How to read the Energy chart?

Well, I always like to direct people to the FAQ on the TeslaMotors forums.
FREAKIN AWESOME QUESTIONS (FAQ) | Forums | Tesla Motors

As to the energy chart questions:
The "Instant" versus "Average" is related to what it uses to calculate that estimated miles left on the right side of the graph. I find "Instant" to be completely useless. If you accelerate hard, that will immediately go way down. When you pull your foot back and coast, it will immediately go way up. If you start up a hill, it will immediately go way down. It jumps around so much based on whatever is going on in the current few seconds, that it doesn't give you real useful information.

"Average", however, is really useful. See on the left, where you can pick to show the energy consumption from the last 5, 15, or 30 miles? "Average" will use that measurement to calculate a real distance left that you can go based on how you have been driving for that last while. The Rated Miles number in the display is based on a fixed consumption rate of about 300 watt hours per mile or maybe just under. But if your real driving is consuming at 340 watt hours per mile, you are going to be running through those Rated Miles faster than a 1 to 1 ratio, so the energy Average prediction is good for showing you what you really have left, based on what you're really doing.

For trips, I definitely use that. I keep the destination in nav, showing how many actual miles I have left to go, and then I can see in the energy screen how many projected miles I can go based on the actual driving conditions. It's perfect for seeing if you're still maintaining a good extra buffer, or if you're starting to cut it close and maybe need to slow down some.

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Oh, and whether it has reserve? Basically not really. In older versions of the car's firmware, there used to be a pretty decent bit below 0, but after version 5.9, they reworked the algorithms some, and I think moved that capacity up in the usable range, but made it so there really isn't much below 0. It's kind of unfortunate that used to be one way and then got changed, because a lot of people started quoting this amount below 0 that was available, and then the stories started cropping up of people who relied on it getting stuck when the car shut down just 1 or 2 miles after 0, when they thought they had more. So basically, please don't count on it.

Rocky_H said:

For trips, I definitely use that. I keep the destination in nav, showing how many actual miles I have left to go, and then I can see in the energy screen how many projected miles I can go based on the actual driving conditions. It's perfect for seeing if you're still maintaining a good extra buffer, or if you're starting to cut it close and maybe need to slow down some.

Click to expand...

If you drive in any sort of mountains, the projected average is next to useless. I can show you 30 mile averages of 0 Wh/mi going downhill and 700 Wh/mi going uphill, but I know those hills don't last for 100's of miles.

If you go to the trip tab on the energy screen, it projects the remaining energy needed to the destination including elevation changes and average highway speeds. Before you start, the trip page seems to use standard conditions (speed, temperature, wind, etc), so you have to add your own buffer for bad conditions. OTOH, it seems to project the rest of the trip based on the last several miles of driving including all of those issues. I find this very useful. My normal usage is to maintain better than 10% SoC margin at the destination. If the projection looks like less than that, I slow down, drive 5-10 miles and look again. Continue these cycles and you can quickly adjust your driving speed and style to get you to a destination comfortably. This is best done at the beginning of a trip. If after a few adjustment cycles, I still can't get a comfortable buffer, then I start looking for alternative charging.

This can also be useful to have enough charge to be comfortable leaving a Supercharger, but not to waste time overcharging into the taper. My normal practice is to put in the next destination while charging. i then look at the Energy/trip screen and charge until I have 10-15% buffer with neutral conditions; put in extra buffer until you get comfortable with this technique. If conditions look bad ahead, I will put in extra buffer before leaving the Supercharger.

YMMV, especially with bad conditions!!! When I know there is a blizzard, stiff headwinds, etc down the road, I always add extra buffer.

As to the reserve beyond zero, some people have driven a few miles beyond zero and some have had the car stop with a few rated miles left. Don't go there. Leave yourself some buffer.

While I hadn't read your post on this Cottonwood, I used this same methodology last week driving from DC to Charlotte. Very effective. Now, my route wasn't complicated and I had plenty of options for charging so I wouldn't be stranded, but it is a good habit to learn and makes things that much more comfortable when taking a trip. Thanks!

I second the comments from Cottonwood and Cyclone. As a new owner, I was feeling my way through this on my first extended trip this past week and was working my way to the method Cottonwood described.

I would add one caution. If you are using the beta trip planning feature that automatically routes you through superchargers en route to your destination, the car will try to minimize the time spent at a supercharger by giving you a prompt that you have sufficient charge to continue when it projects you can get to the next supercharger with a 2-3% reserve buffer. (Note that the car continues to charge with the prompt up or even after dismissing the prompt, until you manually stop charging, which is a good thing.) IMO that reserve is a bit thin, especially if you expect to drive faster than the software assumes. My DES told me that the software "should" adjust to reflect my driving "style" over time but to be cautious initially. I added buffer to take me to 10% and was glad I did.

I will be taking my first really long distance trip next week and the comments by Cottonwood confirmed my feeling that I should use the trip tab on the Energy bar rather than rely on the beta trip planner, with all of its problems.