Watts (W) to Horsepower (British) (hp) conversion

What is Watts?

Watts are the standard unit of power in the International System of Units (SI). Power, in physics, is the rate at which energy is transferred, used, or transformed. Therefore, a watt measures how quickly energy is being used or generated. It's a crucial unit for understanding electrical circuits, energy consumption, and various physical processes.

Definition and Formula

One watt is defined as one joule per second. In other words, if one joule of energy is used or produced in one second, that's a power of one watt. The formula for calculating power in watts is:

$$P = \frac{E}{t}$$

Where:

• $P$ is power in watts (W)
• $E$ is energy in joules (J)
• $t$ is time in seconds (s)

In electrical circuits, power can also be calculated using voltage and current:

$$P = V \cdot I$$

Where:

• $P$ is power in watts (W)
• $V$ is voltage in volts (V)
• $I$ is current in amperes (A)

James Watt and the History of the Unit

The watt is named after James Watt, a Scottish inventor and mechanical engineer who significantly improved the efficiency of steam engines in the late 18th century. His innovations were crucial to the Industrial Revolution. The unit "watt" was adopted in 1882 as a tribute to his contributions to the development of power and energy technologies.

Real-World Examples of Wattage

• Light Bulbs: Traditional incandescent light bulbs are often rated in watts (e.g., 60W, 100W). These numbers indicate how much electrical power the bulb consumes. LED bulbs, being more energy-efficient, produce the same amount of light with much lower wattage.

• Household Appliances: Microwave ovens (e.g., 1000W), refrigerators, and washing machines all have wattage ratings. This rating tells you how much power they draw from the electrical supply when in use.

• Electric Motors: The power of electric motors, such as those in power tools or electric vehicles, is often measured in watts or kilowatts (1 kW = 1000 W).

• Human Power: A healthy human can sustain about 75 watts of power output for an extended period. Athletes, during short bursts of extreme exertion, can generate over 1000 watts.

Interesting Facts

• Kilowatt-hour (kWh): Utility companies typically bill consumers based on kilowatt-hours (kWh), which represents the amount of energy used over time. One kWh is equal to 3.6 million joules. A 100-watt light bulb left on for 10 hours consumes 1 kWh of energy.

• Power vs. Energy: Power (watts) is the rate at which energy is used, while energy (joules or kWh) is the total amount of energy consumed over a period of time.

• Power Factor: In AC circuits, power factor is the ratio of real power (watts) to apparent power (volt-amperes). It indicates how effectively electrical power is being used. A power factor of 1 means that all the power is being used effectively, while a power factor less than 1 indicates that some of the power is being wasted. Learn more at Fluke's guide to power factor.

What is Horsepower (British)?

Horsepower is a unit of power, commonly used to measure the rate at which work is done. Specifically, it quantifies the amount of energy transferred or converted per unit of time. The "British" qualifier often distinguishes it from other variations, such as metric horsepower.

Definition and Origins

British horsepower (hp) is defined as the power required to raise a weight of 33,000 pounds by one foot in one minute. This is equivalent to 550 foot-pounds per second.

The concept of horsepower was popularized by James Watt in the late 18th century. He used it to market his improved steam engine, comparing its power output to that of horses, which were a common source of power at the time. Watt determined that a typical horse could turn a mill wheel 144 times in an hour, or 2.4 times a minute, with a radius of 12 feet. This led to his calculation of 32,400 foot-pounds per minute, which he then rounded up to 33,000 foot-pounds per minute for marketing purposes.

Formula and Calculation

The formula for calculating horsepower is:

$Horsepower = \frac{Work}{Time} = \frac{Force \times Distance}{Time}$

Where:

• Work is measured in foot-pounds (ft⋅lb)
• Force is measured in pounds (lb)
• Distance is measured in feet (ft)
• Time is measured in minutes (min)

Thus, 1 hp = 33,000 ft⋅lb/min or 550 ft⋅lb/s

Conversion to Other Units

Horsepower can be converted to other units of power, such as:

• Watts (W): 1 hp ≈ 745.7 W
• Kilowatts (kW): 1 hp ≈ 0.7457 kW
• Metric Horsepower (PS): 1 hp ≈ 1.0139 PS

Real-World Examples

• Car Engines: The power output of car engines is commonly measured in horsepower. For example, a typical family sedan might have an engine that produces around 150-200 hp, while a high-performance sports car can generate over 500 hp.
• Motorcycles: Similar to cars, the power of motorcycle engines is often specified in horsepower. A small motorcycle might have around 20-30 hp, while a high-performance sportbike can have over 200 hp.
• Lawn Mowers: The power of lawn mower engines is typically measured in horsepower. A small push mower might have around 3-4 hp, while a larger riding mower can have 20 hp or more.
• Industrial Machinery: Many types of industrial machinery, such as pumps, compressors, and generators, are rated in horsepower. The horsepower rating indicates the amount of work the machine can perform in a given amount of time.

Interesting Facts

• James Watt, the Scottish inventor and mechanical engineer, is credited with developing the concept of horsepower to market his steam engine.
• The term "horsepower" is still widely used today, even though modern engines are far more efficient than the original steam engines that Watt designed.
• There are several different definitions of horsepower, including mechanical horsepower (which is the same as British horsepower), metric horsepower, and electrical horsepower. These different definitions can lead to some confusion, so it's important to be aware of which definition is being used.
• The amount of horsepower required to perform a particular task depends on the amount of work that needs to be done and the time in which it needs to be done. For example, it takes more horsepower to accelerate a car quickly than it does to maintain a constant speed on a level road.

Horsepower in Everyday Life

Understanding horsepower can help you make informed decisions when purchasing vehicles, machinery, and other equipment. It can also give you a better appreciation for the amount of work that different types of engines and machines are capable of performing. For more information on power and energy see this page on Energy Education.