You're standing in a hardware store looking at a shop vac that claims it has "5 Peak HP," then you glance at your microwave which is rated in watts, and suddenly nothing makes sense. If you’ve ever tried to figure out exactly how many watts are in a horsepower, you probably got a quick answer from Google: 746.
That’s the standard. It’s the "mechanical" or "imperial" horsepower used in the U.S. and the UK. But honestly? That number is just the tip of a very weird, very old iceberg.
James Watt—the guy the "watt" is actually named after—basically made the whole thing up to sell steam engines. He needed a way to tell coal miners how many horses his machine could replace. He watched some ponies working a mill, did some generous rounding, and decided that one horsepower was the ability to lift 33,000 pounds one foot in one minute.
If you convert that mechanical work into electrical units, you get $745.699$ watts. We round it to 746 because life is short and decimals are annoying.
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The Metric Twist: Why 735.5 exists
Most people don't realize that horsepower isn't a universal language. If you're looking at a German car or a piece of industrial equipment from mainland Europe, they use "metric horsepower" (Pferdestärke or PS).
It's slightly different. A metric horsepower is defined as the power to lift a 75-kilogram mass against Earth's gravity at a velocity of one meter per second. When you do the math on that, you get roughly 735.5 watts.
It's a tiny difference—about 1.5%—but it matters. If you’re an engineer designing a power grid for a factory full of European pumps, that 1.5% discrepancy across a hundred motors will blow a fuse or melt a breaker. It's the kind of detail that keeps electricians up at night.
Different Flavors of the Same Unit
We also have "boiler horsepower," which is a whole other beast. Used in North America for industrial heating, one boiler hp is equal to 9,810 watts. It’s huge. It’s the amount of energy required to evaporate 34.5 pounds of water into steam in an hour.
Then there’s "hydraulic horsepower." If you’re working with fluid power, you calculate it using pressure and flow. But at the end of the day, whether it’s steam, water, or electricity, we are just measuring the rate of doing work.
The Electric Vehicle (EV) Identity Crisis
Tesla, Rivian, and Lucid are changing how we talk about power. For a century, we bought cars based on "hp." Now, we see battery specs in kilowatt-hours (kWh) and motor outputs in kilowatts (kW).
One kilowatt is exactly 1,000 watts.
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To convert from a car's kilowatt rating to its horsepower, you take the kW and multiply it by 1.34. So, a motor rated at 100 kW is roughly 134 horsepower.
Why does this matter? Because of efficiency losses.
When you put 746 watts of electricity into a motor, you don't get 1 horsepower of "work" out the other side. Friction happens. Heat happens. Most standard industrial motors are about 80% to 90% efficient. This means to get 1 actual horsepower of mechanical turning force, you might actually have to pull 850 or 900 watts from the wall.
The "Peak Horsepower" Marketing Scam
You’ll see this on vacuum cleaners, blenders, and air compressors. A manufacturer will claim a "3 HP" motor on a device that plugs into a standard 120-volt household outlet.
Let's do some quick math.
A standard US outlet is usually on a 15-amp breaker. $120\text{ volts} \times 15\text{ amps} = 1,800\text{ watts}$.
If how many watts are in a horsepower is 746, then a "3 HP" motor would need 2,238 watts just to run. That would trip your breaker instantly.
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"Peak HP" is a measurement of the motor's power at the exact millisecond the motor stalls or starts up—before the breaker has time to pop. It's a legal but misleading way to make a small motor sound like a beast. In reality, that 5 HP shop vac is probably putting out about 1.5 HP of continuous, actual work. Always look for the "continuous duty" rating or just look at the Amps. The Amps don't lie.
Practical Steps for Sizing Your Power Needs
If you are trying to calculate power for a DIY project, a home backup generator, or a solar setup, stop relying on the horsepower sticker.
- Check the Nameplate: Look for the "Full Load Amps" (FLA). This is the real-world current the device pulls when it's working hard.
- Calculate Running Watts: Multiply those Amps by your voltage (usually 120V or 240V).
- Account for Inrush: Electric motors need 3x to 5x their running power just to start turning. If your motor is 1 HP (746 watts), you need a generator capable of a 3,000-watt "surge" to get it spinning.
- Use the 746 Rule for Estimates: For quick mental math, treat 1 HP as 0.75 kW. It’s a clean fraction that keeps you in the right ballpark without needing a calculator.
Understanding the conversion is about more than just trivia. It's about knowing why your treadmill flickers the lights when it starts or why a European car's specs look slightly different on a dyno. Power is power, but the labels we put on it depend entirely on who is doing the selling.