Let me simplify this for you such that we might get agreement....
it seems there is a portion of people what implies "horsepower" with no modifier as defined to be "horsepower the car makes somewhere in the system"
...
I think this is a better description.
That is too loose, which is why I went more specific in specifying the motor shaft. Under that criteria you can go all the way upstream to the chemical power of the battery (before internal resistance is accounted for) and get 400V*1300A = 520kW / 697hp (rather than ~320V*1300A = 416kW / 558hp at the battery output after energy is lost to heat via internal resistance).
Do you not agree that if the vehicle has "691 hp" labelled that somewhere in the system it should be measurable as such?
If the vehicle has "691hp" labeled, the standard convention is that it describes the power of the engine (or motor) rated with various attachments to it depending on the standard used. That was how horsepower was specified and defined since the car was invented (again google definition: "a unit of power equal to 550 foot-pounds per second (745.7 watts). the power of an engine measured in terms of this.").
You can go look up some articles on cars coming out. The way they describe horsepower is in reference to the engine. Here's an example just posted a couple hours ago:
Here’s the really important thing about the new 2016 Mazda CX-9: a 2.5-liter turbocharged four-cylinder engine with 250 horsepower and 310 pound-feet of torque.
http://truckyeah.jalopnik.com/the-2016-mazda-cx-9-turns-your-family-hauling-into-a-tu-1743383090
The focus on the engine is why horsepower has continually been specified by automakers on the output shaft instead of the wheels. The way it is measured is also always starting with the engine on a test bench and then adding attachments to it (rather than taking a car apart to measure at shaft). If we really wanted to describe the car as a whole, there is really only one horsepower metric that matters: wheel horsepower.
While my example of SAE gross power was continually brushed off, I note that it does not match the criteria above: "horsepower the car makes somewhere in the system". Yet Ram still uses that standard today in their diesel trucks and says "385 hp" on their website with no asterisks or footnotes of any kind.
Basically my point is "horsepower the car makes somewhere in the system" is the opinion of some people here on what "horsepower" means when used without a modifier. However, I don't agree that is the fundamental definition of horsepower.
You can go through the entire list of existing horsepower rating standards used by the auto industry:
https://en.wikipedia.org/wiki/Horsepower#Engine_power_test_standards
You will find
every one of them meets the definition of "horsepower at the output shaft of the engine/motor", but there are plenty that
do not meet "horsepower the car makes somewhere in the system" because they don't necessarily have all accessories attached or with factory standard accessories/settings.
Some examples: SAE J1995 gross, SAE J2723 certified (SAE J1995 gross section), ECE R85 (electric drivetrain), ISO 2534, JIS D 1001 gross.
If not, then why not just say the motors support infinity horsepower and then let customers take you to court when in the year 3999 GWh batteries are available and the motors fail at around 800 hp?
This is a straw man. Motors (and in Tesla's standard, the motor controllers/inverters) can't support infinity horsepower. Motors are limited by rpm, operating voltage, and thermal (and by extension current) limitations. Motor and electric drivetrains are commonly specified by power independent of the battery. "Motor power" is but one example. Here's some examples previously posted:
UQM 150kW system:
https://uqm.com/products/full-electric/prototype/commercial-vehicles/
AC Propulsion 75kW and 150kW systems:
http://www.acpropulsion.com/products-drivesystem.html