Overwhelmingly most of it. Most generators run on diesel and not gasoline, and I gave up trying to figure gallons per KwH after a few minutes of googling. But you're talking about converting heat to mechanical energy with a ton of lossage, and then mechanical to electricity with a ton more lossage.

The original article isn't interesting. You may as well say that your mouse has enough energy to recharge your iphone for 10 years, provided we found a way to convert the mass into energy.