*Saul Griffith is an inventor and entrepreneur. He did his PhD at MIT in programmable matter, exploring the relationship between bits and atoms, or information and materials. Since leaving MIT, he has co-founded a number of technology companies including www.optiopia.com, www.squid-labs.com, www.instructables.com, www.potenco.com, and www.makanipower.com.*

How do we measure energy and power?

If you would like to quantitatively understand the relationship between your lifestyle, global energy use, and climate change, you need to establish the language with which you can translate between these things. There are many different ways we use energy, many different ways we produce energy, and many different consequences environmentally. Power and energy are being measured around us all of the time. You get your electricity bill in kilowatt hours (kWh), your gas bill in Therms or British Thermal Units (BTUs), your car's performance is measured in horsepower, and your lightbulbs are rated in watts. To compare these things you need a common set of units, and we've already encountered 4 different units (kWh, BTU, Hp, W), and two different concepts - energy and power -- and we've only just started.

The first problem with comparing these things is that some of them (BTUs and kWh) are measures of energy consumed, and some of them (horsepower and watts) are measures of power. To add to this confusion, some of them are measures of primary energy (barrels of oil equivalent, or metric tons of coal), some are measures of net electrical power at your outlet (W), some are measures of thermal energy or heat, and some are measures of net mechanical power (Hp at the wheels of your car). To wade your way through all of this, you need an intuition for the difference between energy, and power. Energy can actually be an abstract concept, while people often have a more intuitive understanding of power-- "my car has 200 horsepower!˝

Energy is required to do work. Work is the exertion of a force over some distance. You perform work on an apple when you lift it from the ground to a table. It takes roughly 1 joule of energy to lift an apple from the ground to the table. It takes 1 watt (1 joule / second) to lift that apple from the ground to the table in one second. Energy is the measure of how much work can be done, whether it be moving apples, heating your house, or driving your car. You transform energy from one form to another when you do work. For example, you convert the chemical energy contained within gasoline to mechanical energy of rotating the crankshaft when it is burnt in an internal combustion engine. The energy that doesn't make it to the crankshaft is converted to heat. That's why your engine gets hot.

Power is the rate at which you consume energy or do work. Lifting the apple onto the table quickly requires more power than doing it slowly, but the same amount of work is performed. A more powerful car engine can accelerate you to 65 mph faster than an engine with less power, but they both get you to 65mph.

If I were powering the laptop I was writing this on by lifting apples from the floor to the table, I'd have to be lifting a crate of 40 apples every second to do so. That's quite a lot of work. Energy is a quantity, whereas power is a rate.

Quantitative comparison of aspects of your life (or 7 billion peoples' collective lives) could be made in terms of energy or power (or even carbon). If you use energy, you are bound to ask questions about the time period: is it the amount of energy in a month? Or over a lifetime? It was those questions that convinced me to start thinking in terms of power rather than energy. The rate at which your lifestyle uses energy is a convenient measure that gives you a single number to think about your energy use, power consumption, and ultimately environmental impact.

But having decided to talk about power, we still needed to decide upon the right units to talk in. Should it be kilowatt hours per day? Horsepower? BTUs per month? Watts? Kilowatt hours per day measure the use of electricity well. Horsepower measures the use of mechanical power well. BTUs per month describe the use of heat well. Watts, however, are universal, and are in fact the scientific standard as defined by the Système Internationale, so we decided to use them to measure our lives. Even though I'm talking in Watts, you'll still need to think occasionally about energy, especially in the embodied energy of objects. It isn't easy, but it is necessary. At least we are down to only two units, and they are fundamental: Watts (Power - rate), and Joules (Energy - quantity).

Trying to understand the global energy system requires understanding power use on many different scales. Billions of people each use thousands of watts of power, and the way they use that power and get that power varies enormously. It's very difficult to have an intuition or understanding of all these different units and numbers. We all have a rough understanding of the amount of power in a light bulb. We have a sense of the power of an automobile. We speak of powerful winds. Many people have stood at the side of Hoover Dam or Niagara Falls and have been awed by the raw power in front of them.

Wikipedia nicely lays out the power consumption of various activities at different orders of magnitude.

Wikipedia provides examples of the energy required to do different things at different scales.

This Wikipedia page contains an excellent converter between various energy and power units.

Now, everyone else talks about "Carbon Footprint." Carbon dioxide is the problem, isn't it? If so, why am I talking about energy and power, joules and watts, instead of CO2 and PPM?

The best answer to this is that calculating their "carbon footprint" merely makes people want to reduce their carbon footprint. Yes, the carbon is a problem, but let's imagine that it wasn't (perhaps even wish that it wasn't!). Calculating my lifestyle in 2007 on Wattzon, I needed 18kw of power. If 6.6 billion people used that much energy, the world would use more than 100TW. Global world energy production currently is 15-18TW. It is extremely unlikely that we are going to be able to make more than 100TW of power, fossil-fuel-based, green, nuclear, or otherwise. On top of reducing carbon footprint, people are going to have to simply use less energy -- hopefully while improving their lives.

As I'll explain later, the production of non-carbon emitting energy, say by using solar panels, requires a very large area of land. By talking about power instead of carbon, we will help you understand the trade-offs of all the various methods of producing humanity's power -- even the renewable energy hopefuls aren't perfect. If there is a not so subtle subtext to my blog posts, it is that the energy challenge is a game of trade-offs and compromises. It's actually a design problem; the analogy I like to use is that we are designing the garden that is earth, and we are choosing where to put the rose beds, the organic veggies, the compost heap, and the irrigation system. The choices we make in the design will effect the quality of the garden, and its variety.

There's another, less obvious reason why I talk about power instead of carbon. The carbon footprint thing leads to a shell game: "I drive a lot, so I have a large footprint. I buy an electric car so now I've reduced my footprint." Well, maybe ... it depends on where the energy came from and how big your electric car is. If you got the power from a coal power plant and it is an electric SUV, you are still using about the same amount of power and producing about the same amount of CO2. If you drive a 6000lb SUV at 75 mph, you're going to burn a lot of energy. (This is also ignoring the embodied energy required to build your shiny new electric car). The hope is that if you do your accounting in energy and power, then there's a better chance of being grounded in a number that's not process-based and so doesn't tempt you just to switch the process (eg. from gas in your tank to coal at a power station). We'd like to inspire people to solve this problem by making intelligent consumer choices, not trying to buy things to solve the problem that ultimately exacerbate it. The solution is as much about more efficient and lower-energy ways of doing things as it is about making carbon-free power.

For reference, here is a table of the amount of CO2 produced making 1 million joules (1 MJ) from different processes:

Natural Gas - 53 g/MJ

diesel - 69 g/MJ

gasoline (petrol) - 67 g/MJ

coal - 83 g/MJ

These emission ï¬gures are taken from DEFRA's Environmental Reporting Guidelines for Company Reporting on Greenhouse Gas Emissions.

This Wikipedia page contains an excellent converter between various energy and power units.

To begin estimating your own power consumption, you can use Wattzon.