# Why Use SI Units?

### Give this a try!

 Quickly, decide which of the values on the right is the largest. Hint: All of them represent solar intensity. (A)    117 calories per square centimeter per month,  (B)    254 BTU per square foot per minute,  (C)    2 MW generated per 130 acres of solar collector, or  (D)    1/2 cord of white oak per acre per year? Now decide which of the values on the right is the largest. They are also units of solar intensity. (A)    50 watts per square meter  (B)    100 watts per square meter  (C)    0.12 watts per square meter, or  (D)    200 watts per square meter

 Here, it is childs' play to decide which is the largest, because all the values have the same units.  For many decades, the official system of units in The United States has been the International System of Units, called SI (Systeme Internationale). Canada changed abruptly to this system about 20 years ago, and now only the old people remember what a foot is, or what a gallon is. In the US, the reluctance to use sensible units has been very strong!    The main impediment to understanding solar energy is the proliferation of bizarre units.  Do everything in SI units and everything becomes very simple.