A Way to Think About Converting Units of Measurement

By Julio Friedmann

One of the more obnoxious issues facing energy specialists is the constant need to convert units. We have to convert megawatts to kilowatt-hours to tons CO2 to cost of electricity in order to tell a story. For those of you paying attention, these units are not the same (in order, they are units of power, electric energy, emissions and dollars).

To help sort this out, I find it helpful to use a single, big, coal-fired power plant -- about 1,000 megawatts (MW). The one here is the Mountaineer plant in West Virginia, which is about 1,300 MW.

To help frame the discussion, a 1,000 MW power plant emits between 5 to 8 million tons of CO2 every year. The range relates to the kind of coal, the kind of plant, the altitude and other factors. I find it helpful to simplify this way: 1,000 MW coal power = 6 million tons of CO2/y.

Most natural gas plants emit half that, so: 1,000 MW natural gas power = 3 million of tons CO2/y.

Many people think about this in terms of cars on the road. Roughly speaking, 250,000 cars in the U.S. emit 1 million tons of CO2 each year. Given that: 1,000 MW coal power = CO2 emissions from 1.5 million cars.

A few additional helpful framing units: 1 gallon of gasoline contains 5 pounds of carbon and will emit 18 pounds of CO2.

To put this in perspective, the carbon content of a gallon of gas and a five-pound bag of charcoal are about the same.

When burning fossil fuels for energy, it's helpful to think of the emissions from any fuel-based substances as the equivalent amount of energy produced. In crude, kicking-around numbers: 1 unit coal emits = 1.5 units oil emits = 2 units natural gas. (Note: this means that natural gas still emits plenty of greenhouse gas.)

One final thought: Many people have a hard time getting their brains around the benefits of efficiency and conservation. Dr. Robert Socolow of Princeton did some excellent work trying to explain how big and complicated the energy/climate challenge is, in particular what applying today's technology can do.

In talking about automobile efficiency, he ran this case:

  • There are about 800 million cars in the world today.
  • By 2055, there will likely be about two billion cars.
  • If there is NO improvement in efficiency, then those cars will emit an addition four billion tons CO2/year.
  • Today, these cars have an average efficiency of between 25-30 miles per gallon.
  • If we increase the car efficiency everywhere worldwide to 60 miles per gallon, there is no increase in emissions and we effectively save four billion tons CO2/year we were gonna own.
  • Doubling car efficiency while doubling the number of cars does not reduce emissions at all.

This is why I have good days and bad days. I explain that in my next two posts.

Julio Friedmann is the Carbon Management Program Leader for Lawrence Livermore National Laboratory, and the technical leader for the clean coal consortium under the U.S.-China Clean Energy Research Center.