As you can see, an incandescent light bulb is highly inefficient. Most of the energy that goes in is not used for usueful work, but is lost as heat. You could calculate the efficiency of the light bulb by using this formula, .
This is just the rewritten formula of η = (Eout/Ein)* 100% (since E = P * t). Ein is the total amount of energy in joules that you put in. Since η never equals 100%, some energy is always lost as heat.
Edit. It's not a very well written sentence in my view. Energy is not represented by fuel. Fuel can be burnt to produce energy and heat.
Student or Learner