Data Source and Methodology
This calculator provides precise conversions based on internationally recognized standards. The primary authoritative source for these conversions is the NIST Special Publication 811 (2008 Edition), "Guide for the Use of the International System of Units (SI)", published by the National Institute of Standards and Technology (NIST).
All calculations are strictly based on the definitions and conversion factors provided by this source. You can access the publication directly at: https://www.nist.gov/pml/special-publication-811
The Formulas Explained
The term "horsepower" can refer to several different measurements. This tool allows you to convert the three most common types, each with a distinct conversion factor.
1. Mechanical Horsepower (hp)
This is the most common definition, often used for automotive engines and industrial machinery in North America and the UK. It is based on the work of James Watt and is defined as 550 foot-pounds per second.
2. Electrical Horsepower (hp)
This value is used specifically for rating electric motors. In the United States, it is legally defined as *exactly* 746 Watts.
3. Metric Horsepower (PS / cv)
Known as *Pferdestärke* (PS) in German, *cheval-vapeur* (cv) in French, or *cavallo vapore* (CV) in Italian, this is the standard definition used in most of Europe and Asia. It is defined as the power needed to raise 75 kilograms by one meter in one second.
Glossary of Variables
- Kilowatt (kW): The standard (SI) unit of power, equal to 1,000 Watts. It is used globally for nearly all official power ratings, from appliances to vehicles.
- Horsepower (hp): An imperial unit of power. As seen above, its value differs depending on whether it is mechanical, electrical, or metric.
- Mechanical hp: The standard imperial horsepower, common in US/UK automotive contexts.
- Electrical hp: Used specifically for electric motors, legally defined as 746 W.
- Metric hp (PS): The metric equivalent of horsepower, common in European and Asian vehicle specifications.
How it Works: A Step-by-Step Example
Let's convert the power of a car engine rated at **150 hp (mechanical)** to kilowatts.
- Select the Type: Choose "Mechanical Horsepower (hp)" from the dropdown, as this is the standard for a car engine in the US.
- Identify the Formula: The correct formula is $kW = \text{hp} \times 0.745699...$
- Enter the Value: Type "150" into the "Horsepower (hp)" field.
- Get the Result: The "Kilowatts (kW)" field will instantly update with the result.
Calculation:
$$150 \text{ hp} \times 0.74569987... \text{ (kW/hp)} = 111.8549... \text{ kW}$$The calculator will show a rounded value, such as **111.855 kW**.
Frequently Asked Questions (FAQ)
What is the difference between hp and bhp (brake horsepower)?
Brake horsepower (bhp) is a specific measurement of an engine's mechanical horsepower *before* any power is lost to the drivetrain (gearbox, alternator, etc.). For conversion purposes, 1 bhp is equivalent to 1 hp (mechanical). The term "brake" simply specifies *where* the power was measured.
How many horsepower are in 1 kilowatt?
This depends on the type of horsepower.
- $1 \text{ kW} \approx 1.341 \text{ mechanical hp}$
- $1 \text{ kW} \approx 1.340 \text{ electrical hp}$
- $1 \text{ kW} \approx 1.360 \text{ metric hp (PS)}$
Why does my car's registration document (V5C in UK) show a different kW value?
Vehicle registration documents, particularly in Europe and countries using the metric system, almost always use **Metric Horsepower (PS)** as their "horsepower" reference. The official power rating will be in kilowatts (kW). If you convert the kW value back, it will match the PS rating, not the mechanical hp rating. For example, a "100 hp" car in Germany is actually 100 PS, which is $100 \times 0.7355 = 73.55 \text{ kW}$.
Which conversion should I use for my electric car (EV)?
Electric car (EV) power is almost exclusively rated in kilowatts (kW). If you see a "horsepower" rating for an EV, it is most likely a conversion *from* kW for marketing purposes. In this case, they typically use $1 \text{ kW} = 1.341 \text{ mechanical hp}$ or $1 \text{ kW} = 1.360 \text{ metric hp (PS)}$, depending on the market.
Is kilowatts (kW) the same as kilowatt-hours (kWh)?
No. This is a very important distinction.
- Kilowatt (kW) is a unit of power (how *fast* energy is used).
- Kilowatt-hour (kWh) is a unit of energy (how *much* energy is used over time).
Tool developed by Ugo Candido.
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