The calculator from the EIA is quite comprehensive, it lists NG as well as propane separately.
The same agency where that document can be found has a glossary of terms they list a cord of wood as:
Cord of wood: A cord of wood measures 4 feet by 4 feet by 8 feet, or 128 cubic feet.
http://www.eia.doe.gov/glossary/glossary_c.htmAlso note there is disclimer below, see notation #3. I believe they are basing the 20 million BTU per cord on seasoned hardwood as that is what previous charts I have seen from government agencies baseing it on.
As for the gas that depends on what you're paying for the coal or the gas, if you're getting coal delivered for $160 a ton which is the going rate locally then it's very easy to justify. There's really no comparison. Gas or Oil has never been cheaper than coal locally, both have come close but as far as I know they have never beaten it and when they did come close it was usually short lived. Lastly keep in mind the spot prices for NG have nearly doubled since the beginning of the year. I beleive the local gas company just requested a 20% increase and unless it drops they will undoubtedly be asking for more increases and will get them.
"Hey, Homer, I'm worried about the beer supply. After
this case, and the other case, there's only one case left."
Barney from the Simpsons