Power Converter — Watts, Kilowatts & Horsepower

A dependable power converter matters most in the moment you're comparing a car's horsepower rating against a motor's kilowatt rating, or trying to size an air conditioner rated in BTU per hour against a generator rated in watts. Power measures the rate at which energy is delivered or consumed, and — much like pressure and energy — it is expressed in a surprising number of competing units depending on the industry, the country, and even the era the equipment was designed in.

This free tool from Arb Digital converts between seven common power units — watt, kilowatt, megawatt, mechanical horsepower, metric horsepower, BTU per hour, and foot-pounds per second — using exact conversion factors anchored to the watt, the SI unit of power, so every result is precise regardless of which units you're moving between.

What This Power Converter Does

Enter a value, choose the unit it's currently in, and choose the target unit. The tool multiplies your input by the source unit's watt-equivalent to get a value in watts, then divides by the target unit's watt-equivalent to produce the final answer. Because two different definitions of "horsepower" are in everyday use — mechanical (imperial) horsepower and metric horsepower — the result panel deliberately shows your value converted into both simultaneously, alongside plain watts and kilowatts, so you can immediately see how much the two horsepower conventions actually differ for your specific number.

How to Use the Power Converter

1. Enter your value. Type the number you want to convert, such as 100 for a 100-watt light bulb.
2. Select the "From" unit. Choose watt, kilowatt, or whichever unit your source data is in.
3. Select the "To" unit. Pick mechanical horsepower, metric horsepower, or any other supported unit.
4. Read the instant result, along with four bonus conversions shown in the panel below it.
5. Use the swap button to reverse the "from" and "to" selections in a single click.

The Formula Behind Every Power Conversion

The watt (W) is the SI unit of power, defined as one joule of energy transferred per second. Every other power unit here has an exact watt-equivalent: mechanical (imperial) horsepower is defined as exactly 550 foot-pounds of work per second, which works out to 745.6998715822702 W, while metric horsepower (also called PS or CV) is defined as 75 kilogram-force-meters per second, equal to exactly 735.49875 W. One BTU per hour equals 1,055.05585 J ÷ 3,600 s, or approximately 0.29307107 W, and one foot-pound per second equals 1.3558179483314 W. These figures are documented in the NIST reference on SI and non-SI units. Because the tool always converts through watts first, a conversion from metric horsepower directly to BTU per hour is exactly as accurate as watts to kilowatts.

Mechanical Horsepower vs. Metric Horsepower

This is the single biggest trap in power conversions. "Horsepower" sounds like one universal unit, but there are actually two distinct definitions in common use, and they differ by about 1.36%. Mechanical horsepower (sometimes called imperial or hydraulic horsepower) equals 745.6999 W and is the figure typically quoted for US-market vehicles, most English-language engine specifications, and many pump and motor datasheets. Metric horsepower — abbreviated PS (Pferdestärke) in German, CV (cheval-vapeur) in French, or simply "hp" in some European and Japanese spec sheets — equals 735.49875 W. A car advertised at "300 PS" in a European brochure is actually about 296 mechanical horsepower, not 300. The gap is small per unit but adds up quickly at higher power figures, which is why performance comparisons across regions can be subtly misleading unless you know which horsepower definition is being used. This converter shows both simultaneously so you never have to guess.

Sizing HVAC and Appliances in Watts, kW, and BTU/hr

Air conditioners and heaters in North America are usually rated in BTU per hour for cooling/heating capacity, while the electricity they draw to run is measured in watts or kilowatts. These are not interchangeable: BTU/hr describes the rate of heat moved, while watts describe electrical power consumed — an air conditioner's "efficiency" (its SEER or EER rating) is essentially the ratio between the two. The U.S. Department of Energy's guide to estimating appliance energy use is a useful reference when converting between an appliance's power draw and its expected running cost. If you need to go a step further and estimate the energy consumed by an appliance over time (rather than its instantaneous power), pair this tool with our Energy Converter, which handles watt-hours and kilowatt-hours.

Motors, Generators, and Industrial Equipment

Electric motors are commonly rated in both watts/kilowatts and horsepower depending on the manufacturer's home market — a "5 hp" pool pump motor, for instance, is rated at roughly 3.73 kW using the mechanical horsepower convention. Generators, similarly, are often labeled with a kilowatt rating for their electrical output and sometimes a horsepower rating for their engine. Foot-pounds per second, meanwhile, shows up mostly in older US engineering texts and some torque-and-speed calculations, since power equals torque multiplied by rotational speed.

• Automotive: engine output is typically mechanical horsepower (US/UK) or metric horsepower/PS (Europe, Japan).
• HVAC: cooling/heating capacity in BTU/hr, electrical draw in watts or kW.
• Home appliances: power consumption labeled in watts, sometimes kilowatts for larger appliances.
• Industrial motors: rated in kW or horsepower depending on regional convention.

Common Mistakes to Avoid

• Treating mechanical and metric horsepower as identical. They differ by roughly 1.36% — small on paper, but enough to mismatch spec sheets from different regions.
• Confusing BTU (energy) with BTU/hr (power). Only BTU per hour is a valid power unit; a plain BTU figure belongs on our Energy Converter instead.
• Assuming 1 kW always equals about 1.34 hp without specifying which horsepower. That figure is correct for mechanical horsepower (1 kW ≈ 1.341 hp) but slightly different for metric horsepower (1 kW ≈ 1.360 PS).
• Ignoring efficiency losses. A motor's rated output power is not the same as its electrical input power — the difference is lost as heat, and this converter only handles unit conversion, not efficiency.

Related Free Tools From Arb Digital

Since power, energy, force, and pressure are closely linked physical quantities, you may also want our Energy Converter for joules and kWh, our Force Converter for newtons and pound-force, and our Pressure Converter for psi and bar. For everyday conversions, try the Length Converter and Weight Converter. See everything at our free online tools hub.

Power vs. Energy: Why kW and kWh Are Never the Same Thing

Power and energy are among the most frequently confused pairs of physical quantities, largely because their units look so similar on a page: a kilowatt (kW) and a kilowatt-hour (kWh) share the word "kilowatt," yet they measure fundamentally different things. Power is a rate — how quickly energy is being transferred at a given instant, measured in watts or kilowatts. Energy is a total — how much has been transferred altogether over some period, measured in joules, watt-hours, or kilowatt-hours. The relationship between them is simple: energy equals power multiplied by time (E = P × t). A 2 kW electric heater run for one hour uses 2 kWh of energy; the same heater run for three hours uses 6 kWh, even though its power rating of 2 kW never changes. This is precisely why an appliance's power rating alone can't tell you its running cost — you also need to know how long it runs, which is the job of our Energy Converter rather than this tool.

This distinction matters most whenever someone tries to directly compare a "kW" figure with a "kWh" figure as though they were interchangeable, which happens surprisingly often in casual conversation about solar panels, batteries, and electric vehicles. A solar panel might be rated at 400 W (its instantaneous power output under ideal test conditions), while the electricity it actually generates over a day is reported in kWh (its accumulated energy output, which depends on sunlight hours and weather). Similarly, an EV's motor might be rated at 150 kW of peak power, while its battery capacity is rated in kWh — the power rating tells you how quickly it can accelerate, while the battery's kWh rating tells you how far it can travel before recharging. Confusing the two leads people to badly misjudge things like expected electricity generation, driving range, or charging time.

How Appliance and Engine Power Ratings Are Really Quoted

A "power rating" printed on a nameplate or spec sheet is rarely as absolute as it looks. Many electrical appliances quote a nominal or rated power that assumes typical operating conditions, while their actual draw can vary — a refrigerator's compressor cycles on and off, drawing its rated wattage only while running, and a laptop charger is typically rated for its maximum possible draw rather than what it draws during light use. Audio equipment adds another wrinkle: speaker and amplifier wattage is sometimes quoted as continuous (RMS) power, sometimes as a short-term peak figure that can be several times higher, and manufacturers don't always make clear which one they mean, making direct comparisons between brands unreliable without reading the fine print.

Engine horsepower has its own quoting conventions worth knowing. A manufacturer's advertised horsepower figure is typically measured "at the crank" — the output of the engine itself on a test stand — rather than "at the wheels," where drivetrain losses (through the transmission, differential, and other components) typically reduce the power that actually reaches the road by a meaningful margin. Dyno-tested "wheel horsepower" figures are consequently lower than manufacturer-quoted crank horsepower for the same vehicle, which is a common source of confusion when comparing an official spec sheet against an independent dyno test. None of this affects the accuracy of the unit conversion itself — 100 hp is exactly 100 hp regardless of where it was measured — but it's worth knowing which kind of figure you're actually converting.

Frequently Asked Questions

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