Efficiency can be defined as the ratio of useful output energy to the amount of input energy.
Efficiency = Useful output energy / Input energy.
Dimensional formula of output energy = M1L2T-2
Dimensional formula of input energy = M1L2T-2
Putting these values in above equation we get,
Dimensional formula of Efficiency = M0L0T0.
Efficiency has no SI Unit. (As both values are of energy).