How to Convert HP to Watts
Converting horsepower to watts is the most important power unit translation in mechanical engineering — it's how you size a generator for a 50 HP pump (37,285 W), how you compare a 2 HP garage door opener to its 1,500 W nameplate, and how you evaluate whether a 5 HP air compressor will trip a 30 A / 240 V circuit (7,200 W available vs. 3,728 W draw — safe with headroom). 1 mechanical HP = 745.7 watts. Multiply HP by 745.7. This is the mechanical/imperial HP standard. Metric HP (DIN PS, CV) uses 735.5 W; electrical HP uses exactly 746 W; boiler HP uses 9,809.5 W. When reading a European motor datasheet that says "15 kW," that's 20.1 mechanical HP — but only 20.4 metric PS. The 0.3 HP gap between the two standards may determine whether a motor meets specification in a contract written to imperial HP.
The Conversion Formula
W = HP × 745.7
Worked Examples
Example 1: Convert 5 HP to watts
5 × 745.7 = 3,728.5 W (a typical 5 HP air compressor motor)
Example 2: Convert 200 HP to watts
200 × 745.7 = 149,140 W (149.1 kW) (a mid-size sedan engine)
Example 3: Convert 0.5 HP to watts
0.5 × 745.7 = 372.9 W (a small sump pump or pedestal fan motor)
Common HP to Watts Conversions
| HP | Watts |
|---|---|
| 0.25 HP | 186.4 W |
| 0.5 HP | 372.9 W |
| 1 HP | 745.7 W |
| 5 HP | 3,728.5 W |
| 10 HP | 7,457 W |
| 50 HP | 37,285 W |
Related Unit Converters
Frequently Asked Questions
How do I convert HP to watts?
Multiply horsepower by 745.7 (mechanical HP). Formula: W = HP × 745.7. For metric HP, multiply by 735.5. For electrical HP, multiply by 746.
How many watts does a 1 HP electric motor actually draw?
A 1 HP motor delivers 746 W of mechanical output, but draws 800-950 W of electrical input depending on efficiency (typically 78-93% for NEMA design B motors). The nameplate "1 HP" refers to output, not input — the FLA (Full Load Amps) rating on the nameplate tells you the actual electrical draw.
What is the difference between mechanical HP, metric HP, and electrical HP?
Mechanical HP (imperial) = 745.7 W — standard for US/UK engines. Metric HP (PS/CV) = 735.5 W — standard in EU/Japan. Electrical HP = 746 W exactly — a convenient round number used for motor nameplates. The difference is small (~1.4%) but significant enough that a German 100 PS engine is only 98.6 imperial HP.
Engineering Context
The HP-to-watts conversion governs every motor-driven system specification. NEMA MG 1 defines motor horsepower standards in the US, but IEC 60034-1 uses kW — a 7.5 kW IEC frame 132 motor is the exact same physical size as a 10 HP NEMA 215T frame, but the EU version is rated 0.05 HP less (7.5 kW = 10.06 HP). In pump systems, the water horsepower (WHP) formula is: WHP = Q × H × SG ÷ 3,960, where Q is flow in GPM and H is head in feet. Converting WHP to motor input watts: W = WHP × 745.7 ÷ η_pump ÷ η_motor. A 100 GPM pump at 150 ft head with SG=1.0 and combined efficiency of 65% needs: (100 × 150 × 1 ÷ 3,960) × 745.7 ÷ 0.65 = 4,339 W input — so a 5 HP (3,728 W) motor is undersized; you need a 7.5 HP unit. Generators follow the same logic in reverse: a 20 kW standby generator is 26.8 HP at the shaft, but the engine must produce ~35 HP (26,100 W) to overcome alternator efficiency (~85%) and still deliver 20 kW electrical. The difference between 20 kW nameplate and 26.1 kW input is 6.1 kW (8.2 HP) of pure thermal loss — exactly why generators have large cooling fans and derating curves above 40°C ambient. In EVs, the 745.7 factor is built into every vehicle controller: when a Tesla driver sees "300 kW" on the display during Supercharging, the car's on-board charger is drawing the equivalent of 402 HP — more power than the drive motors themselves consume at highway cruise. This is why DC fast charging cables are liquid-cooled: 300 kW at 400 V is 750 A, enough to melt an uncooled CCS connector in under 60 seconds.
More: watts to hp · hp to kw · kw to hp · kwh to joules · Guide