I think the reasonable way to approach this question is to set the denominator to the MSRP less the discounts that a new car purchase will enjoy. Then the fractional depreciation is equal to 1 - (used_car_price)/the_amended_MSRP)
Example:
MSRP 50k
Tax-credits: 10k
Manufacturer Discount: 5k
Used_car value: 25K
Then the depreciation is 1 - 25/35, = to 10/35
Note that Tesla's insistence in selling cars at MSRP gives them a built-in advantage in the "how much depreciation game" compared to other EVs. The LEAF is a great example, particularly from early 2017 when 10k discounts were offered by Nissan in addition to the $7.5 federal tax credit and whatever state credits were available. In my state of Colorado it is $5k. The MSRP was about $32, but the out the door price in Colorado before taxes was (32-10-5-7.5) = $9.5k. In two years lets guess that a person can sell the car for $6.5 in Colorado and the same rebates are in effect.
- The standard way of calculating depreciation suggests that the car lost 25.5/32 = 79.7% of its value
- My way of calculating depreciation calculates out to 1 - (6.5/9.5) = 3/9.5 = 31.8%
- And of course the owner depreciation cost is $3k which is not expected at first glance from a car that drops like a rock in value.
Compare that last value of $3k depreciation loss to a Tesla Model S/X that costs about $20k depreciation after a couple of years. Each person will have to make a value judgement of how much more value a Tesla is compared to a LEAF. For me it is about double or
maybe triple, so the normalized depreciation compared to a LEAF would in my mind be $6k - $9k and I would conclude that the Tesla depreciates considerably faster than a LEAF.
I have never shopped for non Japanese brands so my impression of a consumer expectation of 10-20% off MSRP may be off. If it is correct then oddly enough the EV tax credit is not too far off from the standard discounts in ICE sales and the generic depreciation fractions can be read without amendment. Each car model of course may have its own market quirks and the generic calculation may be waaay off, as the LEAF example demonstrates.
Phew. If anybody reads even half of this post I'll be amazed.