Kilowatts to Horsepower: Converting Generator and Motor Ratings

What a kilowatt and a horsepower each measure

Both are units of power, which is the rate of doing work. Power is not the same as energy. A kilowatt is a rate; a kilowatt hour is a quantity of energy. Confusing the two is the single most common reading error on a generator or battery spec sheet.

A watt is the SI unit of power, and a kilowatt is a thousand watts. Kilowatts are the standard unit for electrical load, and you will see them on generator output figures, motor plates and pump curves. In the United States you will also often see the same information given as watts and amps rather than kilowatts.

Horsepower is the older unit, still standard in the United States for engines, outboards, compressors and shaft-driven pumps. It survives because buyers are used to it, and because engine makers have decades of catalog history in it. It is a general remark rather than a technical fact that the unit was originally pitched against the output of a draft horse, so treat that story as background colour, not specification.

Because both units measure the same physical quantity, converting between them is a single multiplication. No efficiency, no assumptions, no fudge factor. That only becomes complicated when the two numbers on the page are measured in different places, which the generator section below deals with.

The conversion formula, both directions

One kilowatt equals about 1.341 mechanical horsepower. So:

HP = kW x 1.341

kW = HP x 0.7457

That is the whole thing. The two factors are reciprocals of each other, so you can divide by one instead of multiplying by the other if that is easier to do in your head. For a rough field estimate, multiplying kilowatts by 1.34 or by four thirds gets you within a percent, which is closer than most nameplate tolerances anyway.

Worked example. A pump motor plate reads 5.5 kW. 5.5 x 1.341 = 7.4 HP. Going the other way, a 25 HP outboard is 25 x 0.7457 = 18.6 kW. If you want the conversion without a calculator, remember that 3 kW is roughly 4 HP and scale from there, then check the table below.

Quick reference table for common ratings

These are the ratings that turn up most often on generators, pumps, compressors and small engines. Figures use the mechanical horsepower conversion factor and are rounded to one decimal place.

The relationship is linear, so scaling works: double the kilowatts and you double the horsepower. One caution, though. If you scale a value taken from this table, you carry its rounding error with it, and multiplying by ten multiplies that error by ten too. For anything where the exact figure matters, run the multiplication on the unrounded number instead of scaling a rounded one.

Why mechanical, electrical and metric horsepower differ

There is more than one horsepower. Three definitions turn up on equipment you might buy, and they are close enough that the difference hides easily and far enough apart to matter on a large machine.

Mechanical horsepower, sometimes called imperial or brake horsepower, is the one behind the 1.341 factor above. It is defined as 550 foot-pounds-force per second, which works out to about 745.7 watts. This is the default in the United States for engines and outboards.

Electrical horsepower is defined as exactly 746 watts. It is used in the United States for electric motor ratings. The difference from mechanical horsepower is small enough to ignore in almost any practical sizing job.

Metric horsepower, written PS, CV or ch depending on the country, is defined as 75 kilogram-force metres per second, about 735.5 watts. It is about 1.37 percent smaller than mechanical horsepower. On a 400 HP engine that difference is about 5.5 horsepower. If you are comparing a European-specified engine against an American one, check which horsepower each figure uses before you conclude one is stronger.

Reading generator labels: running watts, starting watts and engine horsepower

A generator carries at least three power numbers, and none of them converts into the others with the 1.341 factor.

Running watts, sometimes called continuous or rated output, is the electrical power the generator will supply indefinitely. This is the number you size your loads against. Starting watts, also called surge or peak, is a short-duration figure for the inrush an electric motor pulls as it spins up. It is higher, and it is not available continuously.

Engine horsepower is the mechanical output of the engine turning the alternator. It is always larger than the electrical output in equivalent units, because the alternator is not perfectly efficient and the engine also has to drive its own cooling, fan and accessories. So a generator advertised at 7.5 kW output on a 13 HP engine is not a contradiction. Convert the 7.5 kW and you get about 10.1 HP of electrical output, and the gap between that and 13 HP is the conversion loss plus engine overhead.

The practical rule: never size your loads off the engine horsepower. Use the running watts. And when a listing gives you only engine horsepower, treat it as an upper bound on what could ever come out of the outlets, not as the output figure.

Sizing a generator or pump for what you actually run

Add up the running wattage of everything that will be on at the same time, then find the largest single starting load in the group and add that surge on top. That gives you the peak the generator has to survive, and the running total gives you the continuous rating it has to hold.

Many appliances list wattage on their data plate. Many others list only volts and amps, in which case volts x amps gives you volt-amps, which for a resistive load such as a heater or an incandescent lamp is close enough to watts. For motors, compressors and anything with a transformer, volt-amps run higher than watts because of power factor, so the wattage figure is the one to hunt for if you can find it.

Motor-driven loads are what catch people out. A refrigeration compressor, a well pump or an air compressor draws several times its running current for a moment at startup. If two of them can start at the same time, size for both. Sequencing the loads by hand, so the pump is not starting while the compressor is already spinning up, is often cheaper than buying a larger generator.

Two more things reduce what you actually get. Altitude and high ambient temperature both cut engine output, and manufacturers commonly publish derating guidance for their own machines. Treat that as general advice and read the figures in your own engine's manual rather than applying a rule of thumb. Second, a generator held near its rated output continuously runs hotter and burns more fuel per hour than one loaded moderately, so leaving headroom is not wasted money.

Motors, volts and why current is the number that sizes wiring

Power tells you what a machine can do. Current tells you what wire, fuse and switch it needs. They are related but you cannot size a circuit from a kilowatt figure alone, because the same power can arrive at different voltages.

For a DC circuit, watts = volts x amps. Rearranged, amps = watts / volts. The same power delivered at double the voltage requires roughly half the current. That is why 24 volt systems use smaller conductors than 12 volt systems for the same job, and why a lot of marine DC gear is specified in amps rather than in watts at all. The Samlex America SDC-30 step-down converter GearPort stocks, for instance, is specified at 30 A continuous with a 35 A current limit, not in kilowatts.

AC three-phase adds a square root of three term and a power factor, which is why a three-phase motor plate lists kilowatts, amps, voltage and power factor together. For sizing purposes, use the amps on the plate rather than deriving them from the kilowatt figure.

Once you know the amps and the round-trip length of the run, you can pick a conductor size. Our marine wire size chart covers that part, and the marine wiring basics guide covers circuit protection and connections.

Common mistakes when comparing two machines on paper

Comparing an engine's horsepower to a generator's kilowatt output. This is the big one, and it is covered above. Convert both to the same unit and compare like against like, or better, compare the two output figures and ignore the engine numbers entirely.

Mixing metric and mechanical horsepower. A 150 PS engine and a 150 HP engine are not the same engine. Check which definition a spec sheet uses, particularly on imported equipment.

Confusing input power with output power on a pump. A pump can be rated by the electrical power it consumes or by the mechanical power at the shaft, and hydraulic output at the discharge is lower again. Compare pumps by flow at a stated head, not by their power rating. Our bilge pump sizing guide goes through why the flow figure on the box is not the flow you get.

Ignoring a horsepower rating that is a hard fitment limit rather than a performance claim. Some hardware is rated by the engine it can carry, like the Sea-Dog Poly Rail Mount Motor Bracket 327150-1 in white that GearPort stocks, which is rated for small two-cycle outboards up to 8 HP. Anodes are similar: kits are often specified by engine make together with a horsepower band, as with the Tecnoseal 21101AL aluminium kit in the catalog, listed for Yamaha 150-200HP outboards. Converting those numbers to kilowatts does not help you, because the rating is a fitment envelope, not a measurement.

On boats, there is also a plate to check. Under 33 CFR Part 183 Subpart C, manufacturers of monohull boats less than 20 feet in length must display a capacity plate showing a maximum horsepower rating. Whether an operator may exceed it, and by how much, is a matter of state law, so check your own state's rules rather than assuming the federal marking regulation settles it.

Common questions

How many horsepower is 1 kW?

One kilowatt is about 1.341 mechanical horsepower. So 1 kW is a little over one and a third horsepower. For a quick mental estimate, multiply kilowatts by 1.34, or by four thirds, which gets you within about a percent. That is well inside the tolerance of most nameplate ratings.

How do I convert horsepower to kilowatts?

Multiply horsepower by about 0.7457. A 25 HP outboard is 25 x 0.7457, which is 18.6 kW. Dividing horsepower by 1.341 gives the same answer, since the two factors are reciprocals. Use whichever you find easier to remember.

Why is a generator's kW output lower than its engine horsepower?

Because they measure power at different points. Engine horsepower is mechanical output at the crankshaft. The kilowatt figure is electrical output at the outlets, after alternator losses and after the engine has driven its own cooling and accessories. Always size your loads against the running watts, never against engine horsepower.

What is the difference between mechanical, electrical and metric horsepower?

Mechanical horsepower is about 745.7 watts and is the US default for engines. Electrical horsepower is exactly 746 watts and is used for electric motors. Metric horsepower, written PS or CV, is about 735.5 watts, roughly 1.37 percent smaller than mechanical. On a 400 HP engine that gap is about 5.5 horsepower.

Are running watts and starting watts the same thing?

No. Running watts, also called continuous or rated output, is what a generator supplies indefinitely, and it is the number you size loads against. Starting watts, also called surge or peak, is a short-duration figure covering the inrush when an electric motor spins up. It is not available continuously.

Can I work out the amps a machine draws from its kilowatt rating?

For DC, yes: amps equals watts divided by volts. For AC, particularly three phase, you also need power factor and a square root of three term, so use the amps printed on the motor plate instead. Current, not power, is what determines wire gauge and fuse size.

Sources

• Derived from the definition of mechanical horsepower as 550 foot-pounds-force per second, approximately 745.7 W: 1000 / 745.7 = 1.341. NIST Special Publication 811, Guide for the Use of the International System of Units.
• Reciprocal of the mechanical horsepower conversion factor; mechanical horsepower equals approximately 745.7 W per NIST Special Publication 811.
• NIST Special Publication 811, Guide for the Use of the International System of Units, conversion factors for power.
• NIST Special Publication 811, Guide for the Use of the International System of Units (horsepower, electric).
• NIST Special Publication 811, Guide for the Use of the International System of Units (horsepower, metric).
• Arithmetic from the NIST values: (745.7 - 735.5) / 745.7 = 1.37 percent; 400 x 0.0137 = 5.5.
• SI Brochure, 9th edition, BIPM, derived units and prefixes.
• Definition of electrical power in a DC circuit, P = V x I (Joule's law).
• Standard three-phase power relationship P = sqrt(3) x V x I x power factor.
• 33 CFR Part 183 Subpart C, Safe Loading, US Coast Guard boat manufacturing standards.
• GearPort catalog listing, Samlex America 24VDC-12VDC Step-Down Converter SDC-30.
• GearPort catalog listing, Sea-Dog Poly Rail Mount Motor Bracket 327150-1 White.
• GearPort catalog listing, Tecnoseal Anode Kit 21101AL Aluminum Yamaha 150-200HP Outboard Motor.