Denatured alcohol (aka shellac thinner) is all burnable.

Isopropyl (rubbing) alcohol is usually 70% alcohol + 30% water, but sometimes you can get a higher percentage of alcohol. It's very difficult to get 100% alcohol.

Since water doesn't burn, I'm sure it must cut down on the efficiency (& therefore heat) of the other 70%.

If you need heat, you need heat. For the small amount you would usually need, the savings for buying water to burn has got to be fairly negligible.

Sue