Thus energy may be regarded as the universal currency of the Sciences, and we can follow it through its various transformations by means of a sort of accounting procedure which can be highly informative. To do this, we need to use the right kind of units: and, rather predictably, the traditional units of energy are in a fine, rich state of muddle.
Mechanical engineers have tended to use foot-pounds, physicists are addicted to ergs and electron-volts, chemists and dietitians like to use calories, but our gas bills come in therms and our electricity bills in kilowatt-hours. Naturally all these are mutually convertible, but nowadays there is a good case for using the SI unit of energy, which is the Joule, that is the work done when one Newton acts through one metre.

Structures- or Why things don't fall down - JE Gordon