Differences with Tesla Motor's estimated charging power output vs. SDGE?

[earlyretirement]

I had a quick question that I figured some of you experts would be able to answer.


So I got my dedicated SDGE TOU meter hooked up and used it for the first time yesterday. I plan on keeping very detailed notes/diary to track the usage, miles charges and kWh charges by SDGE. It's easy as the meter started out at 0.00.


So I plugged it in to start after 12:30 AM when I have super peak rates.


On Tesla's website: Tesla Charging | Tesla Motors


They estimate that charging it 170 miles with a HPWC with dual chargers would use about 55.9 kWh and take about 2 hours 53 minutes to charge.


Well, I charged exactly 168 miles (I couldn't get it exactly on their estimator) yet I went out to look on my SDGE meter and it says I used 66 kWh of power last night. If it was a smaller difference I wouldn't care but does anyone know what accounts for such a big percentage difference with what Tesla says would be estimated and what was actually charged by SDGE?


I'd really appreciate some thoughts as it got me curious. Thanks in advance.

 

[Merrill]

What I did is got info from mknox on how to compute kw used per hour. Multiply volts times amps and that will give you kw used per hour, then determined how long you charged and multiply that by you factor above. Example 240x40=9600 or 9.6kw. This will tell you how close you are, you cannot go by Tesla numbers.

 

[earlyretirement]

What I did is got info from mknox on how to compute kw used per hour. Multiply volts times amps and that will give you kw used per hour, then determined how long you charged and multiply that by you factor above. Example 240x40=9600 or 9.6kw. This will tell you how close you are, you cannot go by Tesla numbers.

Ok..thanks for this explanation. I also heard from Cosmacelf on another thread where he posts this explanation below which makes sense as well.

The energy that gets put into the battery is always going to be smaller than what is taken from the grid due to transmission and conversion losses. You lose some energy in just the wires (they do heat up somewhat). You lose a lot more energy in the on-board car charger than converts 240V AC to 400V DC for the battery.

 

[stevezzzz]

Vampire losses also figure into the discrepancy: charging has to make up for vampire losses as well as adding to Rated range.

 

[gregincal]

I'll post my usual rule of thumb: Energy from the wall = energy used driving * 1.15 (charging loss) + 3 (vampire loss)

In your case 55.9*1.15+3 = 67 so you are a bit under my estimate.

 

[earlyretirement]

Thanks for all this information. It's helpful for rookies like me.

 

[liuping]

I'll post my usual rule of thumb: Energy from the wall = energy used driving * 1.15 (charging loss) + 3 (vampire loss)

In your case 55.9*1.15+3 = 67 so you are a bit under my estimate.

Is that vampire loss with 5.0 or are you still on 4.5?

 

[gregincal]

Is that vampire loss with 5.0 or are you still on 4.5?

4.5. Very few people have 5.0 yet. When I do I'll update my rule of thumb.

 

[earlyretirement]

4.5. Very few people have 5.0 yet. When I do I'll update my rule of thumb.

In case I didn't mention it, I have version 5.0 that came with the car when I picked it up about 2 weeks ago. Also, I didn't charge the car for 2 days and I was pleasantly surprised that the vampire loss only seemed to be about 1/2 miles per night. So there is obviously a big improvement from what people were mentioning before.

 

[gregincal]

In case I didn't mention it, I have version 5.0 that came with the car when I picked it up about 2 weeks ago. Also, I didn't charge the car for 2 days and I was pleasantly surprised that the vampire loss only seemed to be about 1/2 miles per night. So there is obviously a big improvement from what people were mentioning before.

Ah, we'll that explains why your usage was less than my estimate. I'd be interested if you have a day where you don't drive how much energy is used at night (I don't trust the rated miles stuff).