Energy Units & Scales

High-tension power lines like these in Freeport, CA carry much
of the world's electricity

In 2010, the world consumed 524 quads of energy.

Don’t know how much that is? It’s about the same as 153,570 terrawatt-hours. Still don’t know how much that is?

Units are intended to give a number a sense of scale, but a little familiarity with a scale is necessary to make them useful. Is 500 kBTU more than 500 kWh? Does a wind turbine produce closer to 5 kW of power or 5 MW? How many power plants does it take to produce a terrawatt anyway?

 Most people have a good understanding of a single scale in each of the four basic measures- length, mass, time, and temperature. But new scales and units are tough to grasp, and this is why much of the US has yet to adopt the metric system- not because we can’t convert, but just because we’re not familiar with standard benchmarks, such as the distance "a hundred meters". What really helps is to develop an internal scale with more familiar objects and values. Randall Munroe from XKCD has a handy guide for interpreting metric units, using some familiar and some not-so-familiar scales.

But, for the average citizen, we don't have many familiar benchmarks in the realm of energy and power scales- especially when we get into larger scales. Maybe you know that a standard, inefficient incandescent bulb draws 60 Watts of power, but if you were asked to estimate what draws 6,000 Watts, you'll probably have more trouble (and answering 100 light-bulbs doesn't count). So below, you’ll find a handy guide to understanding energy units- and maybe you’ll be able to get the answers to some of the questions above.

Energy and Power

Energy is at its core a measure of work that can be done. In the physics world, work can mean a lot of things- moving a solid object, generating heat, or powering the electronic computer on which you're reading this. Energy can be used to describe electricity, or power from other means- the chemical energy a fuel or food contains, or the change in temperature of any object or fluid.

Lets start with two common scales to measure energy: the kilowatt-hour (kWh) and the British thermal unit (BTU). Conventionally, we use the kilowatt-hour most often to describe electrical energy, and the British thermal unit to describe thermal energy, or heat- as you may have deduced from the latter's name. Despite the convention, they are effectively interchangeable as measures of energy. If you've ever payed your electric bill, you'll recognize the kilowatt-hour- it's used universally across the US. The BTU on the other hand has it's roots in quantifying heating fuels, and a single BTU is defined as the theoretical energy required to heat a single pound of water one degree Fahrenheit (There are 8.33 pounds of water in a gallon).

A kWh is significantly larger than a BTU- in fact, 3,412.14 times larger. That's why it's sometimes more useful to report BTUs in kBTUs, a thousand BTUs- just as a kilowatt-hour is a sum of 1000 watt-hours.

1 kWh = 3.41214 kBTU

The average lightning bolt carries 1400 kWh (4775 kBTU)  of energy

So now we know some basic terms about energy. But when you are talking about energy, and especially electricity, you've got to also mention Power. While energy is a measure of total work that can be done, power is a measure of the rate at which energy is delivered- energy divided by time. Power is incredibly important for electronics, because they need a very stable source of power- too little energy at a time and a device won't function, too much and it could be completely fried- so yes, it's definitely important.

Power is most often talked about in the watt (W). You'll notice similarities in the terms “watt” and “watt-hour”. That's because a watt-hour is the amount of energy used by a device which draws one watt of power for one hour (yes, it's that easy). Similarly, an object that draws one kilowatt will consume a kilowatt-hour in one hour.

So now we know a little bit about both energy and power. But how much is a lot?

Household appliances will usually describe their power draw in watts somewhere on their packaging, which can be a good thing to pay attention to as a conscientious consumer. Here are some standard power levels of common household electronics:

Remember, however, that energy is consumed only when a device is on. Even though a microwave uses on average a kilowatt and a half of power, you probably run it less than 15 minutes a day, and your lights will be on for hours- more then likely, they're the greater contributor to your utility bill. And because lights are used so often, LED light bulbs- four times as efficient as incandescent bulbs- can save some serious electricity.

Fuels and Energy Content

The biggest home energy expense, however, is often heat- and more often then not, heat generated with electricity. Two-thirds of US homes use some sort of fuel- whether it be natural gas, heating oil, kerosene, or wood- to heat their homes. These fuels are all different and contain different amounts of energy.

Lets take a look at these different fuels:

Natural Gas 

Unless you have an electric stovetop, your stove likely burns natural gas-
emitting distinctive blue flames.

Natural gas is the most popular fuel for home heating- in much of the country it is delivered via efficient underground pipelines. Unlike most of the other fuels used in the US natural gas is, well, a gas- and actually more buoyant than ambient air. Natural gas is often measured by the considerably odd unit of the centum cubic feet (Ccf), meaning 100 cubic feet (From the latin prefix instead of the internationally accepted “hecto-”, from greek). 100 Ccf is most notably pretty close to 1 Therm of natural gas- defined as the amount of gas that contains 100 kBTU of energy. 

The conversion of Ccf to Therms actually varies based on the purity of the gas in question- but typically averages to about 1.023 therms, as of 2011. So we get the final conversion:

1 Ccf Natural Gas = 1.023 Therms = 102.3 kBTU

It's handy to bring this back to our definition of BTUs- 100 cubic feet of natural gas could heat 1000 pounds of water 102.3 degrees Fahrenheit, excluding efficiency losses. Put another way, with that much gas you could completely boil away about 11 gallons of water.

Fuel Oil 

Fuel oil is most often delivered by tanker truck

Fuel oil is the second most popular non-electric heating fuel in the US. Typically used in areas without gas pipelines, it has to be delivered by truck. Fuel oil is similar to other liquid fuels, such as diesel oil and gasoline, but with a slightly different chemical makeup. Fuel oil is sold by the gallon, and although the energy content can vary by composition, on average each gallon contains a little more energy than a ccf of natural gas.

1 Gallon Fuel Oil = 140 kBTU

Wood

A single cord of wood is quite a lot

I'm going to bet you know what wood is- it comes from trees, and all that. It's one of the first fuels used by humans, but now it's mostly used in rural states where it is cheap and abundant. It's often sold in bulk by the cord- which is defined as a stack of wood 4 by 4 by 8 feet, or around 128 cubic feet (some air space). A cord can vary in energy content by type of wood and even how it's stacked- but as an estimation, a cord of wood typically contains 12,000 kBTU.

1 Cord Wood = 12,000 kBTU

There's a lot of energy in a cord of wood- but a cord is a lot of wood. Don't worry, we'll normalize all these fuels to weight at the end of this section.

But before we do, there's a lot of fuels used in the US that aren't typically home heating fuels but are certainly worth mentioning. I'll bet you've heard of them.

Coal

 

Coal has dominated US electrical generation for decades, but natural
gas has begun to overtake it in 2013.

Coal is the waning king of US electrical generation- and one of the US's most plentiful natural fuels. It's a black rock rich in organic matter, and like oil, comes in several grades and purities. Coal is hardly used in small quantities, and is most often reported by the short ton- 2000 pounds.

1 Short Ton Coal = 28,000 kBTU

Gasoline

 

While cars running on electricity, natural gas, and even hydrogen do exist,
gasoline is by far the popular fuel choice

Chances are, if you're a human with access to a computer and the internet, you've used gasoline directly before. As the primary transportation fuel worldwide, it's hard to escape it. Gasoline is sold by the gallon (in the US), and is very similar to fuel oil. The kind of gasoline we use in cars is typically a little less viscous and easier to pump and transport than fuel oil, but contains slightly less energy.

1 Gallon Gasoline = 125 kBTU

Both fuel oil and gasoline are both generated from the same petroleum precursor- and the distinction between them is a little 'fluid'. There are many different petroleum fuels with varying energy contents and properties- but the numbers reported here are closest to US convention.

Uranium 

Uranium ore goes through considerable processing
before it can be used as a fuel

Nuclear power operates entirely differently than any other power source humanity currently operates. While renewable energy systems exploit natural energy gradients, and fossil fuels are burned to release energy, nuclear power captures energy from the nuclear decay of atoms. This results in radioactive waste- but also an extreme amount of energy. Because uranium can be processed and used different ways, defining the thermal heat content of uranium isn't exactly easy, but the world nuclear association reports:

1 Pound Uranium = 21,500 kBTU

That's a lot- much more energy per weight than any other fuel. 

Now we know some standard energy benchmarks for a lot of different fuels- but to be fair to those fuels, we should really standardize them by weight:

Standardized energy per weight is the energy-density of a fuel, a good indication of roughly how easy it is to transport and store the fuel. The higher the energy density, the less physical fuel is required to generate the same amount of energy. If you need to produce 1000 kBTU, it might be easier to transport 30 kg of gasoline than 115 kg of wood.

A Note on Tons:

Tons are a bit of a sticky unit- and the mainstream media misuses them often. A “ton” can mean several different things- so here's a little clarification:

In the US, a short ton is most common. This is equal to 2000 pounds. Interestingly, this is a unit of weight and not mass, but the distinction rarely matters.

Internationally, most often considered is the metric ton (or tonne)- which equals 1000 kilograms. As 1 kilogram is 2.205 lbs, a metric ton is actually bigger than a short ton.

Officially accepted in the UK is the long ton, which, for inexplicable reasons, is equal to 2240 pounds, making it the largest of the bunch.

It's incredibly important to understand what ton you're talking about when reading scientific literature- and if it isn't stated, consider the country of origin of the source material. Note that not all news articles get this right. Either way, here's a handy guide to how big tons are in relation to each other:

Scaling Up

Now we have a good idea of energy units and how much energy different fuels contain. Lets take a look at what that means on a large scale:

The average US home used 90,000 kBTU of energy in 2009. If the home was all-electric, this would be close to 26,300 kWh over the course of the year. On average, this amount of energy cost a home $2,024 USD.

Annual Household Energy Consumption = 90,000 kBTU = 26,300 kWh

This energy use varies a lot by size, type, and location of the home. But according to EIA stats, that's the average. And in 2009, there were around 113 million occupied homes in the US- so what does that mean in total? It means we need a bigger energy unit.

A Quad is equal to a quadrillion (10^15) British Thermal Units, or a trillion kBTU. At large scales, it's absolutely necessary. A Terawatt-hour is equal to 1 billion kilowatt-hours. 

1 Quad = 1,000,000,000,000 kBTU

1 TWh = 1,000,000,000 kWh

1 Quad = 239.07 TWh

So yes, a quad is bigger- but both are astronomically large. In 2009, US comes consumed:

Total US Residential Energy Consumption = 10.183 Quads = 2,434.4 TWh

That's a lot. But in actuality, most homes are powered by a mix of these fuels, or electricity generated by them. Electricity is typically less than half of energy use in homes, however- on average, 11,320 kWh.

The Electricity Grid

Electricity on the energy grid is wholly different. The largest uses are large industrial and commercial complexes- and the US electricity grid hit a full 3,883 TWh in 2011. We've got to power that somehow.

Solar panels are one of the cheapest options for home generation

Sunforce sells a small, 1 square meter solar panel with a maximum output of 130 Watts for about $700. Typical solar panels in the US have a variable capacity factor from 13-19%- so, over the course of a year, it can be expected to generate somewhere between 150 and 220 kWh. It's not a whole lot, but it'll take the edge off of your electrical bills.

Yearly Solar Panel Generation = 150 - 220 kWh

Wind turbines have cropped up in lots of areas across the US
recently, sparking debates about appropriate locations to place them.

A typical, modern wind turbine has a maximum generation capacity of 2.5 MW. Wind turbines are rarely operating at full capacity, but the average turbine of this size will produce somewhere close to 5,475 MWh in a year, enough to generate the energy consumption of 480 homes.

Yearly Wind Turbine Generation = 5,475 MWh

Hydroelectric Power is the largest sector of
renewable energy in the world today

Commercial hydroelectric plants are a great, reliable source of electricity. While they vary in size, the average US plant has a maximum output of just under 20 MW – close to that of 8 wind turbines- and an average US capacity factor of 46%, meaning that it's generating power much more often than wind. That means we're generating 80,000 MWh in a year- powering over 7,000 homes. Pretty good.

Average Yearly Hydroelectric Plant Generation = 80,000 MWh

Fossil fuel plants are the mainstay of the US electrical grid

US fossil fuel & nuclear reactors can vary pretty wildly in size, but large, grid-baseload units can max out with a capacity of almost 1 GW. That's quite a bit- equivalent to the maximum generation of 400 wind turbines. Because these units consume costly fuel, many only run when they're needed, but a baseload coal or nuclear power plant can operate at full output 85% of the time and generate 7.4 TWh in a year.

Large Coal/Nuclear Plant Generation: 7,450,000 MWh

That's a lot. These large baseload plants generate electricity around the clock to keep you supplied- and each one produces enough electricity to power 658,000 homes. 

OK- now we know how big power plants are. Lets crank this up a notch.

The National Energy Flow

We've already mentioned that the US consumed 3,883 TWh in 2011. It in fact generated 4,054 TWh- some electricity is lost in transmission, and some is actually bought and sold across our borders with Canada and Mexico- but we're a net importer, with just over 47 TWh imported, the majority of it from Canada. That means that about 5.32% of electricity put onto the grid was wasted, lost through inefficiency. 

Generating 4,000 Terrawatt-hours isn't easy, and the US has used a mix of generation strategies to do it. 

In 2011, the US generated:

1.8 TWh from Solar Energy

15.3 TWh from Geothermal Energy

19.2 TWh from Biomass and Waste Burning

30.2 TWh from Petroleum & Oil

37.4 TWh from Wood & Wood Derived Fuels

120.2 TWh from Wind Energy

319.4 TWh from Hydro Electric Power

790.2 TWh from Nuclear Electric Power

1,013.7 TWh from Natural Gas

1,733.4 TWh from Coal

25.7 TWh from Other Sources & Unconventional Fuels

And what about our fuels? Well, we used some of them in the generation of the electricity above- but we used more than just that to generate heat for our homes, mechanical energy for our cars and energy for industrial processes and manufacturing. In sum total, we managed to consume a lot:

702,000,000 short tons of Coal
243,935,000,000 Ccf of Natural Gas
282,944,000,000 gallons of Petroleum

Overall, all of these energy sources contributed to a total use of just over 78 Quads of primary energy in 2011. That's 78,000,000,000,000,000 British Thermal Units.

Now that is a pretty big number.