HP to Watts Calculator

To convert horsepower to watts, multiply by 746 for electric HP: 1 HP = 746 watts. So a 5 HP motor is 5 × 746 = 3,730 watts (3.73 kW). Two other definitions exist: mechanical (imperial) horsepower is 745.7 W, and metric horsepower (PS/CV, common in Europe) is 735.5 W. For most electrical work — motors, pumps, and machinery in North America — use the electric horsepower factor of 746 W.

The choice of factor matters only at the sub-percent level for a single horsepower, but it compounds for large machines and precise specifications. The calculator above lets you pick the exact definition. To reverse the conversion and turn watts back into horsepower, divide watts by the same factor: watts ÷ 746 gives electric HP.

Horsepower is a unit of power, like the watt, so the conversion is a simple multiplication by a constant:

Three standard definitions of horsepower are in use, each with its own factor:

• Electric horsepower = 746 W (exactly). Defined for electric motors; this is the value used on virtually all North American motor nameplates and in the National Electrical Code.
• Mechanical / Imperial horsepower = 745.7 W. The original James Watt definition (550 foot-pounds per second); used for engines and mechanical power.
• Metric horsepower (PS, CV, pk) = 735.5 W. Defined as 75 kilogram-force-metres per second; standard in Europe and Japan for engine ratings.

The differences are small — about 1.4% between electric and metric HP — but selecting the right one keeps large-motor and engine figures accurate. To convert the resulting watts to volts or amps, use the watts to volts calculator or watts to amps calculator.

These examples show the conversion for each definition, with arithmetic shown.

Example 1 — 1 HP electric motor: 1 × 746 = 746 W = 0.746 kW. This is the benchmark most people memorize.

Example 2 — 5 HP pool pump (electric): 5 × 746 = 3,730 W = 3.73 kW. At 240 V and power factor 0.9, this pump draws roughly 3,730 ÷ (240 × 0.9) ≈ 17.3 A of input current — though motor input exceeds shaft output due to efficiency.

Example 3 — 100 metric HP engine: 100 × 735.5 = 73,550 W = 73.55 kW. A European car advertised at "100 PS" produces about 73.5 kW, which equals roughly 98.6 electric HP — a small but real difference from the metric figure.

Example 4 — 10 HP mechanical: 10 × 745.7 = 7,457 W = 7.457 kW. Using the electric factor instead would give 7,460 W, a 3 W difference — negligible for most purposes but worth matching to the correct standard on spec sheets.

A crucial distinction: horsepower on a motor nameplate usually refers to mechanical shaft output, not electrical input. Because motors are not 100% efficient, the electrical watts drawn from the supply exceed the output watts.

For example, a 5 HP electric motor outputs 3,730 W at the shaft. If the motor is 88% efficient, it draws 3,730 ÷ 0.88 = 4,239 W from the supply. This input figure — not the shaft output — determines wire size, breaker rating, and utility billing.

Typical motor efficiencies: small single-phase motors 70–85%, three-phase induction motors 85–93%, premium-efficiency (NEMA Premium / IE3) motors 90–96%. When sizing conductors or a generator, always convert HP to shaft watts, then divide by efficiency to get true input demand. Combine this with the watts to amps calculator to find the supply current, and the amps to kW calculator to relate it back to panel capacity.

To go from watts back to horsepower, simply divide by the same factor:

Using the electric definition, watts ÷ 746 = HP. For example, a 2,000 W device is 2,000 ÷ 746 = 2.68 HP. A 1,500 W vacuum motor is about 2.0 HP. Because 1 kW ≈ 1.34 electric HP, a quick mental estimate is to multiply kilowatts by 1.34 to get horsepower, or multiply HP by 0.746 to get kilowatts.

Handy reference points: 1 kW = 1.341 electric HP = 1.360 metric HP. 1 electric HP = 0.746 kW. These reciprocals let you convert either direction quickly. For engine ratings quoted in kilowatts (common on modern European vehicle documents), divide by 0.7355 to recover metric horsepower (PS).

1. Enter the horsepower. Read it from the motor or engine nameplate. Decimal values like 0.5 or 7.5 HP are fine.
2. Select the horsepower type. Choose Electric (746 W) for North American motors and electrical work, Mechanical (745.7 W) for imperial engine ratings, or Metric (735.5 W / PS) for European and Japanese engine specs.
3. Read the watts and kilowatts. The primary output is watts; the secondary outputs show kilowatts, the factor used, and the equivalent in metric horsepower.
4. Adjust for efficiency if needed. For electrical demand, divide the shaft output by the motor efficiency to find true input watts.

The result updates live. Use it to size electrical service for motors, compare engine ratings across measurement systems, or translate machinery specs into kilowatts for energy calculations.

Translating horsepower into watts bridges the mechanical and electrical worlds:

Electrical service sizing. Motor nameplates list horsepower, but conductors and breakers are sized in amps derived from watts. Converting HP to watts (then to amps) is the first step in applying NEC motor-circuit rules.

Generator and backup power. To run a well pump, air compressor, or shop tool rated in HP, you must know the running watts (HP × 746 ÷ efficiency) and the surge watts, which can be 3–7× higher at startup for motor loads.

Automotive and engine comparison. European cars are rated in PS (metric HP) or kW, while American cars use SAE horsepower. Converting to a common unit — watts or kilowatts — lets you compare vehicles fairly.

HVAC and pumps. Compressor and pump motors are specified in HP. Converting to kilowatts feeds directly into energy-cost and efficiency calculations, and pairs with the watts to volts calculator to verify supply voltage.

Solar and off-grid systems. Well pumps, water pumps, and shop equipment are the loads that most often determine how big an off-grid inverter and battery bank must be, precisely because their horsepower ratings translate into large surge and running watts. A 1.5 HP submersible pump outputs about 1,119 W at the shaft, but its electrical draw is higher after efficiency losses, and its inrush at startup can momentarily hit three to seven times that figure. Converting HP to watts first, then dividing by motor efficiency, then adding a surge margin, is the disciplined way to avoid an inverter that trips every time the pump cycles.

Fitness and appliance ratings. Consumer products from treadmills to shop vacuums and blenders advertise "peak horsepower" figures that often exceed what the device can sustain continuously. Converting the honest continuous rating to watts (HP × 746) and comparing it to the appliance's actual amp draw on a 120 V circuit is a quick reality check: a genuine 2 HP continuous motor would need about 1,492 W of output, which is more than a standard 15 A outlet can supply once efficiency is included — a sign the "peak" number is marketing rather than sustained capability.

The table below lists common motor and engine sizes converted to watts and kilowatts using electric horsepower (746 W), the standard for electrical work. Use it as a quick lookup when sizing circuits, generators, or energy budgets, and cross-reference the metric-horsepower column when reading European or Japanese specifications.

Notice how the numbers scale linearly: every horsepower adds exactly 746 W (electric) or 735.5 W (metric). A fractional-horsepower motor like 1/3 HP is 249 W, small enough for a standard 120 V branch circuit, while a 50 HP industrial motor at 37.3 kW demands three-phase service and heavy conductors. The kilowatt column is what feeds directly into energy-cost math — multiply by run hours and your rate using the watts to kWh calculator to estimate operating expense.

The horsepower unit dates to the 1780s, when James Watt needed a way to sell his steam engines to customers who thought in terms of the draft horses the engines replaced. Watt measured that a strong horse could turn a mill wheel to lift roughly 550 foot-pounds of weight per second, and he defined one horsepower as that rate of work. Converting those imperial units into SI gives 745.7 watts — the mechanical horsepower still used today.

The electric horsepower of 746 W is a slight, deliberate rounding adopted so that motor ratings map cleanly onto electrical calculations; the tiny 0.3 W difference from mechanical HP is immaterial in practice. The metric horsepower arose separately in continental Europe, defined as the power to lift 75 kilograms one metre in one second under standard gravity, which works out to 735.5 W. Because it is defined in metric units, it differs from the imperial value by about 1.4%.

Understanding this history clarifies why three factors exist and when each applies. Electrical engineers and electricians in North America use 746 W because it aligns with code and motor nameplates. Automotive engineers quoting SAE figures use the mechanical 745.7 W lineage. European and Japanese manufacturers advertise PS (Pferdestärke) or CV (cheval vapeur), both equal to 735.5 W. When comparing a "150 HP" American engine to a "150 PS" European one, the American figure is genuinely larger — 150 electric HP is 111.9 kW versus 110.3 kW for 150 PS. The calculator above keeps these straight so you never mix standards.

• Using the wrong HP definition. Mixing metric PS with electric HP introduces a 1.4% error — significant for large engines and precise specs.
• Confusing shaft output with electrical input. Nameplate HP is usually mechanical output; the supply must provide more watts due to motor inefficiency.
• Ignoring startup surge. Motors draw several times their running watts at startup; generators and inverters must handle this surge.
• Rounding 746 to 750. Small rounding accumulates across many motors or large ratings; use the exact factor.
• Forgetting power factor when finding amps. Converting the resulting watts to current on an AC motor requires the power factor.

For the downstream electrical conversions, use the watts to amps calculator, the volts to watts calculator, and Ohm's law explained.