*Comparing and visualizing common forms of energy*

### What's a kilowatt-hour?

Although we still don't know what energy is at its most fundamental level, we know a lot about it. Energy is that mysterious thing that allows changes to happen. Energy can be transformed from one form to another. We measure energy in many different units, but the SI unit of energy is the joule (J), equal to the energy required to exert a force of one newton on a mass through a distance of one meter.

One joule of energy transferred every second is called a watt (1 J/s = 1 W). A watt is a unit of power, which is energy transferred in a given amount of time. One watt delivered for an hour is equal to 1 watt-hour (1 watt for 1 hour = 1 Wh), which is a unit of energy. Since there are 3,600 seconds in one hour, 1 Wh = 3,600 J. I tend to use watt-hours or kilowatt-hours (1 kWh = 1,000 Wh) as a standard unit of energy because it's the unit on our electric bills that most people are familiar with. One watt-hour is very roughly the amount of energy required to lift an adult up two flights of stairs.

1 kWh = 1,000 watt-hours

1 MWh = 1,000,000 watt-hours 1 GWh = 1,000,000,000 watt-hours

##### Ready for a fun quiz?

Use your intuition.

**What contains more energy?**

1 cup of gasoline or a car traveling 100 mph?

1 hamburger or a 100 lb anvil raised 10,000 feet off the ground?

The amount of energy required to heat 1000 lbs of steel to its melting point (2500°F) or the amount of solar energy that hits an average American house in a day?

Energy comes in many different forms that can be converted. Some common forms include gravitational potential energy, kinetic energy, elastic energy, magnetic energy, chemical energy, nuclear energy, thermal energy, electrical energy, radiant energy, and several others.

Forms of energy can be subdivided in different ways, but perfect classification is not that important. For example, a system's macroscopic kinetic and potential energies together are called mechanical energy. However, kinetic energy at the microscopic level is called thermal energy because it's the incoherent movement of the individual particles in all different directions.

### Energy Equivalence

The following is a list of different forms of energy and a description of what that looks like in practice.

1 kWh of electrical energy – An EV sedan driving 4 miles.

1 kWh of chemical energy in gasoline – A gas-powered sedan driving 1 mile.

1 kWh of thermal energy – Roasting a chicken in the oven.

1 kWh of thermal energy – Heating 5 gallons of water from room temperature to 150°F.

1 kWh of chemical energy – A large hamburger.

1 kWh of radiant energy – The sunlight that hits 1.5 square feet of ground per day in Utah.

1 kWh of kinetic energy – A large school bus traveling 55 mph.

1 kWh of chemical energy – Half a pound of firewood.

1 kWh of gravitational potential energy – Raising a man 3 miles above the ground.

1 kWh of gravitational potential energy – Raising an SUV 500 feet above the ground.

1 kWh of chemical energy – Half a cup of gasoline.

1 kWh of radiant energy – The light from 20 LED light bulbs on for 10 hours.

1 kWh of electrical energy – Running a home air conditioning system for 12 minutes.

1 kWh of nuclear energy – 0.05 milligrams of fissile Uranium.

1 kWh of thermal energy – Hot water for 4 minutes of showering.

1 kWh of chemical energy – A man running 6 miles.

This small sampling gives us some perspective on how much energy is contained and used in everyday things. All of these things represent 1 kilowatt-hour of energy. It's always funny to see how little energy a human is capable of putting out compared to the energy sources we take for granted. A person would have to work hard on a rower or a stationary bike for 167 hours just to produce the same amount of energy contained in 1 gallon of gasoline (that can be bought for $3.50!)

While energy can be converted between all of these forms, the conversion process is never 100% efficient, unless our goal is to convert some form of energy into heat. Low-quality heat, heat that's about the temperature of the surroundings, is the energy tax paid by any energy conversion.

So before you go screaming "see, we need nuclear!", we must take into account that we cannot convert all of the energy in the fissile uranium into usable energy. Most of that energy from the nuclear fissions is lost as unusable heat.

Understanding how much energy is contained in different things—and the __entropy__ of that energy—is critical to designing energy storage systems.

### Energy Storage Systems

With the shift to intermittent renewable energy, energy storage is the key to matching energy supply with demand. There are several considerations when designing energy storage systems, including size, cost, and "what's this going to look like?" Let's look at a few of the most well-known options.

##### Pumped Hydro

Water is pumped to a higher level to store energy as gravitational potential energy, and released back through turbine generators to a lower level to recover the energy. A pumped hydro facility that can store 1 gigawatt-hour of energy must pump 640 billion gallons (about 970,000 Olympic swimming pools) up 500 feet in elevation. This is feasible and is being done on a much larger scale.

##### Raising Weights

A few companies are experimenting with lifting heavy blocks to store energy and then lowering them when energy is needed. If we took a 10-foot concrete cube weighing 150,000 lbs and raised it up 100 feet, it would store 5.6 kWh of electricity. To store 1 GWh of energy, we'd need to lift 180,000 of these blocks 100 feet off the ground.

##### Lithium-Ion Batteries

To store 1 GWh in lithium-ion batteries, 260 Tesla Megapacks would need to be used. Each Megapack stores 3.85 MWh, is 29'x5'x9', and costs about 1 million dollars.

##### Thermal Energy Storage

Sand, rocks, bricks, water, or some other medium is heated to high temperatures; then the heat is recovered later. It's worth noting that the stored thermal energy's exergy content (energy quality) is relatively low. So even if we can extract much of the heat back from the thermal energy storage system, it's not of high enough quality to be converted to electricity or do much of anything besides space and water heating. Sand has a heat capacity of about 6.1 Wh per cubic foot °F, so to store 1 GWh of heat, we would need to heat 1.37 million cubic feet of sand from 80°F to 200°F. That's a cube of sand about 110 feet on each side.

These energy storage systems are just a sampling of those that exist and are simply rough calculations to give us an idea of the scale of what it would look like to store 1 GWh of energy in different forms. When evaluating energy storage systems, it's important to consider other factors such as round trip efficiency, exergy storage, and other practical implications.

For more perspective, the average American uses about 80,000 kWh of energy per year. So 1 GWh of energy storage is enough for about 12 Americans for a year.

##### Quiz results:

**1 cup of gasoline has 2.09 kWh of chemical energy.**A car traveling 100 mph has 0.47 kWh of kinetic energy.**1 hamburger has 814 Wh of chemical energy**. A 100 lb anvil raised 10,000 feet off the ground has 377 Wh of gravitational potential energy.The amount of energy required to heat 1000 lbs of steel to its melting point (2500°F) is 383 kWh.

**The amount of solar energy that hits an average American house in a day is 1070 kWh.**

##### Questions for you:

How did you do on the quiz? Are the results surprising?

Looking at energy equivalence of different things, what is something that stands out to you?

What other ideas do you have for energy storage systems?

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