Help me understand Wh/mi

3V Pilot

Top-Contributor
Joined
Sep 15, 2017
Messages
1,274
Location
Oro Valley, AZ
Tesla Owner
Model 3
Country
Country
#1
Okay, I understand that Wh/mi is the amount of energy it took to drive a certain amount of miles. What I'm trying to figure out is how to relate that to cost per mile or how to compare it to MPG of an ICE car. There is a recent pic of a Model 3 showing 254 Wh/mi. If my electricity cost 12 cents per kWh what math would I need to do to figure out cost per mile? I understand that this number might change based on how I drive and maybe some current owners could help out here, is 254 an average number?

I'm trying to figure all this out because I might switch my electricity to a time of use plan once I get my car. I haven't done it yet because they charge $19 a month to be on the plan, I don't use much electricity right now and rates are not that much better on the plan. I drive about 300 miles a week just back and forth to work, don't know if I'd save more than $19 a month once I start charging this car. I'd like to have a better understanding of how to calculate some of these crazy new electric car numbers. And please keep it simple, after all, I am a gear-head and I've sniffed exhaust fumes for too many years to understand higher math....LOL!

For you math geniuses out there, right now I pay 12 cents per kWh (up to 800kWh then it goes to 13 cents per kWh).
On the TIme of Use Plan I would pay 07 cents per kWh off peak but the on peak rate is 19 cents.

I'd like to know, just based on driving to work and back how much I might save if I changed plans.....
 

BigBri

Top-Contributor
Joined
Jul 16, 2016
Messages
1,066
Location
Toronto, Ontario
Tesla Owner
Model 3
Country
Country
#2
I could be wrong but it should just be 0.254KWH/M so it'd be 1KW to do 3.9~miles. Sounds about right as I get around 7.5km/KWH in the summer and around 5.7 this time of year in a Leaf. My average this month is 192WH/KM
 

mig

Well-Known Member
TOO Supporting Member
Joined
Jul 10, 2017
Messages
243
Location
California
Tesla Owner
Model 3
Country
Country
#3
$.12/kWh /(3.9 mi/kWh) = $0.031/mi

300 miles will cost about $9.30 at this rate

$0.07/kWh/(3.9 mi/kWh) = $0.018/mi

300 miles will cost about $5.40
(Barring dumb error on my part)

Yes, 254 Wh probably is an average and real world will vary (but in my experience possibly for the better!)
 

3V Pilot

Top-Contributor
Joined
Sep 15, 2017
Messages
1,274
Location
Oro Valley, AZ
Tesla Owner
Model 3
Country
Country
#4
Okay, so based on the above math I wouldn't even be making up the $19 a month it would cost me. That is good info and what I was looking for, thanks!

Just one other question though, how do you get from 254 Wh/mi to the fact that it takes 1 kWh for 3.9 miles? I don't understand how that works and would like to be able to figure that out if my average "mileage" is different. Is there any good website or spreadsheet that has a calculator for all these numbers? Something I could just input my average Wh/mi and electric rates that would spit out cost per mile and cost to recharge for X number of miles or any other useful figures.
 

3V Pilot

Top-Contributor
Joined
Sep 15, 2017
Messages
1,274
Location
Oro Valley, AZ
Tesla Owner
Model 3
Country
Country
#6
254 Wh/mi is 1 mi/.254 kWh = 3.94 mi/ kWh
Thank you!.....that is almost easy enough that even I could do it!. Still wondering if someone could come up with a spreadsheet that would crunch all these numbers for me. I know, it's probably not that difficult to accomplish if it's as easy as whats shown above, I'd just like to have something I could plug different numbers into and see how the effect the outcome. Different Wh/mi, different electric rates, different miles driven per month and what it all would cost per mile and per month.
 

SoFlaModel3

@Teslatunity
Moderator
TOO Supporting Member
Joined
Apr 15, 2017
Messages
9,814
Location
Florida
Tesla Owner
Model 3
Country
Country
#9
I know you got your answers, but interesting topic. I’ve come to the realization that my costs will go way down each month but never in my mind took it a step further than a high level estimate of about $120/month in savings.

So right now gas in my area is $2.80/gallon. My car goes ~350 miles on 15 gallons. That’s 23.333 miles per gallon. Finally that’s $0.12/mile.

For electricity in my area we’ll go with the similar $0.12/kWh. If the car uses 254 Wh/mile, then that’s $0.03/mile.

That means each mile costs me 75% less “fuel”.

I currently spend $150/month on gass and was figuring around $30/month for electricity. It looks like $37.50 may be the safer estimate.

Of course early days I may have lead right foot syndrome and it could be worse ;)
 

Dr. J

Private
TOO Supporting Member
Joined
Sep 1, 2017
Messages
1,191
Location
Fort Worth
Tesla Owner
Model 3
Country
Country
#10
You could extend the calculation beyond fuel cost to total ownership cost by adding maintenance, depreciation and financing costs. And anything I've left out.
 
4

4701

Guest
#11
Number instrument cluster shows is not the number you pay for. You pay more.
It also applies to some ICE vehicles that do not measure fuel consumed while stationary.
For example my BMW uses fuel heater, that fuel is not metered. But I still have to buy it.
In case of electric vehicle, you pay at the meter, not battery output.
So there can be more energy used if vehicle is preconditioned.
Also add 10% as charging losses (10% more metered than charged to battery).
 

Dan Detweiler

Top-Contributor
Joined
Apr 8, 2016
Messages
1,573
Location
Canton, Georgia
Country
Country
#12
Bottom line...you're going to spend about 25-30% of what you do now in fuel. Then you can start adding in ancillary stuff like oil changes, brake wear, etc.

Dan
 

NOGA$4ME

Top-Contributor
Joined
Sep 30, 2016
Messages
659
Location
Durham, NC
Tesla Owner
Model 3
Country
Country
#13
Of course early days I may have lead right foot syndrome and it could be worse ;)
A lead foot on its own will not hurt your efficiency (much). You'll spend more on tires of course...

Jack rabbit starts off the line hurting efficiency is an internal combustion engine issue and doesn't apply the same way for electric motors, thanks to the torque curve of a motor. There are minimal losses due to increased IR drop when you floor it, but nothing as near significant as the inefficiency of a gas vehicle.

If the light turns green at an on-ramp to a highway and you accelerate to 70mph, you use pretty much exactly the same amount of energy to reach that speed regardless of whether you do it in 5.5 seconds or 55 seconds.

That said, obviously any time you accelerate you are expending energy. So if you are in stop and go traffic and having to accelerate, followed by using your brakes, followed by accelerating, etc. then all those accelerations will add up (and even though you can regen, you never get as much back out as you put in). If you are accelerating off the line only to have to immediately slow down at the next light or when you catch up to the cars ahead, then yes, you are driving inefficiently. It would be more efficient to accelerate at a rate just fast enough that you can coast and avoid the repeated accelerations.

(Having said that, unless I can directly see otherwise, my mindset is that I may be just as likely to MAKE the next light as I might be to miss it, so I may as well gun it!)
 
Joined
Sep 30, 2017
Messages
20
Location
Santa Cruz, CA
Country
Country
#14
I don’t blame you for still being slightly confused by people’s overly complicated way of inverting the numbers twice. Try this: 254 wh/mile is .254 kWh/mile. That’s basically 1/4 kWh/mile. If your electricity is 12 cents per kWh it will cost you about 3 cents per mile.
 

NOGA$4ME

Top-Contributor
Joined
Sep 30, 2016
Messages
659
Location
Durham, NC
Tesla Owner
Model 3
Country
Country
#16
Yeah, inverting wh/mile to Miles/kWh makes the math much easier. For simplicity sake, I just plan on 4 miles/kWh, which is about what I get in my i3.
Like you, I'm used to dealing with miles/kWh because that's how my car reports it. But for figuring out cost, Wh/mile (or more directly, kWh/mile) is actually much more convenient. Just multiply the number by the number of miles you drive and voila, you have your cost.
 

BigBri

Top-Contributor
Joined
Jul 16, 2016
Messages
1,066
Location
Toronto, Ontario
Tesla Owner
Model 3
Country
Country
#17
These stats might be interesting/useful. I have a device plugged into the ODB port on the car that collects a ton of data from it. I'm part of a pilot program that tracks EV owners routes/charging habits so the data can be provided to utilities and (presumably sold) to third parties that are trying to install charging networks.

I've had my Leaf since July but it spent 2-3 weeks in the garage as the charging port on it was damaged fresh from the dealership. So I've more had it from around mid August. My EVSE at home tracks my overall savings of gas VS my electricity cost. I update the gas prices every few weeks and calculate my average electricity cost when my bill comes in. My average seems to be around 15.2C/KW.

My EVSE has calculated about $697 in savings from August til now in terms of gas savings. I can say I have noticed a BIG difference. My electricity bill is generally around $130 and that includes the car. Last month it was $100 and that was including 1500km of driving.

car2-png.4265

car-png.4266
 

danzgator

Top-Contributor
Joined
May 17, 2017
Messages
612
Location
Largo
Tesla Owner
Model S
Country
Country
#19
@Mike Land FWIW I drive about 35 miles a day to and from work, plus whatever weekend driving I do. I charge about 30-40% of that at work for free. I think our rates are about the same in Florida, as in AZ, and I am hard pressed to find a noticeable difference in my electric bill pre-Tesla. This isn't a very scientific answer, but I can't imagine that I'm paying more than $15-20 more per month to charge my car.
 

BigBri

Top-Contributor
Joined
Jul 16, 2016
Messages
1,066
Location
Toronto, Ontario
Tesla Owner
Model 3
Country
Country
#20
@BigBri - can I ask what EVSE you have and what pilot program you're enrolled in? The information you are provided seems very useful... :)
This is the EVSE: https://emotorwerks.com/store/residential/juicebox-pro-40-smart-40-amp-evse-with-24-foot-cable

The program is called Charge the North so it's a Canadian thing. Might be something similar in your area or even just an ODB tool you can buy. I think Leaf Spy enables you to get similar stats out of the Leaf but I've got no experience with it as it's like $25 for the app.