Efficiency of the boiler is nothing but ratio of rate of energy absorption by water to form steam to rate of energy release by the combustion of fuel.
Boiler efficiency = ms*(h1-h2)/mf*CV
ms = steam generation rate (kg/s)
mf = fuel burning rate (kg/s)
CV = calorific value of fuel
h1,h2 = enthalpy at entry and exit of the boiler