What is Horsepower Rating: Good Explain


So, sometimes I look at my traffic and links here, and the post about horsepower ratings gets consistent traffic, and is linked by a number of discussions all over the charts – even appliance repair forums.  This one, though, is from the Tesla Owners page, and it’s awesome.  So I’m linking it.  Sort of a meta-link, since he links me.  The Wonders of the Interwebs.

Seriously, it’s probably the most concise, to the point, and factually correct explanation of power ratings, torque ratings, how gas and electric motors generate power, and even the historical significance of the convention.  Just say “no” to the comments, though.  I guess even the Tesla forum gets its trolls.


For electric motors the power is often rated as to how much power you can supply to them safely so that engineers can design systems around them. Public didn’t really come that much in contact with electric motor details and engineers were happy. Electric motors are VERY efficient compared to ICE and the difference between input and “output power” isn’t that huge – it is kind of like a sales tax.

Internal Combustion Engines are way more complicated beasts and are horrendously inefficient when compared to electric motors. They leak and squirt that input power everywhere. They can’t even process it equally efficiently across various speeds. Because of this their “input power” was never really measured in power units but rather in their fuel consumption. Even that is rarely stated – we mostly see various “good efficiency” fuel consumption numbers that don’t relate to the power listed. The HP rating that we get for ICEs is the “output power”.



2 responses to “What is Horsepower Rating: Good Explain

  1. Interesting perspective, and spot on. We think of battery HP in an EV as the main indicator of power, perhaps because it’s the easiest number to determine. Calculating the output power in an EV where you need to factor in motor and drive train efficiency, complicates things a bit.

    Those of us that drive or ride electric already know the efficiency of electric vs. ICE simply by the cost of fuel. But I never realized that it’s an input versus output rating. hmmm, no wonder that 1/2 gallon tank of electrons gets me so far.

  2. The idealised “maximum acceleration” and “maximum output power” graphs are flat-out wrong, whether intentional or not.

    #1 gearing reduces wheel torque but spreads the torque curve over a wider range of wheel speed. Ignoring drag and other factors, acceleration is proportional to wheel torque.

    The graph should appear with the torque curve for each gear being broader and shorter.

    See figure 3 from this article written by Marc Fenigstein.

    #2 A motor producing 92 Nm at 4000 RPM is also producing 38 kW of power. The gear selected changes the location of the power peak and width of the power curve relative to wheel speed.

    The graph should appear with the power curve for each gear being broader but the same height.


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