[jfim]

Maybe your utility company bills in joules, but mine does in kWh. You can easily figure how many kilowatts your bulb does (divide by 1000), and you can come up with an estimate of how many hours you use it in a day/month/year, which is then easy to turn into an actual dollar amount.

A joule is just a watt second instead of a kilowatt hour, and the 3600 factor (seconds per hour) is really annoying to use in mental math.