Calculating an equivalent $/Gallon for the Model S

[HankLloydRight]

This is just for fun, and not meant to be a rigorous scientific analysis.

But I wondered if I could come up with an equivalent $/gallon figure I could tell people what it's actually costing me to drive my Model S.

Here are my assumptions:

Battery: 85KWh (ignoring the 3-5KWH non-bricking margin)
Full battery Range: EPA Rated: 265 miles
MPGe: EPA: 89 mpg (from the window sticker)
Current fuel cost: $4.40/gallon (premium)
Energy cost: $.066 per KWh (I just signed up for a PPA plan with SolarCity for power at 6.6cents/KWH)
Actual A/C power used to fill 85KWH battery: 100KWH (this is just a wild-ass guess, I really don't know how much "wall power" in KWH it takes to fully charge an 85KWH battery. If anyone has a good figure, please let me know)

So if the eMPG is 89, that means the battery stores the equivalent of 2.97 "gallons" of energy.

So to figure the KWH/Gallon, I take the 100 KWH/Battery and divide by 2.97 "gallons"/full battery == 33.58 KWh/gallon.

If power costs me $.066 per KWH, that would be an equivalent of $2.21/"gallon".

That seems high at about only half the cost of premium fuel. Where did I go wrong?

 

[ElSupreme]

That is probably correct. You have to remember that you get ~4-5 times the distance on a single 'gallon' than an equivalent car. So your driving costs would be even lower than a gasoline vehicle.

 

[HankLloydRight]

That is probably correct. You have to remember that you get ~4-5 times the distance on a single 'gallon' than an equivalent car. So your driving costs would be even lower than a gasoline vehicle.

Oh snap, that's right. So it's not a good comparison. I have to incorporate my current car's MPG (12-15 MPG) to make it a better analysis.
thanks!

 

[Merrill]

I keep it simple, take the cost in electricity for miles driven and then divide those miles by the equalivent miles per gallon your ice car gets, or would have and come up with how many gallons you would have used. The divide the cost in electricity by those gallons used and you have your actual cost per gallon. Example for my last year: drove 10000 miles with a cost for electricity of $150, my ice if I'm lucky would get 18 mpg, so that is 555 gallons use. Divide actual cost of $150 by 555= 27 cents per gallon.

 

[HankLloydRight]

Ok, so my ICE car gets 15 MPG @ $4.4/gallon= 29 cents per mile

The MS gets 89 MPG @ $2.21/gallon = 2.4 cents/mile.

I feel better now.

But I'd still really like to derive an equivalent $/gallon. hmmm.

 

[SlyWombat]

Exactly, if you have magic fairy extract that costs $5,000 per gallon, but you get 50,000 miles on that gallon, who cares what it costs , it is better to show the cost per mile, so it costs you $2.21/89 = $.0248 (two and half cents per mile) vs $4.50/20 = $.225 (twenty two and half cents per mile), so you get 10x better cost performance.

 

[HankLloydRight]

Ok, taking miles/gallon into account, I calculated that the MS "fuel" costs 37cents/gallon.

And it's much more simple than my first post.

ICE: 265 miles@15MPG= 17.7 gallons@$4.4/gallon= $77
MS: 265@89MPG == 2.97 "Gallons" @2.21/"gallon" === [email protected] === $6.60

Divide the $6.60 cost by the original 17.7 gallons of ICE fuel == $.37/gallon.

Thanks everyone!

 

[JohnQ]

Exactly, if you have magic fairy extract that costs $5,000 per gallon, but you get 50,000 miles on that gallon, who cares what it costs , it is better to show the cost per mile, so it costs you $2.21/89 = $.0248 (two and half cents per mile) vs $4.50/20 = $.225 (twenty two and half cents per mile), so you get 10x better cost performance.

This is what I tell people who ask. My energy cost is 20 cents per mile to drive my most recent ICE and 5 cents per mile to drive the S. Doesn't answer the OP's original question but I find putting it in terms of cost per mile makes sense to most people.

 

[HankLloydRight]

This is what I tell people who ask. My energy cost is 20 cents per mile to drive my most recent ICE and 5 cents per mile to drive the S. Doesn't answer the OP's original question but I find putting it in terms of cost per mile makes sense to most people.

I think if you ask 100 people what their actual cost per mile is, very few would know, but if you ask them what they last paid per gallon of gas they paid at the pump, most would know that. That's what I was wanted to compare to, and I was able to derive above (37 cents/gallon -- pre 1970 energy crisis prices!).

 

[Zythryn]

You are making it far too complicated.

For talking points when discussing your cost, don't worry about the 89mpge.

These are the important numbers.
Your cost for electricity 6.6cents/kWh
Your old car's mpg 15.
Miles/kWh (generally about 3)

So to go 15 miles in your old car cost 1 gallon, presently $4.40.
To go 15 miles in your new car it takes 5 kWh (15miles/(3miles/kWh)).
5kWh costs 33 cents.

So one gallon in your old car costs $4.40 and $0.33 in your new.

 

[AndyM]

I usually tell people this:
My gasoline bill each month was $250 for my driving pattern.
When I switched to Tesla, that went to zero, and my electric bill only went up $45 a month, same driving patterns.. sometimes more, since now its so much more fun to drive!

 

[qwk]

The easiest comparison for me is noting the fact that 10kwh=30 miles of rated range when charging on 240V. These are well to wheel numbers, so there is very little fudging.
I then use the rated range number at the start of the trip and subtract the rated range at the end. This gives me a very accurate cost per trip when compared to the actual miles traveled, and it also takes efficiency into account. The 89mpge number is not all that accurate, because with light foot I can easily get much better.

 

[tga]

This is just for fun, and not meant to be a rigorous scientific analysis.

But I wondered if I could come up with an equivalent $/gallon figure I could tell people what it's actually costing me to drive my Model S.

Here are my assumptions:

Battery: 85KWh (ignoring the 3-5KWH non-bricking margin)
Full battery Range: EPA Rated: 265 miles
MPGe: EPA: 89 mpg (from the window sticker)
Current fuel cost: $4.40/gallon (premium)
Energy cost: $.066 per KWh (I just signed up for a PPA plan with SolarCity for power at 6.6cents/KWH)
Actual A/C power used to fill 85KWH battery: 100KWH (this is just a wild-ass guess, I really don't know how much "wall power" in KWH it takes to fully charge an 85KWH battery. If anyone has a good figure, please let me know)

So if the eMPG is 89, that means the battery stores the equivalent of 2.97 "gallons" of energy.

So to figure the KWH/Gallon, I take the 100 KWH/Battery and divide by 2.97 "gallons"/full battery == 33.58 KWh/gallon.

If power costs me $.066 per KWH, that would be an equivalent of $2.21/"gallon".

That seems high at about only half the cost of premium fuel. Where did I go wrong?

On http://www.fueleconomy.gov/ the US Dept of Energy uses 33.7 kWh/gal, so your numbers are close. Remember that that an internal combustion engine is notoriously inefficient, wasting a lot of gasoline's potential energy as heat. The numbers that often get thrown around are 85% wasted as heat, with only 15% used to move the car.

 

[WeazL]

Here on Oahu we pay $0.36/kWh for electricity. That comes out to $0.15 per mile, or $2.25 per gallon for gasoline equivalent (compared to $4.40 per gallon at the pump). Paradise is expensive!

Solar is very popular here, so in reality, many owners actually pay less, or nothing at all to charge their MS since it's offset by their solar productivity.

 

[astrotoy]

You are making it far too complicated.

For talking points when discussing your cost, don't worry about the 89mpge.

These are the important numbers.
Your cost for electricity 6.6cents/kWh
Your old car's mpg 15.
Miles/kWh (generally about 3)

So to go 15 miles in your old car cost 1 gallon, presently $4.40.
To go 15 miles in your new car it takes 5 kWh (15miles/(3miles/kWh)).
5kWh costs 33 cents.

So one gallon in your old car costs $4.40 and $0.33 in your new.

That makes it simple, thanks. I have it even easier (although not as cheap). Our off peak EV rates (midnight) are always 10 cents per kwh. So I can see my cost per mile on the info screen. If I am using 350 watts/mile, my cost is 3.5 cents per mile. My Lexus LS costs about 25 cents per mile for gas and another 3 cents per mile for oil (change every 5000 miles). So my old car cost about 8 times as much to drive.

 

[patn]

To restate what others have said already - the cost of the energy should be roughly comparable no matter what form it takes (otherwise most industries would migrate from one to the other). e.g. you can burn gas to make electricity. The main advantage of the Tesla is that due to the inherent efficiency of electric motors, regenerative braking, better design, etc. the car just goes farther on the same amount of energy... And I would add performs better as well...

 

[MattMDK]

I also have a p85+. I use my trip meter to figure my consumption.

2560 miles driven used 968.1 kw at 9 cents per kw. = $87.12 in electricity costs.

My previous car used premium gas ($4.50 p/gal) and averaged 15 mpg.

2560 divided by 15 = 170 gallons x $4.50 = $765 in gas.

Gas Car $765
Tesla S $87

I have had the car for exactly 2 months so have saved $339 a month in fuel charges, including the cost of electricity.

 

[russman]

My calculations came out to be about 5x cheaper to drive the Tesla when comparing $.09 per kwh to $4 gas

 

[AustinPowers]

Here on Oahu we pay $0.36/kWh for electricity. ... Paradise is expensive!

Still, cheaper than Germany, we pay around $0.40/kWh for electricity - tendency rising sharply. And we are far from paradise ;-)

 

[David_Cary]

To restate what others have said already - the cost of the energy should be roughly comparable no matter what form it takes (otherwise most industries would migrate from one to the other). e.g. you can burn gas to make electricity. The main advantage of the Tesla is that due to the inherent efficiency of electric motors, regenerative braking, better design, etc. the car just goes farther on the same amount of energy... And I would add performs better as well...

Except that oil (ie gasoline) is a way more expensive form of energy than anything else. That is why we don't generate electricity from oil in the US anymore. On a BTU basis, Coal and NG duke it out for the cheapest in the US which is why electricity is generated from them the most.

Electricity is valuable because it can carry energy from a variety of sources - whether it be solar, hydro, coal. Gasoline must come from oil so you are forced into an inflexible source that is currently (and for the foreseeable future) expensive.

So the main advantage of the Tesla is using non oil sources of power, not "better design".