to transform energy by burning fuel and converting the heat into useful work. There are limits to how efficiently we can transform heat into mechanical work (for more about how heat engines work, see my essay about gas turbines).

The thermal efficiency of an engine is the fraction of heat energy it can transform into useful work. Coal power plant typically operates at around 30 to 40% thermal efficiency. A combined cycle gas turbine will hit closer to 60% thermal efficiency. A gas-powered car, on the other hand, operates at around 25% thermal efficiency. The large fraction of energy lost by heat engines is why some thermal electricity generation plants list their capacity in MWe, the power output in megawatts of electricity.

Most other losses aren’t so egregious, but they show up at every step of the energy transportation chain. Moving electricity along transmission and distribution lines results in losses as some electrical energy gets converted into heat. Electrical transformers, which minimize these losses by transforming electrical energy into high-voltage, low-current before transmission, operate at around 98% efficiency or more. Here’s a table with conversion efficiencies of various pieces of energy infrastructure:

The low thermal efficiency of ICE cars and heat engines in general and the high efficiency of electrical equipment (especially things like heat pumps) are the biggest counterweight to the high energy capacity of hydrocarbons. The gas tank on an ICE car technically stores much more energy than a Tesla battery pack but only a small fraction of that gasoline energy can be converted into useful motion. Switching to EVs, even if that electricity is still provided by burning fossil fuels, could save large amounts of energy (and thus carbon emissions), as it could mean switching from a 25% efficient gasoline engine to a 60% efficient combined cycle gas turbine. And of course, with electric vehicles, there’s the possibility of powering them by non-carbon emitting sources of electricity like solar or wind.⁶ There’s a similar calculus at work for replacing gas furnaces with heat pumps.

Variability and storage

The energy system needs to supply enough energy to meet demand at any given time. And whenever a system supplies something to meet demand, it’s helpful for it to have storage for excess supply. Demand might fluctuate greatly over time, and storage can act as a buffer to smooth out the demand peaks, meaning you only need to build infrastructure to meet average demand. Without storage, meeting demand might mean overbuilding infrastructure, much of which would only be necessary at brief moments of peak demand.

Conveniently, hydrocarbons are very easy to store. Coal power plants have huge piles of coal next to them. Natural gas gets stored in huge underground caverns, large tanks, or old gas fields, totaling 5 trillion cubic feet of natural gas storage in the US alone. Gas stations have underground tanks that contain tens of thousands of gallons. The strategic petroleum reserve can hold 714 million barrels of oil in huge underground salt caverns. Total US energy storage is around 22 quads, or close to 25% of US annual energy consumption.

However, electricity historically couldn’t be stored easily or cheaply. While hydrocarbons can be stored in a big tank, for electricity you need huge, special pumped hydroelectric facilities or comparatively expensive batteries.⁷ Total US grid energy storage, mostly in the form of pumped hydropower, is today around 600,000 megawatt-hours, or 0.002 quads: roughly 10,000 times less than hydrocarbon energy storage. The lack of storage has historically meant that electrical generation must match supply on a minute-by-minute basis, which incentivized having large, connected power grids to smooth out variations in demand and reduce the requirements for infrastructure.

The requirement to balance supply and demand in electricity consumption gets more challenging with renewables like solar and wind (and even, to some extent, hydroelectric power) where the supply of it varies outside our control. Unlike a nuclear reactor, which can run almost continuously and have a very high capacity factor, or a gas turbine, which can quickly be turned off and on as needed, solar and wind power supply is a function of a varying environment.

Replacing hydrocarbons with electricity on a large scale requires us to solve this storage problem, which is why things like cheaper batteries, and perhaps startups like Terraform Industries, are so important.

Conclusion

My not-especially-groundbreaking takeaway from this exercise in honing my energy intuitions is that we built our energy infrastructure primarily around hydrocarbons, a technology with a particular set of capabilities and constraints. Hydrocarbons are very energy-dense, easy to move around, and easy to store, which to some extent makes up for the fact that it’s hard to convert them into other types of energy without incurring large losses.

The technology we’ll replace it with will likely not share those particular capabilities: electricity can be converted to different forms of energy with fewer losses, but it's not as easy to store and move around as hydrocarbons are. Decarbonizing doesn’t just mean building lots of solar power and wind — it probably means completely rethinking how our energy infrastructure works.

Another takeaway is that it’s very important to pay attention to growth rates in the deployment of things like solar and batteries. Right now solar generates around 1% of total energy consumed in the US, but US solar installations are growing at roughly 25% annually. If those growth rates continue, solar could very quickly become a large fraction of total energy production. But if they taper off, hydrocarbons will remain the source of most of our energy for much longer.

1. To add to the confusion, Wikipedia recursively defines work in terms of energy: “Thermodynamic work is one of the principal kinds of process by which a thermodynamic system can interact with and transfer energy to its surroundings.”

2. This gas in turn is ultimately the result of sunlight providing energy to plants and animals, which died and were gradually chemically transformed beneath layers of sediment for millions of years.

3. This energy flow diagram shows something similar for China, though with a much larger coal fraction (here we have yet another unit, millions of tons of oil equivalent).

4. Another difficulty with energy is that estimates aren’t necessarily consistent: The LLNL energy estimates seem slightly different than the EIA energy estimates.

5. This will somewhat underestimate heating energy consumption because “other uses” will include some heating operations, like combined heat and power systems.

6. Solar and wind do only convert a portion of input energy into electrical energy, but because this energy would be expended regardless (the sun shines and the wind blows whether solar cells and wind turbines are there or not), their low conversion efficiencies don’t have the same meaning, even aside from questions of carbon emissions.

7. Neither pumped hydro nor batteries are technically electrical energy storage; the former is gravitational potential energy and the second is chemical energy. But they can be quickly and efficiently converted to electricity and back.