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Please check my math (cost of charging)

Patrick W

Active Member
Mar 17, 2015
1,478
886
SLC, UT
I just returned home and the car shows that I used 4.4 kWh of power today.

So since the power company charges 13¢ per kWh does that mean today I used 57¢ worth of electricity? (4.4 kWh x 13¢ = 57¢)

The battery in the car is an 85 kWh. So if I run it down to basically flat the cost to charge it to 100% would be $11.05? (85 kWh x 13¢ = $11.05)

Is all that basically correct?

Thanks!
 

andrewket

Well-Known Member
Dec 20, 2012
5,704
1,525
More or less correct. Charging efficiency comes into play. To increase the state of charge of your battery pack by 4.4kWh will require approx 4.7kWh of energy (the S is approximately 93% efficient.)

Additionally, you can't actually use all of your 85kWh pack. Tesla reserves some capacity to protect it. Assume at the most you'll be able to use 74kWh.
 

Patrick W

Active Member
Mar 17, 2015
1,478
886
SLC, UT
More or less correct. Charging efficiency comes into play. To increase the state of charge of your battery pack by 4.4kWh will require approx 4.7kWh of energy (the S is approximately 93% efficient.)

Additionally, you can't actually use all of your 85kWh pack. Tesla reserves some capacity to protect it. Assume at the most you'll be able to use 74kWh.

Thanks for the info.

Guess I'm just a numbers-OCD person but I would like to know how much each charge costs. I've heard somewhere that someone makes a meter that can be installed that tells how much power is used but my electrician has not been able to find one. (Any pointers on where one can be found would be appreciated)
 

jerry33

(S85-3/2/13 traded in) X LR: F2611##-3/27/20
Mar 8, 2012
19,730
22,812
Texas
Thanks for the info.

Guess I'm just a numbers-OCD person but I would like to know how much each charge costs. I've heard somewhere that someone makes a meter that can be installed that tells how much power is used but my electrician has not been able to find one. (Any pointers on where one can be found would be appreciated)

There are several TED, eMonitor, and OpenEnergyMonitor are a few. Your electrician didn't look very hard (or at all). Search for "home energy monitoring system"
 

jgallops

Member
Nov 21, 2013
61
21
Florida
I've heard somewhere that someone makes a meter that can be installed that tells how much power is used but my electrician has not been able to find one. (Any pointers on where one can be found would be appreciated)

Your electrician must not be very creative. Mine had to convert a meter base to a sub-panel. Was a ground/neutral issue that had to be sorted out. I have a KWH meter that shows total power used for the life of the Tesla!
IMG_0020.JPG


This is just before we got the Model S, so the meter is zero!
 

InsaneDriver

Member
Apr 4, 2015
233
49
Miami, FL
Your electrician must not be very creative. Mine had to convert a meter base to a sub-panel. Was a ground/neutral issue that had to be sorted out. I have a KWH meter that shows total power used for the life of the Tesla!View attachment 107371

This is just before we got the Model S, so the meter is zero!


I like this. I might have to have my electrician come back out and install a meter. I won't have lifetime Useage but you have to start somewhere.
 

David99

Active Member
Jan 31, 2014
4,850
7,024
Brea, Orange County
The motor is about 90% efficient (depending on speed and load and weather it's a dual motor or not), the battery has about a 92% efficiency (round trip) and the onboard charger also has about 90% efficiency. Some losses happen in the cables and wiring between the meter and the car as well. Also there are some losses of the car just sitting and running it's systems. Those don't show up in the trip meter at all. So there are a lot of things adding up that are not accounted for in the display.

If I remember correctly, some people who have kept track of the power usage closely say the consumption at the meter is about 1.3 times of what the car shows for a trip.
 

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