where part of the energy is converted into work and part is lost to heat. BTUs: "The British thermal unit (symbol BTU or Btu) is a traditional unit of energy equal to about 1,055 joules." BTU is a measurement of energy. You're right that the energy density of ethanol is lower than gasoline, but if you're suggesting it is a 'more efficient fuel' for home heating versus as an automotive fuel, that's nonsense.