In a blog post published yesterday, Tesla CTO JB Straubel addresses Model S acceleration and horsepower concerns recently expressed by Tesla owners and car publications. Some have suspected and even accused Tesla of misrepresenting the power output of the Model S for a while, and now Straubel explains the logic behind Tesla’s horsepower and performance calculations…

Based on the name alone, horsepower is arguably an outdated unit of measurement and Tesla’s CTO argues that it’s even less useful when talking about electric vehicles. For reference, 1 horsepower is equal to 550 foot-pounds per second (745.7 watts).

Straubel said:

“Defining electric power in terms of horsepower is not very intuitive. Kilowatts or Megawatts are a much more useful unit. Electricity alone can’t generate physical motion the way a horse or a fuel-burning engine does. An electric motor converts electricity into motion. Think of electric power as flowing much like fuel flows from a tank to an engine. Various situations (low state of charge, cold temperatures, etc.) can reduce this flow of electrons below the ultimate capability of the electric motor.”

This is the main point Straubel is making – that the electric motor’s power output is consistent, but dependent on the battery pack’s capacity, which is inconsistent because of the state of charge (SoC). Even though the company advertises the true horsepower of its motors, the real results can vary depending on the state of charge of the battery pack:

The difference is most obvious to drivers when the battery is at a very low SoC. In this state, the chemical reactions generate less voltage and less equivalent horsepower, even though the physical electric motor hasn’t changed. The maximum torque the electric motor(s) are capable of is nearly unchanged as the battery horsepower changes even though the maximum shaft horsepower is reduced as the battery horsepower reduces.

On the rear wheel drive single motor versions of the Model S, the company simply advertises the shaft horsepower rating of the motor. When it comes to the dual motor all-wheel drive versions of the Model S, the company combines the rating of the two motors.

This is something most, if not all, Model S owners knew, but the main complaint is that a Model S can achieve the advertised horsepower only in the best possible conditions. While Tesla has been pushing the limits of its motors, the amount of times where the battery can follow is decreasing. Straubel is being upfront about it:

As we have pushed the combined motor horsepower higher and higher, the amount of times where the battery chemical horsepower is lower than the combined motor horsepower has increased.

Tesla’s CTO doesn’t seem to indicate that the company plans on changing its method to calculate horsepower, but Tesla is acknowledging the situation and argues that this is the most “straightforward” way of calculating the car’s power.

You can read the full blog post here.

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