Horsepower (Water) to Kilocalories (th)/minute

Water HP = (Flow in GPM × Total Head in feet) / 3,960. The magic number 3,960 comes from the weight of water (8.33 lb/gal) × 33,000 ft·lbf/min per hp ÷ 8.33 × the appropriate conversions. For example: 200 GPM at 80 feet of head = 200 × 80 / 3,960 = 4.04 water hp. If the pump is 75% efficient, you need 4.04 / 0.75 = 5.39 shaft hp, so you'd select a 7.5 hp motor (next standard NEMA size up).

Because the motor must provide shaft hp, not water hp. If your application needs 3 water hp but the pump is only 65% efficient, you need 3 / 0.65 = 4.6 shaft hp — meaning a 5 hp motor minimum. Underestimating this gap leads to overloaded motors, overheating, and premature failure. Pump efficiency varies from 30% (tiny pumps) to 90% (large, well-designed centrifugal pumps), so the water-hp-to-shaft-hp gap ranges from 10% to 70%.

Water hp is the useful hydraulic output — the power actually delivered to the fluid as pressure and flow. Brake hp (bhp) is the shaft input power measured at the coupling. The ratio is pump efficiency: η = WHP / BHP. A pump rated at 10 bhp with 80% efficiency delivers 8 water hp; the other 2 hp is lost to friction, turbulence, and heat in the pump. When specifying pumps, you calculate the required water hp first, then divide by efficiency to find the necessary brake hp.

Rarely. International pump engineering uses kilowatts for both hydraulic power and shaft power. The formula becomes: Hydraulic Power (kW) = (Flow in m³/h × Head in meters × density × g) / 3,600,000. European pump catalogs list "hydraulic efficiency" as a percentage relating input kW to output kW. The concept is identical — water hp is just the US customary wrapper around it. If you see "water horsepower" in a specification, it's almost certainly an American or Canadian document.

Municipal water supply: 50–500 water hp per pump station. Oil pipeline pumping: 1,000–10,000 water hp per station. Mining dewatering: 100–5,000 water hp. Hydroelectric dam penstocks deliver thousands of water hp to turbines (in reverse — the water does the work). The largest single pump installations, like the pumps protecting New Orleans from flooding, can exceed 10,000 water hp each — moving millions of gallons per minute against levee heights.

A MET (Metabolic Equivalent of Task) is the ratio of activity metabolic rate to resting metabolic rate. Sitting = 1 MET; walking = 3.5 METs; running = 8–12 METs. Researchers prefer METs because they normalize for body weight — a 50 kg woman and a 100 kg man both register 8 METs while running at the same pace, even though their raw kcal/min differ by 2×. This makes METs portable across populations. To get kcal/min from METs: multiply METs × body weight in kg × 0.0175. The Compendium of Physical Activities lists METs for over 800 activities, from accordion playing (1.8) to wrestling (6.0).

Cross-country skiing uphill can hit 15–20 kcal/min (1,050–1,400 W metabolic), making it one of the highest sustained metabolic rates in sport. Rowing and swimming at race pace reach 12–18 kcal/min. Cycling at elite level sustains 15–25 kcal/min. But the absolute champion is sprint running: Usain Bolt's 100m final produced roughly 80–100 kcal/min of metabolic power for 9.58 seconds. Of course, no one sustains that for long.

VO₂ max (maximum oxygen consumption) converts to kcal/min via the caloric equivalent of oxygen: 1 liter of O₂ consumed ≈ 5 kcal. An elite endurance athlete with VO₂ max of 80 mL/kg/min (70 kg person = 5.6 L/min) can sustain roughly 28 kcal/min at maximum effort. An untrained person at VO₂ max of 35 mL/kg/min maxes out around 12 kcal/min. This is why fit people can sustain higher power outputs — they literally process more oxygen.

Because their energy accounting is in kilocalories: food energy in kcal, basal metabolism in kcal/day, exercise expenditure in kcal/min. If a client eats 2,000 kcal and you want them to "burn 500 kcal," it's immediately useful to say "run at 10 kcal/min for 50 minutes." Saying "exercise at 700 W" is technically correct but meaningless to most clients. The kcal/min rate connects directly to the dietary energy balance equation.

Yes — EPOC (excess post-exercise oxygen consumption) is measured as elevated kcal/min above resting rate after exercise. After intense interval training, your metabolic rate might stay 0.2–0.5 kcal/min above baseline for 12–24 hours. That sounds tiny, but over 24 hours it adds up to 200–700 extra kcal — a meaningful amount. However, the fitness industry wildly oversells this: moderate exercise barely budges EPOC. You need truly brutal intensity to get a significant afterburn.