Boiler horsepower
| Value | 9,809.5 W |
| Status | Conventional / typical value |
| Source | ASME; NIST SP 811 (horsepower, boiler) |
| Categories | ThermodynamicEngineering & Trade |
| nanowatt | 9,809,500,000,000 nW |
| erg per second | 98,095,000,000 erg/s |
| microwatt | 9,809,500,000 μW |
| milliwatt | 9,809,500 mW |
| foot pound-force per minute | 434,106.95 ft·lbf/min |
| BTU per hour | 33,471.403 BTU/h |
| watt | 9,809.5 W |
| kilocalorie per hour | 8,434.6518 kcal/h |
| foot pound-force per second | 7,235.1159 ft·lbf/s |
| thousand BTU per hour | 33.471403 MBH |
| metric horsepower | 13.337208 PS |
| horsepower (mechanical) | 13.154756 hp |
| electrical horsepower | 13.149464 hp (elec) |
| kilowatt | 9.8095 kW |
| ton of refrigeration | 2.7892836 ton |
| boiler horsepower | 1 bhp |
| million BTU per hour | 0.033471403 MMBTU/h |
| megawatt | 0.0098095 MW |
| gigawatt | 0.0000098095 GW |
| terawatt | 9.8095000e-09 TW |
| solar luminosity | 2.5625653e-23 Lsun |
Learning zone
Boiler horsepower is a rating of heat output, not shaft power, and it is 13 times larger than mechanical horsepower — 9.81 kW against 0.746 kW. The definition comes from the 1876 Centennial Exhibition in Philadelphia, where a committee decided that the boiler serving a good steam engine of one horsepower had to evaporate about 34.5 pounds of water per hour "from and at 212 °F", meaning feedwater already at boiling point. Multiply 34.5 lb/h by the 970.3 BTU/lb latent heat and you get the defining 33 475 BTU/h.
The unit is still printed on North American firetube boiler nameplates, where it is a convenient proxy for physical size — roughly 5 square feet of heating surface per boiler horsepower. Actual fuel input is higher than the rating by the reciprocal of efficiency, and feedwater below 212 °F needs extra sensible heat, so a 100 bhp boiler at 80 % efficiency burns closer to 4.2 million BTU/h of gas.