One issue which constantly frustrates me about discussions of energy efficiency is the completely disorganized use of units.

FPL estimates it will cut its natural gas use by 1.3 billion cubic feet each year, the consumption of 18,000 American homes. It will also cut carbon emissions by 2.75 million tons over 30 years, the equivalent of taking 19,000 cars off the road.  (from NYT http://www.nytimes.com/2010/03/05/business/05solar.html?pagewanted=2&sq=solar&st=cse&scp=4)

So what? How much energy are we talking here?  18,000 American homes?  1.3 billion cubic feet of natural gas?  Is 2.75 million tons of carbon big or little?  How does that compare to the scope of HUGE problems like global warming?  How does that compare to micro-scale issues like the idle wattage of my cell phone charger?

We need to speak in consistent units in order to get a sense of how different energy issues compare to each other.

In his excellent book "Sustainable Energy: without the hot air", David McKay proposes the following:

     The unit of energy I have chosen is the kilowatt-hour (kWh). This quantity is called “one unit” on electricity bills, and it costs a domestic user about 10p in the UK in 2008 [Jonah: the residential cost in the US circa 2009 was 11.6¢/kWh]. As we’ll see, most individual daily choices involve amounts of energy equal to small numbers of kilowatt-hours.

     When we discuss powers (rates at which we use or produce energy), the main unit will be the kilowatt-hour per day (kWh/d). We’ll also occasionally use the watt (40 W ≈ 1kWh/d) and the kilowatt (1 kW = 1000 W = 24 kWh/d), as I’ll explain below. The kilowatt-hour per day is a nice human-sized unit: most personal energy-guzzling activities guzzle at a rate of a small number of kilowatt-hours per day. For example, one 40 W lightbulb, kept switched on all the time, uses one kilowatt-hour per day. Some electricity companies include graphs in their electricity bills, showing energy consumption in kilowatt-hours per day. I’ll use the same unit for all forms of power, not just electricity. Petrol consumption, gas consumption, coal consumption: I’ll measure all these powers in kilowatt- hours per day. Let me make this clear: for some people, the word “power” means only electrical energy consumption. But this book concerns all forms of energy consumption and production, and I will use the word “power” for all of them.

     One kilowatt-hour per day is roughly the power you could get from one human servant. The number of kilowatt-hours per day you use is thus the effective number of servants you have working for you.

McKay's brief discussion of units is available (along with the rest of the full book!) online, here: http://www.inference.phy.cam.ac.uk/withouthotair/c2/page_24.shtml

Calculating Carbon Emissions

Use of consistent units also allows us to easily translate into carbon emissions for any activity.  McKay has a great translation chart here: http://www.inference.phy.cam.ac.uk/withouthotair/cL/page_371.shtml