Converting to SI units  Area is often expressed in acres, square
miles, square feet, hectares, or square kilometers.
Energy is often measured in calories, kilocalories, or BTU (when people talk about heat) or in ergs, footpounds, or kilowatthours (when people talk about mechanical energy), or in barrels of oil or tons of TNT (chemical energy). Power is often measured in BTU per hour (heat), horsepower (mechanical). Time is measured in many common units, minutes, hours, days, or years. Imagine how many ways you can dream up to express solar intensity, which must be in units of energy divided by area divided by time, or power divided by area. Just pick a combination: kilowatthours per day per acre, for example. How about BTU per year per square mile? Unfortunately, people use all those weird units! If you find the table bewildering, remember this: if you measure things in SI units to begin with, you don't have to use any conversion factors whatsoever! How can anybody make any sense of it all? The rule to make sense of it is: 
Make your life simple: Convert all units to SI! 
The tables below give factors for converting miscellaneous units to SI.
Length 
SI Prefixes (Mega, micro and all that) 
Area 
Energy 
Mass (kilograms, tons, ...) 
Power 
Force (Newtons, pounds ...) 
Basic SI Units 
Time (years, days ...) 
