I agree with your sentiment, but not your proposed method. I need to build context for why.
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ICE vehicles are required to post
fuel efficiency per distance (MPG, L/100km, etc.), and the
amount of fuel (gallons, litres) is usually somewhere in the public specs as well. These are well-understood units by most of the population at this point, as well as being something you can actually see (amount) or
find an equivalent physical interpretation of (efficiency).
The equivalent for
amount of "fuel" for EVs would be kWh, and efficiency something like Wh/km or mi/kWh. This "fuel" measure is unfamiliar to almost everyone, but distance is familiar. Further, distance for EVs is
important due to the current lack of fast fueling infrastructure compared to ICE fuel. Marketing distance is therefore a necessity, though not technically desirable.
This has led to confusion because vehicles are magic as far as most people are concerned, and electricity even more so. The common understanding of ICE fuel efficiency (drops at higher speeds, etc.) could very well not apply to the "magical" electricity. And EVs have so much torque, you might not even realize it requires so much more power to ascend a hill for example (because it
feels easy).
(Of note, even this efficiency understanding might not be common for ICE. See: people driving faster to "reach a gas station" when low)
I still struggle with what exactly a kWh is. It's a 100W bulb for 10 hours. It's a 1800W microwave for about 33 minutes. These are very abstract and not perceptibly physical. I get about 132Wh/km on my drives. Is that
good? Is that like a 20MPG car, or a 50MPG car? I still can't answer that. We have very little reference for EVs.
I would
love if they primarily published Wh/mi or Wh/km, and then the usable battery capacity. This would be equivalent (ish) to what they publish for ICE vehicles. The EPA even does this, but the Wh/mi they show is based on total efficiency with a standardized charge setup - for Model 3, it's about 89% efficient. The result is where the car tells you that you are driving at the
rated efficiency (say, 234Wh/mi), the EPA says that number is more like 263.
Both numbers are correct in their own way - the EPA is telling you what to expect in
fueling costs (since the charging efficiency matters here) from the wall outlet to the battery, whereas the car is telling you how much energy it is taking from the
battery to move.
It's messy, confusing, and you can paint a lot of different pictures. EVs are just more complicated to represent than an ICE vehicle that goes through a physical quantity of matter.
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Regarding daily usable range, this is a tricky one.
I'll advocate to anyone that they need to take the current advertised range and only take 80% of it. This is to reflect using 10-90%: not above 90% daily to preserve the battery, and above 10% for your sanity to make it to the next charge (you do this for ICE as well, fueling stations are just more common).
Now, will we always need to subtract 10% for daily charging to preserve health? Perhaps a manufacturer bakes this 10% buffer in and we don't need to consider it, and charging to "100%" is fine. Or, maybe battery developments in the future mean that storing it at 100% isn't a problem.
A sort of "Daily recommended usable" could suffice here, but I struggle with how to define it in a way the manufacturers would be on board with. Event the 90% daily recommendation from Tesla isn't wholly scientific and precise - it's rounded to 10% increments! What would the threshold be for what is an OK state of charge? What if the battery chemistry changes? This is so incredibly hard to define (in a way that allows adaptation to future technologies and advancements) that I believe any effort spent doing so would be wasted. At least from a regulatory standpoint - obviously, per-model, fan communities and the manufacturers
do communicate best practices and such, and you can infer what states of charge are OK.
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Now, regarding temperature effects on range. I agree they need to convey this impact somehow, but I'm unclear on how. EVs with resistive heaters (like Model 3, etc.) will use a lot more. EVs with heat pumps (Hyundai/Kia, etc.) will use less in
some temperature ranges, but ultimately will be just as bad as a resistive heater at lower temperatures. Future EVs may even include combustion heaters - this is actually a very effective and efficient use of fossil fuels.
Any testing or standardization of range marketing with respect to cold temperatures needs to account for today's realities
and future developments. This problem is easier to tackle from a procedure design perspective, I think. But I do wonder what the requirements are for essentially building huge environment chambers to do these tests. Assuming these don't already exist, they would need to be carefully designed, built, and their value to be clear. This would take a few years.
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And finally, degradation.
Like I mentioned for the other bits, this might change over time. ICE vehicles
also loss efficiency (though not as much as EV fans might have you believe) which impacts their marketed number as well. However, they're not required to publish any sorts of numbers on this. And again, like the other bits, I would struggle with how they would define this. Some engines would behave especially poorly, while others would perform much better on average. Sounds... like degradation!
Of course, the battery degradation does hurt more for all the typical EV range reasons. And unlike an ICE vehicle, true degradation is not fixable (at least not cheaply, as it would require at least
parts of the battery to be replaced, which there isn't much precedent for currently and the whole pack is replaced instead). Further, unlike an ICE, routine maintenance isn't much of a factor in preserving the life and efficiency of the powertrain, so less of the liability of decreased performance lies with the customer. It is instead inherent in the design.
But ultimately, like ICE, it would be hard to represent this beyond warranties. New engines are made, new cell chemistries are used. They have expectations, but their true future performance is an educated bet.
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tl;dr: It sucks, but what we have today is best when paired with verbal understandings of degradation and temperature effects. Kind of like we do for ICE vehicles already. I would only prefer additional clear marketing of Wh/mi or Wh/km ratings.
End of essay.
