Electric Car Motors: Horsepower

When it comes to electric car motors, the most commonly asked questions start with: "How many horsepower...?"

You know what I mean...

• How many horsepower do I need to get on the freeway?
• How can a 20 hp electric motor do that, when my little 4-cylinder Corolla has a 120 hp engine, and barely gets up the hills?
• How many horses do I need per pound of car?
• Why does the same electric car motor have different horsepower in different conversions? I don't get that.
• What's the deal on "continuous horsepower" and "peak horsepower" ratings?

The Corolla question (why does my Toyota need every single one of its 120 horses to push its light little body around, while a 24hp WarP 9 in the same car would give it twice the pepper?), that's easy. It's just that the answer kind of makes you want to shake your head and blink.

Answer: Efficiency.

Electric Car Motor Efficiency

When you hear that electric car motors are more efficient than gas-guzzlers, this is what we're talking about. What we mean is "a LOT more efficient". Most of your Corolla's 120 horses are being used to get from 0-35 mph, which gas engines are not that good at - it takes them a while to find their torque.

Electric car motors (especially series wound DC motors like that NetGain WarP 9), on the other hand, have a particular talent for low-end acceleration, which is just one of the many reasons we love them!

The new version of Build Your Own Electric Vehicle by Seth Leitman - recommended reading! - has some really good answers to the horsepower question. Here's the gist:

• Weight to power ratio:
  When you're selecting electric car motors, figure 6-8hp (continuous) for every thousand pounds of finished, loaded vehicle weight. That means figure in the car's curb weight - along with the load of batteries, corn-fed American passengers, and the trunk full of camping equipment. Seth sez: "More horsepower is required for higher speeds, heavier vehicles, and steeper terrains."
• Electric Motor Rating:
  They're rated at their maximum efficiency, which is designated "continuous", although they are capable of short bursts of power 2-4 times greater than their continuous rating (without damaging the motor), called "peak".
• Voltage and Horsepower:
  The horsepower goes up with the voltage. Motors are rated, not only "continuous" and "peak", but at a given voltage, as well. There's no way to figure an EV motor's horsepower without knowing the voltage of the system.

• Over- and Under-powered Motors:
  Don't worry if you get a motor that's a little oversized for your car, that's what the controller is for. DO worry if you get a motor that's a little undersized, because it's bound to fatally overheat sooner or later.
  
  
• The Need for Speed:
  If you like to drive on the freeway, you'll need more horsepower per pound. Seth says it takes four times the horsepower to go 70mph than it does to go 35mph.
  
  

Translating "gas-guzzler engine horsepower" to "electric car motor horsepower" feels a little like translating English to Japanese - not a lot of words in common! Once again, it's - mostly! - all about the voltage.