To understand energy use, start with the basics– how much energy you need every day. You eat about 2000 Calories a day, so lets use that as our baseline because everyone in the world can relate to energy in food for a day. So we define our daily energy unit as: 1 D = 2,000 Calories (per day). 

With that simple definition, we are ready to relate our energy consumption for other activities to that most basic amount of energy needed for us to live. To see how this D unit works, lets start with another basic item: a simple old-fashioned incandescent light bulb rated at 100 watts (W). This bulb is rated not in terms of light it produces (that would be Lumens) but instead in terms of the power it uses, which is continuous energy use. So we need to understand units of Watts (power) compared to energy units of Watt hours (Wh). A Watt is energy per time, so a light bulb that is always on uses 100 W. But what if you only use it for say 1 hour? Then, the energy used in that hour, 100 W times 1 hour , or 100 Wh. If it was on for 2 h, it used 200 Wh and, for 3 hours 300 Wh and so forth. If it is on all day it uses 2400 W h (100 W times 24 h). Lets now compare our definition of D in terms of energy used per day to Wh and kiloWatt hours (kWh) in terms of daily energy units: 1 D = 2000 Calories = 2320 Wh = 2.32 kWh
So your food you eat each day is equal to about the same as a 100 W light bulb turned on for one full day! (Well, technically, for 23.2 hours out of 24 h day.) Your daily food is therefore the about the same as the power consumed by a 100 W light, or in daily energy units, one 100 W light bulb consumes 1.03 D.

Converting different energy units into kWh and D

Now we can consider other energy uses in our lives. For example, the average heating use with natural gas for a home in the US is 72 CCF of gas in a month, or 2.4 CCF of gas per day. A CCF is one hundred cubic feet of natural gas. But other energy consuming activities may be in different units. For example, gasoline might be in BTU (British Thermal Units), and an electric bill in kWh. How can we relate CCFs to other energy units? Many engineers and scientists prefer units of kWh for different types of energy use, so lets start there with expressing some things in the energy used for one day for these things:

• Average daily electricity use for a US house = 30 kWh
• 1 gallon of gasoline used in my car for one day = 120,300 BTU = 35.3 kWh
• Natural gas for daily heating a US house = 2.4 CCF = 73 kWh

Now we can start to see how these things compare to each other. But we still lack a way to appreciate this energy use a kWh is still a pretty abstract unit. Instead, lets put these and other daily energy uses into units of D. For the above divide the given numbers in kWh by 2.32 and you get units of D, so you have:

• Food for 1 day =  1 D
• Running a single 100 W light bulb all day =  1.03 D
• Average daily electricity use for a US house =  13 D
• 1 gallon of gasoline used in an average car (goes 18 miles) =  15.2 D
• Natural gas for daily heating a US house =  31 D

Now these are numbers we can all relate to because the ratio back to the food we eat every day. We can see that gallon of gas uses about 14 times more energy in a day than it takes to provide energy for 1 human being. Our house actually uses less than that energy over a day than we use in a gallon of gasoline. Heating with natural gas heating consumes far more energy than any of these others, which is why heating and cooling in a house is so essential for home energy use. See the Examples Page to learn more about home heating. Here are a few more examples of energy use in units of D:

• Energy for a pot of coffee = 0.035 D
• Washing machine for a house = 0.06 D
• Old refrigerator = 1 D
• New refrigerator = 0.6 D

Lets aim for 20 D

The total energy use per person in the USA is 101.6 D. That is based on 98 Quads of energy used per year (a Quad is a quadrillion BTU). The nice thing about the D unit is that it expresses energy use essentially on a 1 to ~100 D scale. How much should we reduce energy consumption? A study by the Federal Institute of Science in Zurich, Switzerland, suggests that we should aim for 2000 W per person, or about 20 D. A newer study clarifies that only 5 D of this energy could be from fossil fuels. That means in the US we need to reduce energy consumption by 80%, and have only a very small portion of our energy used derived from fossil fuels. That is quite a challenge! For comparison purposes, Swiss citizens use 54 D, about half of what a person uses in the US.

Thinking about what you eat in terms of D

If you see a sandwich and it says it is 720 calories, you might not want to dig into the calculation of how much that is relative to the daily total. But with D units, it is really easy! If you know you need 1 D every day, then just think of food as % of D. For example, that sandwich is 36% of your D for the day. Wouldn’t counting “Calories” be so much easier if everything was labelled in D % units? That way, all food items are a percentage of what you need every day. Here are some examples:

• Breakfast: Denny’s Ham and cheese omelet = 39% D
• Lunch: McDonald’s Big Mac, fries, medium drink = 55% D
• Dinner: Outback Charbroiled Ribeye (12 oz) , baked potato and house salad =  76% D
• Drinks with dinner of (2 medium sodas or 2 beers) = 15% D
  Oops! You just ate 185% of your daily required calories. (And sorry, substituting that sandwich for the burger lunch wouldn’t have helped all that much.) Also, to loose a lb of fat is 3600 Calories, or if you lost one every day, you would need to lose 1.8 D. That seems pretty difficult if you only eat 1 D in a day.
  Lets eat better, smarter, and waste less food. D units can help!

 

Updated: February 8, 2021