A boiler horsepower is not a horsepower. It is a POWER — 33,475 BTU per hour, or $9.81\ \mathrm{kW}$ — which is more than THIRTEEN times the 746 W that the same word means on a motor nameplate. So $Q = 9.81\,\mathrm{BHP}$, where $Q$ is the boiler's gross heat output in kilowatts and $\mathrm{BHP}$ is the bare nameplate rating. A 200 bhp boiler is a 1,962 kW appliance, not a 149 kW one, and reading the two units as the same thing has under-sized real gas trains and real flues.

The same rating also reads as a steam rate: $S = 15.65\,\mathrm{BHP}$, with $S$ the steam production in kg/h. The definition was written in 1889 as 34.5 lb/h from and at 212 °F, and 34.5 lb is 15.65 kg. That qualifier matters: real feedwater arrives far below boiling, so a boiler making its nameplate steam is working harder than the definition admits.

Steam leaves its dissolved solids behind in the drum, exactly as evaporation does in a cooling tower, so the same algebra returns: $B = \dfrac{S}{\mathrm{COC} - 1}$, with $B$ the continuous blowdown in kg/h and $\mathrm{COC}$ the cycles the drum chemistry allows. Boilers run at far higher cycles than towers — often 20 or more — because their makeup is softened or demineralised before it ever reaches the drum.

Two more lines close the plant's water balance. $\%CR = \dfrac{100\,S_c}{S}$ is the condensate return percentage, where $S_c$ is the condensate coming back in kg/h against $S$ the steam sent out. And what does not come back must be replaced: $M = S\left(1 - \dfrac{\%CR}{100}\right)$, the makeup in kg/h. Returned condensate is hot, already treated and nearly free; makeup is cold, must be softened, and brings oxygen with it. Condensate return percentage is the single headline number of any steam plant, and every point of it is money.