Understanding Electric Outboards: Horsepower vs Kilowatt

Understanding Electric Outboards: Horsepower vs Kilowatt

One of the first things that you'll notice if you're thinking about repowering your boat with an electric outboard is that unlike combustion engines, electric outboard motors aren't measured in horsepower, they're measured in kilowatts. What's the difference? Well, horsepower and kilowatts are both a measure of power. If an electric outboard is rated at 25kW, then 25kW is its maximum power output at full throttle. If an internal combustion outboard is rated at 40hp, then 40hp is its maximum power output at full throttle.

There's an easy way to convert power between kilowatts and horsepower. One horsepower is equivalent to 746 watts or .745 kilowatts, and one kilowatt is equivalent to 1.341 horsepower. So, if we multiply the power rating of an electric outboard engine in kilowatts by 1.341, we should get the electric outboard's power rating in horsepower. But you don't have to do much math before you start scratching your head!

For instance, Pure Watercraft manufactures an electric outboard that has a power output of 25kW, and Pure Watercraft states that it is comparable to a 50hp internal combustion outboard. But 25kW x 1.341 only leaves us with 33.5hp. Is Pure Watercraft lying to us? Torqeedo manufacturers at 12kW electric outboard, and Torqeedo states that it's comparable to a 25hp internal combustion outboard. But 12kW x 1.341 gives us just 16hp! Is Torqueedo lying to us as well?

Torqeedo's cruise 12 electric outboard. Torqeedo states that this 12kW motor is equivalent to a 25hp combustion outboard.

It turns out, neither Pure nor Torqeedo is lying to us. The answer lies in the difference between input power, shaft power and propulsive power, and the difference in torque curves between electric outboards and internal combustion outboards. I could try to explain it, but Torqeedo has done an excellent job in their 2023 catalogue; an excerpt is below.

"The most meaningful performance indicator of a drive system is propulsive power, which indicates the power delivered by the motor to drive the boat, while taking all losses, including propeller losses, into account. This method has been used in commercial shipbuilding for nearly 100 years. Manufacturers of combustion engines often advertise less informative measurements, such as the shaft power, input power, or even the static thrust. That wouldn’t be so bad if the differences between power ratings were minimal, but that isn’t the case: a gasoline outboard with an advertised shaft power of 6 HP actually provides a mere 1.6 HP of propulsive power."

"Torqeedo efficiency ratings not only refer to motor efficiency, but also disclose losses in motor, electronics, cables, gears and propellers. Thanks to our focus on optimizing the entire system, Torqeedo motors deliver the highest overall efficiency on the market. When combustion engines burn gasoline or diesel, they primarily use the stored energy to produce heat: 5-15% of the supplied energy is used to propel the boat and the rest is lost due to inefficiencies. A Torqeedo drive converts between 44% and 56% of the available energy into propulsive power, extending range and runtime. A Travel motor can propel a light boat more than 10 nautical miles and only consume the equivalent of a few teaspoons of gas."

"Electric motors can achieve the same propulsive power as combustion engines with a significantly lower shaft power because of the different torque curves they produce. Electric motors deliver ample torque, which is available at any rotational speed. This characteristic allows them to turn large, efficient, high-pitch propellers that would cause an equivalent combustion engine to stall at startup. At Torqeedo, we always compare the actual propulsive power of our motors with combustion engines. A Torqeedo motor specified as a “6 HP equivalent” provides the same power as a 6 HP combustion engine, even though its shaft and input power may be lower."

This image comes from Torqeedo's 2023 North American catalogue

We hope this makes it easier for you to compare an electric outboard to a traditional outboard engine if you're thinking about repowering with an electric outboard. If you have other questions about electric outboards, electric boats, or batteries for electric boats, please send us an email to luke@emoelectric.co and we will do our best to answer your questions!

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