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One boiler Horse Power is defined as the power required to evaporate 34.5 lb (= 34.5 * 0.453 kg = 15.65 kg) of fresh water at 100°C or 212°F in one hour. The boiler HP is used to measure the boiler's output. The unit is an old one and is used in boiler engineering. Boiler HP is equal to approximately 9.8 kW.

It is not used much in all countries. It is used for conveying capacity of boiler that supplies steam to a steam engine.

Boiler efficiency is the effectiveness of the process which converts chemical energy in the fuel into the heat energy of steam.  It takes the low or high calorific value of the fuel which is burnt and the change in enthalpy of water to steam. 
     eta = efficiency = Ms (Hs - Hw) / Mf * CV    where,

                Ms = mass in kg of steam flowing out of boiler per second 
                Mf = mass in kg of fuel burning (= flowing in) in boiler per second
               Hs =  the specific enthalpy of steam in kJ/kg
                Hw = the specific enthalpy of water fed into the boiler (kJ/kg)
                CV = high claorific value of low calorific value (depending on water content in fuel)