For my charging, this is usually around 246V. After about 20 seconds, the current ramps up to 24A and the voltage drops to about 238V.

Yes, normal.

I assume that KW usage calculation is 238V X 24A ?

Yes. It can see no other values. Once it is drawing current it does not use the no-load value (though it does track it in the background to know whether to cut the charging session due to excess voltage drop, I am fairly sure it is not used for power calculations).

What I am paying the power company for is 246 X 24.

Not exactly. You would need to measure at the input (or maybe the output would be good enough) of your meter to see what you're actually being charged for. The meter measures voltage and current and you may have substantial loss in your supply feeder which you would not be charged for if the drop occurs prior to the meter. It's possible the volts into your meter are more like 242V (arbitrary guess).

voltage/current displayed are whole numbers but I assume the onboard power usage calculation has/uses an additional decimal point?

Yes, people have used the API to track this, there's a bit more precision available I think but it's not really necessary to know.

My understanding is the KW hour usage shown on the trip meter is power used during butt time when the car is in either drive or reverse?

Yes.

If I add the 208W to the input side of the equation (after charging to the same percentage after a driving session) and subtract the butt time power usage for the same driving session, this should leave me with the total power costs for functions other that butt time?

Sort of. It's probably pretty inaccurate to do it this way. Really you need to look at your meter and subtract all other loads during the charge event. There's a lot of AC-DC conversion loss as well, and some static losses due to the 250W the car consumes when it is charging which never gets to the battery (that doesn't show up as the kWh added to the battery).

There's a lot of threads around here from me and

@darth_vad3r detailing some of the math involved. I'll try to link them in a second.

This is some quick math for the AWD. You can also check out this thread in general. Note there are some errors in my first post in that thread.

Quick Mileage Cost Calculation, take 2
Here's one discussing the constants involved. Note the post a couple posts below this one with the correct value for the RWD vehicle constants.

Max usable Kwh you've seen on TeslaFi
Here's one with more calculations (but with a 120V outlet, you can ignore that stuff), but the key is the plot link included. In that linked thread,

@darth_vad3r describes what is available from the API. I don't know since I don't use the API directly (though Stats will provide me that exact info for each charge event, as far as I can tell):

Calculating KWh usage from 120v outlet
Frankly, the short (?), simple (?), sweet answer, since in Albuquerque it is not that cold most of the time, is:

1) Start with the kWh/100mi EPA rating for your vehicle. So for AWD is is 29kWh/100mi.

2) Scale that value by the trip meter usage reading in your car, using the constants linked above. For example, for AWD, the constant is 234Wh/rmi (or maybe it is 230Wh/rmi but we are splitting hairs). Then apply a couple (confusing) correction factors. For the AWD the capacity is 79kWh but usable to 0rmi is only 76kWh.

So for example, if you see 250Wh/mi on the trip meter for a long-term average, your usage will be:

250oWh/mi / 234oWh/rmi * (76kWh/310rmi) / (79kWh/310EPAmi)

= 250oWh/mi / 234oWh/rmi * 245Wh/rmi / (255Wh/EPAmi)

= 250oWh/mi / 234oWh/rmi * 0.962EPAmi/rmi = 1.028 EPAmi / mi

*oWh/mi is "output" Wh/mi. Just another unit coined for the Wh that the trip meter displays, since they aren't matched to the input Wh for whatever reason.

** The 76kWh and 79kWh values are directly from the EPA doc for the AWD Model 3. I did round them off but they're close.

(So it follows that to get parity with EPA with AWD you need to get

** 243Wh/rmi **on the

** trip meter** - but note you'll only be able to go

**298 miles** (not 310 miles!) on a full charge to 0%/0rmi with that consumption. This is correct - the EPA test drives substantially past zero to get the rated range, until the vehicle stops moving.) If you want to get 310 miles to 0rmi you obviously have to get 234Wh/mi. (Or maybe 230Wh/mi...splitting hairs)

3) Take that value and multiply by 29kWh/100EPAmi => 29kWh/100EPAmi * 1.028EPAmi/mi = 29.8kWh/ 100 driven miles

4) Then add something like 10% to that if you drive 10k miles per year, 20% if you drive 5k miles per year, etc. This is a total wag and not exact (might be more accurate to scale the baseline EPA number and take the adder from that, rather than scaling your scaled number from step 3).

That's basically it. It spares you having to measure the draw from your meter and you'll find it relatively accurate. If you charge at 120V you have to assign significant efficiency penalty. If you charge at 48A you might do slightly better than 32A (perhaps by a couple %).

This is my best current understanding. I reserve the right to be corrected or correct myself later.