# Forums

## Connecticut 2013 P85 Model S (171K miles) HPWC home charging @ 24A = 18mph @ \$.0817/mile

Submitted for general Forum interest:

Here in Weston, CT our 'Eversource' electric provider charges a \$.219/kWh residential rate. This the total of all electric charges - generation, transmission, delivery etc. - the total cost delivered to my meter.
I charged the car and noted the meter kWh at start and end. I made sure there was negligible other house usage during this daylight period. With a 24A circuit on a HPWC giving an indicated 18mph charge rate, the car attained 134 miles of charge in 459 minutes using 50kWh.
This equates to \$.0817/mile charge cost, totaling \$10.95 for the session. Assuming no driving efficiency losses, if the car gets 100mpg, the cost compared to a gallon of gas would be \$8.17/gallon.

I'm moving to, where? Bighorn? Wyoming?

robgorman | 22 mai 2019

williamweiss21 | 22 mai 2019

Solar? OK, and figure the amortized ROI of that cost into this equation? I'm not remotely smart enough to do that math...
Fortunately, this car has unlimited Supercharger rights, so I'll do that as much as possible and keep it until my last day...

Bighorn | 22 mai 2019

about 8 cents here per kWh and solar is an 8 year payback

williamweiss21 | 22 mai 2019

Hello, my friend... Thanks for your post - makes me feel................sick. Thanks, again!

Silver2K | 22 mai 2019

William

Bighorn is right on payback when staying in your current home. But if you move before the 8 year period you still get your investment back due to the price increase of your home. The increase in home value is around 20k on the market.

https://news.energysage.com/home-solar-power-increase-property-values-ac...

MJP.75D | 22 mai 2019

I’m confused — you seem to be punishing the Tesla for its 100mpg efficiency? In your example, if the Tesla were able to achieve 134 MPGe efficiency your stated cost would be \$10.95/gallon—clearly even “worse”. I think you’re looking at this all wrong....to put things another way, but still using your math, if the Tesla were only capable of 1 MPGe, you would be elated at the equivalent gasoline cost of \$0.08/gallon. Sorry—but I think you’re way off here. If, on the other hand, I’ve misunderstood something, please explain. Thanks.

MJP.75D | 22 mai 2019

A better way to look at this equation is in cost/mile (not gallon) — when doing so, I think you will see the true genius of the Tesla. Good luck. No need to move anywhere but if you do, be sure to drive your Tesla to get there as it is by far the cheapest means of doing so (excluding the original capital investment, of course). Grin on!

Silver2K | 22 mai 2019

100 mpg is equal to \$17.50 in gas equivalent.

This car is compared to cars that get 20mpg. At \$3.50+ (premium gas) per gallon you're saving a good amount. This does not include gas cars losing efficiency as time goes on.

williamweiss21 | 22 mai 2019

@MJP.75D & @Silver2K
You are both right - I was looking at this the wrong way.
I think the shock of my (nearly the highest in the country) electric rate put my thinking into a tailspin. I never actually did an accurate study of the charging data in my garage until now and the "apparent" result caused me to shut down further thinking...

jordanrichard | 22 mai 2019

williamweiss21, I too am in CT (Vernon). Why did you bother getting a HPWC to only then draw 24A? I have the standard NEMA 14-50 and draw 40 amps which gets me 29 miles per hour charging on my 2014 S85.

I pay the same rate as you for electricity and my per mile cost for "fuel" works out to all of 5 maybe 6 cents a mile. To convert that into MPG, I use the local cost of premium gas since a car with this kind of performance would require premium gas and my previous car req'd it. Currently premium is \$3.55 per gallon / \$.06 = 59 mpg.

I never followed/used the "89 MPGe" data point on my original window sticker because that is not based on real world data.

williamweiss21 | 22 mai 2019

@Jordanrichard
I built a new house and the electrician/builder put in the wrong feed to the garage - left me a 30A wire hanging from the wall. They have been fired.

So, I'll get the 100A feed in there some day - In the meantime, I myself hooked up the HPWC to the 30A line they gave me and set it for 24A so at least I can charge overnight... I will email you...

PrescottRichard | 22 mai 2019

Can someone here answer this with any authority- is it more efficient (and therefore cheaper) to charge with the highest amperage you can?

Maybe there’s a point of diminishing returns there too, I suppose some would be happy to have the quicker charge rate regardless of efficiency / cost.

p.c.mcavoy | 24 mai 2019

@PrescottRichard - Everything I've seen says that it is more efficient to charge on AC at a higher amperage rate. There is some fixed energy draw to power the on-board charger, so the longer you need to charge for the same miles/%SOC added to the battery, the more parasitic loss you get from a wall to battery perspective.

You can check that out relatively easily by dialing down the amperage rate in the car, then look at it in a mile per kWh from the wall basis. I don't think you'll find it's a lot say comparing 24A to 40A, both at 240V feed. Where I think you'll see the biggest difference is if you compare charging say at 12A/120V to a 24-40A at 240V session.

AERODYNE | 24 mai 2019

You can check this also with something like Teslafi.

From 120v and 12A to 240v and 40A, I saw about 15% increase in efficiency.

However, I find the cost savings to be minimal with a higher amperage rate. Even at my highest rate, and 100%
home charging, only save about 100/yr.

In practice, my total elect cost has been only been 200 for the last 9k miles, and it cost me about 500 for the 240v instl.

Silver2K | 25 mai 2019

I cant tell you with authority, but a buddy of mine is an electrical engineer and informed me that the higher the amperage the better the efficiency.

jordanrichard | 25 mai 2019

Well, a typical 120v outlet is rated for 15 amps, but the car will draw 12amps. A NEMA 14-50 is rated for 50 amps but will draw 40 amps.

So for my MS 85, 12 amps gets me 4 mph and 40 amps gets me 29 mph. So for just a tad of 3 times the amperage over a 120 outlet, you get 7 times the charging rate using a 40 amp.

Michael Sinz | 25 mai 2019

jordanrichard - be careful comparing the 120v 12amp charging with 240v 40amp charging. The energy difference is:

40 / 12 * 240 / 120 or
40 / 12 * 2 or
80 / 12 or
6.6667 times more energy.

(Volts * Amps = Watts)

There is higher efficiency (to a point) going at higher amperage, but only a bit - the core overhead costs are around the same. It is not just higher is better but at these lower current levels, higher is generally better, as long as all of the cables are up to the rated power.

Tropopause | 25 mai 2019

William,

I'm sure BH will welcome you to charge at his place when passing through WY. ;)

Bighorn | 25 mai 2019

@Tropopause
He served me an awesome dinner when I visited, so of course I’d return the favor.

jordanrichard | 25 mai 2019

Michael Sinz, so 29 miles of range per hour is not 7 times faster than 4 miles per hour? I must of gone to the wrong elementary school.

p.c.mcavoy | 26 mai 2019

@jordanrichard -