Efficiency:

The ratio of the useful work done by a device or machine to the total energy taken up by it is called its efficiency.

OR

The efficiency of a system is the ratio of the required form of energy obtained from a system as output to the total energy given to it as input.

Efficiency= $\frac{(required form of output)}{(total input energy)}$..... (i)

% Efficiency= $\frac{ (required form of output)}{(total input energy)}×100$ ......(ii)

Ideal system/Ideal machine:

           An ideal system is that which gives an output equal to the total energy used by it. In other words, its efficiency is 100 %.

            People have tried to design a working system that would be 100 % efficient. But practically such a system does not exist.

ADDITIONAL INFORMATION