Efficiency is Dimensionless
We express efficiency as a percentage. However, efficiency is dimensionless. In other words, it has no units (Percent is NOT a unit.).
If you look at the efficiency equation at the right, you can see why efficiency has no units. Notice that the SI unit for work out is Joules, which is the same as the SI unit for work in. Work out is on top of a fraction, and work in is on the bottom, and so the Joules on top cancel with the Joules on the bottom of the fraction.