The vast majority of MSs on the road are 85kWh and I wish it was practical for me to have one. However, the realistic decision I face is 40kWh or 60kWh. I have been debating this over the last few months, but I think I have finally made a decision.
It took a lot of forum reading along with many questions of Tesla, but I am confident that the 60kWh is right for me, and I suspect right for many of the people planning about getting a 40kWh.
I apologize in advance for the long post, even though I promise I did edit out some of trail of thoughts.
For purposes of this discussion, I am ignoring the options as that is a value proposition unique to each individual. I will however state, that a bigger battery ended up being more important to me than some of the options.
It took me a long time to really wrap my head around the battery and what it really meant. At the very beginning, I started by using simply equations to compare the batteries before I found out about things like battery reserve to prevent bricking, range vs. standard charge, HVAC usage, etc. After figuring out many (I'm sure not close to all) of the nuances of the MS, I feel the following are very important to maintaining the quality and useful life of a battery: (1) you should not charge in range mode on an everyday basis - and really not more than a couple of times a month (2) you should also not the let battery consistently fall to below 20% before charging.
Even if you agree with the 2 above points, that info is meaningless unless you know (1) how far you drive, on average, and (2) WHAT RANGE YOU CAN EXPECT FROM EACH STANDARD CHARGE. I yelled (2) because it is by far the most difficult to predict and yet the most important factor.
If we take the 208 EPA rating for the 60kWh and assume 145 for the 40kWh (might actually be 150 when all is said and done), we can start to get some real world numbers. Because I plan to charge in standard mode almost all of the time, 208/145 are not realistic. Assuming standard is 90% of EPA, the new numbers are 187 and 130. Another way to look at this is that due to the approximately 5% reserve to prevent briking and the 10% below max standard charge, you really have 51kWh battery (not 60) and a 34kWh battery (not 40). These numbers now begin to tell us something about what the car expects your Wh/mile to be in order to reach these rated ranges. For the 60kWh battery on a standard charge, you really have 51kWh to use to go the estimated 187 miles, which implies a Wh/mile of 272 (51,000 / 187). For the 40kWh battery on a standard charge, you really have 34kWh to use to go the estimated 130 miles, which implies a Wh/mile of 261 (34,000 / 130). The question is how tough are these numbers going to be to achieve. Now I will admit that on average, I would expect the 85kWh to use a higher Wh/mile than the 60kWh or the 40kWh, but not a significant amount if you are looking at real world driving. Tesla has now said that the 40 weighs less than the 60kWh which weighs less than the 85, so this could also help to lower Wh/mile. I have reached out to some 60kWh drivers and they are saying that can get below 300 without a having to be crazy conservative as long as they don't gun it off the line every time. This initially looks pretty encouraging to be pretty close to the EPA numbers.
However, these EPA numbers and those provided by people in forums (mostly CA residents) almost totally exclude 1 extremely important factors that will affect range - use of HVAC. To give my example below some context, the EPA rating for the 85kWh assumes a Wh/mile of 305. However, most everyday drives in the 85kWh appear to be no less than that and for those using HVAC (specifically heat) those numbers are often 350Wh/mile or even 400Wh/mile. Because I live in Missouri, extreme heat and cold are common throughout the year, so the effect of HVAC is a real concern. If we assume you use an extra 50Wh/mile, the standard charge range for the 40kWh battery goes from 130 miles to 109 miles. For the 60kWh battery it goes from 187 miles to 158 miles. Using an extra 100 Wh/mile drops the 40kWh battery to 94 miles and the 60kWh battery to 137 miles.
So, realistically in the winter time and using a standard charge, we are talking about a range of 94-109 miles for the 40kWh and 137-158 miles for the 60kWh. This however, is the range the day you bring the car home. In 8-10 years, I think 15% degradation is a realistic number to use. Therefore, the winter range with a standard charge for the 40kWh drops to 80-93 miles and the 60kWh drops to 116-134 miles.
After looking at these numbers and remembering I don't want to consistently bring the pack below 20% charged, I just don't think the 40kWh option is practical for anyone wanting to drive more than about 70 miles between charges. Although that is still a lot relative to other EVs, it is significantly less than what the 40kWh looks like on the surface. This analysis holds significantly less weight if you live in a more temperate climate, but it could be even worse for those living in Chicago, Detroit, Canada, etc.
Again, I am sorry for the long post, but I wanted to pass along my thoughts and hope to either have enough holes poked in my numbers to make me rethink the 40kWh or help others get the battery they will be happy with now and in another 5-10 years.