Don't Use Useless Definitions
When writing, it pays to be as clear and concise as possible. This sentence (from Alliant Energy's "Monthly Natural Gas Update") is neither:
A decatherm (equal to 10 therms) is enough natural gas to heat an average home for 4.5 days.
It's not clear because I still don't know what a "decatherm" is. The parenthetical definition is useless unless you know what a "therm" is. If you did now what a "therm" is, wouldn't you likely know what the "deca-" prefix meant?
It's not concise because it includes the useless definition. So be both clear and concise: don't use useless definitions.