There's a problem with y for year. The unit y is neither an SI unit nor a unit approved for use with SI. The largest unit that is approved is d, for day.
Bill Potts Roseville, CA http://metric1.org [SI Navigator] -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of J. Ward Sent: Thursday, August 23, 2007 19:32 To: U.S. Metric Association Subject: [USMA:39273] Energy units I've written two prominent experts in energy requesting that they use SI units when talking about energy. One responded that he "entirely agrees," while the other says he's been going back and forth as to which units to use. The biggest difficulty is how to define energy, since, for example, the electricity produced by burning coal is the total thermal energy times the efficiency of converting the heat into electricity. Anyway, here is what I said in my letters. -J Dear ..., I would like to suggest a way to help people understand energy issues more clearly. It's very hard to understand energy issues when every source of energy uses its own units of measurement. It would be much more clear to use a single standard unit for power, preferably terawatts. For example, if you express oil production in TW (instead of barrels per day), coal production in TW (instead of tons / unit time), average solar production in TW (instead of kWh / day or kWh / year), natural gas in TW (instead of therms / unit time), etc. then it becomes very easy to compare all energy sources on a more-equal footing, and also to relate them to consumption. As an additional benefit, the watt is not only the world standard (SI) unit for power, it is also a unit that most people are familiar with from direct experience in their daily lives, and the "tera" prefix has become familiar from computer data storage. Choosing a single unit for energy would be equally helpful. The most relevant time-scale for discussions of fossil fuels and climate change is the year. Thus, if one were already consistently using TW for power, then expressing energy in terawatt years would make comparisons between reserves, production, and consumption extremely quick and simple. For example, 100 TWy of reserves being consumed at a rate of 4 TW implies a time-scale of 25 years. On the other hand, exajoules would be the relevant SI unit for energy. The UN World Energy Assessment uses exajoules to seamlessly compare different energy resources, then uses EJ / year to relate this back to average power to compare with consumption. Either way, I hope you will consider using TW and TWy or EJ and EJ/y to help people understand energy issues more easily. Sincerely....
