Technical

A Primer on Energy vs. Power: The Power of the Big Picture

With great power comes great responsibility

Many of us are privileged to wield shocking amounts of power every day.  I don’t mean political or socioeconomic power here, but rather the myriad physical forms of power around us: the kind that runs your lights, charges your phone, heats your shower, and fuels your body.

Don’t worry: this is not another eco-shaming or a puritanical sermon on the virtues of austerity.  My primary goal is to open your eyes to the variety of powers you have and how they’re all interconnected.  After all, our success as individuals and as a species comes from harnessing various kinds of power.  My second goal is to help you – true to the Mindtribe method – focus on the most important thing(s).  In an increasingly crowded planet with increasingly scarce resources, it’s a very wise thing to understand flows of power.

Today, in part 1, we’ll start with a primer on energy vs. power and expose the power that’s quite literally hiding under our noses: food.

Before we begin, I’d like to remind you that power and energy have different meanings.  We all know that saying, “I drove a mile” versus “I drove at 20mph,” mean different things.  The same goes for power and energy.

Let’s start with energy.  Energy is what it takes to do work.  We measure energy in all sorts of ways: calories (for working out), gallons of gasoline (for moving cars), and number of electrons (for charging phones).  If you don’t quite grok what energy itself is, don’t worry – that’s a topic for philosophers and theoretical physicists, many of whom are still not sure.  Two things are clear: we need it and it’s not free.

Power, in science-speak, is how quickly you use up energy.  A bigger charger can provide more power to charge your iPad more quickly, but it still puts the same amount of energy into the same size battery as a smaller, less powerful charger.

The table below summarizes the difference between power and energy and provides typical units of measure:

Power Energy
Example Quantities 100-watt light bulb burns brighter and uses electricity faster than a 60-watt light bulb. A utility sells you 1 kWh of energy for $0.12. Your iPhone battery holds about 6 Wh (0.006 kWh). 5 gallons of gas. A 2,000-calorie milkshake.
Basic Definition How fast energy is consumed. What it takes to do work.
Units
  • Watts (1W = 1 J/second)
  • Kilowatts (1 kW = 1000W)
  • Horsepower (1 hp = 746W)
  • BTU/hr
  • Joules (J)
  • kilowatt-hours (kWh)
  • British Thermal Units (BTUs)

Ready? Over the course of a couple blog posts, let’s compare the power you control when:

1. Working out intensely

2. Eating burgers and other fast food

3. Driving your car, either aggressively or smoothly

4. Taking a hot shower

5. Leaving your phone charger plugged in (i.e., is “vampire power” real?)

A brief disclaimer: I’m going make some simplifying assumptions here to keep things manageable.  These numbers won’t be 100% accurate, but that’s not the goal.  The goal is to show you how easy it is to get a back-of-the envelope estimate.  For readers who find the following approach elementary, there’s a whole world of research to explore in embodied energy, life cycle assessment, global energy transfer, and energy resources engineering.

For a sense of scale, the average human might exert 100 watts of useful power during an intense, hour-long workout.  A world-class cyclist could output 400 watts.  We burn energy while sedentary, too; the constant speed or power at which we burn energy just to stay alive is called the Basal Metabolic Rate (BMR) and is about 100 watts for a person on a 2,000 calorie daily diet.  Any activity on top of that, such as the exercise situations above, means you’ll need to eat more!

If working out and just being alive burn food energy (slowly, to the chagrin of people trying to lose weight), how quickly do we gain food energy when we eat?  Let’s assume again a 2,000 calorie per day diet; that’s the energy input.  To calculate power, what do we need?  Time!  How long do you spend gaining those 2,000 calories?  In my case, I’ll use 75 minutes across my three meals.  We’re ready to go:Screen Shot 2016-03-03 at 12.40.33 PM

In order to get to watts (Joules per second), we’ll have to convert from calories to joules.  There’s a sneaky gotcha here.  Just as we Americans hold out against the metric system, we also misleadingly print “Calories” on our Nutrition Facts labels when we really are indicating kilocalories, or 1000 calories in scientific terms.  Note how the British label below (from foodlabel.org.uk) uses the accurate specification of “kcal” for kilocalories.

ex

The last ingredient we need is that 1 calorie = 4.184 joules.  Now, we calculate:Screen Shot 2016-03-11 at 2.23.46 PM

While I’m eating, I’m actually gaining energy at a rate of 1.86 kW.  I’m a 1.86 kW (2.5 horsepower) eating machine!  That’s 20 times faster than I burn it while sitting in the office.  Though this is an impressively large figure, competitive food eaters blow me out of the water.  Joey Chestnut, the 2013 Nathan’s Hot Dog Eating Contest champion, ate 69 hot dogs in ten minutes, corresponding to 187 kW.  That’s 250 horsepower.

Large numbers are fun when it comes to playful world records, but sobering when it comes to the true cost of food and its wanton waste.  In our modern food system, convenience comes at an energy price; various sources estimate it could take as much as 10x the calorie value of food in fossil fuel energy to produce and deliver food in America.  Let’s see what that means for my average “food power,” or the average power input that my body needs 24/7.  First, let’s average my 1.86kW, 75-minute eating rate over the entire day:

Screen Shot 2016-03-11 at 2.33.26 PM

Next, let’s apply the 10:1 factor: my 97W rate balloons to 970W of total power input from farm to fork.  To make matters worse, Americans waste unbelievable amounts of food – up to 40% according to the NRDC.  When we factor waste in, 970W becomes 1.62 kW.  Considering that the average American uses 1.4 kW of electricity (a 24/7 average from the 2010 Energy Almanac), it’s clear that food is a serious contender when it comes to big picture power.

What’s a good person to do?  My top tips:

  • Don’t waste food.  I nearly halve my food power needs by eliminating waste.  Think twice before throwing away weeks worth of agricultural growth, resource-intense processing, and oil-laden transportation costs.  You have no excuse for taking too much food when it comes to self-serve buffets and cooking at home.
  • Eat less meat.  You don’t have to be a strict vegetarian to have a large impact – for example, consider becoming vegetarian just for lunch or only on Mondays.  Feeding and raising livestock is incredibly energy intensive compared to plants.  My parents lived in a time when meat was a rare treat and its true environmental cost suggests it should be (not to mention health concerns).  
  • Buy local.  Transporting food takes a lot of energy too.  The NRDC report I quoted above has a great section on this.

This is more than just trivia; by expressing our myriad human activities in one common form, we suddenly see everything in its big picture context.  Your food choices are energy and natural resource choices in disguise, and, as we’ll see next time, carpooling or biking to work really saves a lot of energy!

May this post serve as more than just food for thought, but a call for you to examine your own power and live with greater respect for the resources that sustain you.  See you in Part 2!