Watt to Horsepower (Water)
W
hp
Conversion History
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Quick Reference Table (Watt to Horsepower (Water))
| Watt (W) | Horsepower (Water) (hp) |
|---|---|
| 1 | 0.00134040531175816944 |
| 10 | 0.01340405311758169435 |
| 60 | 0.08042431870549016612 |
| 100 | 0.13404053117581694353 |
| 800 | 1.07232424940653554822 |
| 1,200 | 1.60848637410980332233 |
| 2,000 | 2.68081062351633887055 |
About Watt (W)
The watt (W) is the SI unit of power, defined as one joule of energy transferred per second. It is the universal unit for electrical power, covering everything from a 1 W LED indicator light to a 3,000 W electric shower. Power consumption of appliances, power station output, and solar panel ratings are all expressed in watts or its multiples. One watt equals one volt multiplied by one ampere in a DC circuit, linking power directly to the foundational electrical quantities.
A modern LED bulb uses 8–10 W to produce the same light as a 60 W incandescent. A laptop draws 30–65 W; a microwave oven 800–1,200 W.
Etymology: Named after Scottish engineer James Watt (1736–1819), whose improvements to the steam engine drove the Industrial Revolution. The unit was adopted by the Second Congress of the British Association for the Advancement of Science in 1889.
About Horsepower (Water) (hp)
Water horsepower (hp(W)) equals 746.043 watts and is used specifically in pump engineering to express the hydraulic power delivered to a fluid, as distinct from the shaft power input to the pump. It accounts for the fact that a pump's mechanical input power always exceeds the hydraulic power delivered due to efficiency losses. Pump efficiency is calculated as water hp divided by input shaft hp. The difference between water hp and electric/mechanical hp is negligible (0.05%) for most calculations.
A pump delivering 100 gallons per minute at 50 feet of head produces approximately 1.26 water hp of hydraulic power. A pump rated at 2 shaft hp may deliver 1.6 water hp at 80% efficiency.
Watt – Frequently Asked Questions
How many watts does a phone charger actually use?
A standard USB charger draws 5–10 W, while fast chargers pull 18–65 W and some proprietary ones hit 120–240 W. The charger itself consumes about 0.1–0.3 W even when nothing is plugged in — so-called "vampire power." Over a year, a plugged-in-but-idle charger wastes roughly 2 kWh, costing pennies but multiplied across billions of chargers worldwide it adds up to gigawatt-hours of waste.
Why is a watt called a watt and not a joule per second?
Both are identical — 1 W = 1 J/s — but the watt was named in 1889 to honor James Watt, who quantified engine power decades before the joule was formalised. Giving power its own name made practical engineering simpler: saying "a 60-watt bulb" is far catchier than "a 60-joules-per-second bulb." The naming also followed a 19th-century tradition of honoring scientists with SI units — volt, ampere, ohm, and watt all came from this era.
What wattage does a human body produce?
A resting adult generates about 80–100 W of thermal power, roughly equivalent to an old incandescent light bulb. During intense exercise this spikes to 300–500 W total metabolic output, though only 20–25% becomes mechanical work — the rest is waste heat. This is why a packed lecture hall gets stuffy fast: 200 students produce about 20 kW of heat, equivalent to running 20 space heaters.
How many watts is a lightning bolt?
A single lightning stroke delivers about 1–5 billion watts (1–5 GW) of instantaneous power, but only for 1–2 milliseconds. The total energy per bolt is surprisingly modest — roughly 1–5 billion joules compressed into microseconds, equivalent to about 250 kWh or one month of a US household. You could theoretically power a town for a second, but capturing it is impractical because the pulse is too brief and unpredictable.
What is the difference between watts and watt-hours?
Watts measure the rate of energy flow (like the speed of water through a pipe), while watt-hours measure total energy consumed over time (like the total volume of water). A 100 W bulb running for 10 hours uses 1,000 Wh (1 kWh). Your electricity bill charges per kWh, not per watt — so a 2,000 W heater running one hour costs the same as a 100 W lamp running 20 hours.
Horsepower (Water) – Frequently Asked Questions
How do you calculate water horsepower for a pump?
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).
Why does water horsepower matter when sizing a pump motor?
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%.
How does water horsepower differ from brake horsepower in pumps?
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.
Is water horsepower used outside the United States?
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.
What pump applications require the most water horsepower?
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.