Power unit conversion table
The table lists each supported unit with its exact value in watts, per the SI Brochure and NIST SP 811 conversion factors.
| Unit | Symbol | Watts |
|---|---|---|
| Watt | W | 1 |
| Kilowatt | kW | 1,000 |
| Megawatt | MW | 1,000,000 |
| Mechanical horsepower | hp | 745.6999 |
| Metric horsepower | PS | 735.49875 |
| BTU per hour | BTU/h | 0.29307107 |
- Several other horsepower definitions exist — electrical horsepower (746 W exactly) and boiler horsepower (about 9,810 W) among them; this converter uses the mechanical and metric definitions, which cover automotive and general engineering use.
- Vehicle power figures are also affected by the measurement standard (e.g. SAE net vs DIN), not only the unit — two spec sheets can differ for reasons beyond hp vs PS.
- Electrical installations and equipment ratings are governed by local electrical codes; consult a qualified electrician for any installation decision.
What is a watt and how do power units relate?
The watt (W) is the SI unit of power — one joule of energy per second. It is named after James Watt and defined in the International System of Units maintained by the BIPM. Kilowatts (10³ W) and megawatts (10⁶ W) are simple decimal multiples used for appliances, vehicles and power plants respectively.
Horsepower predates the watt and survives in two main flavors that are easy to confuse. Mechanical (imperial) horsepower is defined as 550 foot-pounds-force per second, which equals 745.6999 W. Metric horsepower — abbreviated PS from the German Pferdestärke, also seen as cv or ch in Romance languages — is defined as 75 kgf·m/s, which equals 735.49875 W. A PS is therefore about 98.6% of an hp: a 300 PS engine is roughly 296 hp, and quoting the wrong unit misstates power by about 1.4%.
BTU per hour is a power unit from the imperial heat world: one British thermal unit per hour equals 0.29307107 W. It remains the standard rating unit for air conditioners, heaters and furnaces in North America — a '12,000 BTU' air conditioner moves 12,000 BTU/h, about 3.52 kW of heat.
How to use this watt converter
- Enter the power value you want to convert.
- Select its unit: W, kW, MW, mechanical hp, metric PS or BTU/h.
- Read the equivalent value in all the other units at once.
- For engine comparisons, note carefully whether a spec sheet quotes hp or PS — the two differ by about 1.4%.
The conversion factors behind the watt converter
Every input is first converted to watts using its exact defined factor, then divided by each target unit's factor. The factors are definitions, not measurements: 1 hp = 550 ft·lbf/s = 745.6999 W; 1 PS = 75 kgf·m/s = 735.49875 W; 1 BTU/h = 0.29307107 W.
Worked example: 1,000 W. In kilowatts this is 1 kW. In mechanical horsepower: 1,000 ÷ 745.6999 = 1.3410 hp. In metric horsepower: 1,000 ÷ 735.49875 = 1.3596 PS. In BTU/h: 1,000 ÷ 0.29307107 = 3,412.1 BTU/h.
Common mistakes
- Treating hp and PS as the same unit — 1 PS = 735.49875 W but 1 hp = 745.6999 W, so a 300 PS engine is about 296 hp, not 300.
- Confusing power with energy: kW is a rate, kWh is an amount. A 2 kW heater running for 3 hours uses 6 kWh.
- Reading air-conditioner 'BTU' ratings as energy — the rating is BTU per hour, a power: 12,000 BTU/h ≈ 3.52 kW.
- Mixing up kW and kVA on AC equipment: kW is real power; kVA is apparent power, and they differ by the power factor.
الأسئلة الشائعة
What is the difference between hp and PS?
Mechanical horsepower (hp) is defined as 550 ft·lbf/s = 745.6999 W. Metric horsepower (PS, Pferdestärke) is defined as 75 kgf·m/s = 735.49875 W. A PS is about 1.4% smaller, so a 300 PS engine equals roughly 296 hp. European spec sheets typically quote PS or kW, while US sheets quote hp — comparing them without converting misstates the power.
How many watts is 1 horsepower?
One mechanical horsepower equals 745.6999 watts (defined as 550 foot-pounds-force per second). One metric horsepower (PS) equals 735.49875 watts (defined as 75 kilogram-force-metres per second). Rounded figures of 746 W and 735.5 W are common in practice.
How do I convert kW to hp?
Divide the kilowatt figure by 0.7456999, or equivalently multiply by 1.3410, to get mechanical horsepower. For metric horsepower multiply by 1.3596. Example: a 100 kW motor is 134.1 hp or 136.0 PS.
What does the BTU rating on an air conditioner mean?
It is shorthand for BTU per hour — a power rating describing how much heat the unit moves per hour. One BTU/h equals 0.29307107 W, so a 12,000 BTU/h unit transfers about 3.52 kW of heat. The rating describes heat transfer capacity, not the electrical power the unit consumes.
Is kW or hp the official unit for engine power?
The SI unit of power is the watt, so regulatory documents in most countries state engine power in kilowatts. Horsepower (hp in North America, PS in much of Europe and Asia) persists as customary marketing usage. Spec sheets commonly show kW alongside one of the horsepower units.
المراجع
- NIST Special Publication 811 (Thompson & Taylor). Guide for the Use of the International System of Units (SI), 2008 — Appendix B conversion factors for horsepower and BTU/h.
- Bureau International des Poids et Mesures (BIPM). The International System of Units (SI Brochure), 9th edition, 2019 — definition of the watt.