Percent efficiency is calculated by determining the actual output and then dividing by the maximum possible output. The result is multiplied by 100 in order to move the decimal two places to the right.
The formula for calculating percent efficiency works in much the same manner as the formula for calculating the percentage of anything in the sense that it is nothing more than the division of the actual by the total. Because of this, it is, by definition, impossible to have a percent efficiency that is greater than 100. In physics, the efficiency of most things is necessarily lower due to friction and the laws of thermodynamics.