holyhippie: (Coffee)
[personal profile] holyhippie
I've been driving my awesome Model S for about 16 months now - and have talked with a lot of people about the tradeoffs around how much it costs to drive this car versus a gas. The fuel cost tradeoffs aren't exactly straightforward - while my experience is that the Tesla is much cheaper to drive than my previous car (a Mazda Miata), charging it isn't free (duh), and if the cost is better or worse depends.

To start off the analysis, let me set a baseline for how much fuel each car uses for a set distance. Electric vehicles are measured in units of watt hours per mile. The Model S I own has averaged 347 Wh/mile over its lifetime so far. Gas vehicles are measured in terms of miles per gallon. Obviously, these vary depending on the car and how you drive - I'll set the comparison here at the approximate average I was getting in the Miata: 28 MPG.

Next, I'm going to set ranges on how much the two different types of power costs. The more simple case really is how much gas costs - I'm using data from here: http://www.gasbuddy.com/gb_retail_price_chart.aspx?time=24 to set the low price of gas at $3.20/gallon and the high price at $3.75/gallon.

How much electricity costs is a very complex subject. Yes, it's regulated by states very carefully, but there is a huge spread in the cost consumers pay depending on the details of the rate plan and when the electricity is used. I'll use data from my own apartment here - in the past year I've paid between $0.04 per KWH and $0.37 per KWH. That's a HUGE variation in price - the most expensive electricity is about 10 times the cost of the cheapest. I can't avoid buying the very expensive electricity (that's the cost I have to pay for powering my house in the evenings) ... but I can avoid putting it in my car.

The next kink in electricity costs is how much it costs from public charging stations. There are two ways to charge for metering the cost of electricity - per KWH used (like your home power bill) or by time. I strongly prefer to use stations that charge per KWH, because then I can easily compare that cost to how much I pay at home. Stations that charge by KWH vary in price. I think I've seen as cheap as $0.20 per KWH. Typical is $0.49 per KWH.

For stations that charge by time ... how good of a deal it is all depends on how many KWH I can get for that time. That varies depending on the station, voltage, etc ... I'll simplify it to saying that about 6 KWH/hour is a good deal.

So all that said ... time for data! First, a table showing how much electricity costs per KWH depending on how much the station charges for the time.

$1.00 $0.17
$1.50 $0.25
$2.00 $0.33

Next, how much it costs to drive a Mazda Miata with 28 MPG for 100 miles, with different gas costs.

$/gal Price/100 miles
$3.20 $11.43
$3.50 $12.50
$3.75 $13.39

OK. There is a baseline we can set for the EV. Now, let's see how well the Tesla Model S does at 347 watt hours per mile.

$/kwh Price/100mi
$0.04 $1.39
$0.07 $2.43
$0.10 $3.47
$0.13 $4.51
$0.17 $5.90
$0.18 $6.25
$0.32 $11.10
$0.33 $11.45
$0.37 $12.84
$0.49 $17.00

The interesting thing here is that it is not always cheaper to drive the Model S, depending on electricity price. I certainly don't want to charge at home using the most expensive power at $0.33 per KWH and up. And I absolutely don't want to charge at public stations charging $0.49 per KWH. And for those charging by time - $1/hour is an OK deal, but not the greatest. $1.50/hour is marginal, $2/hour is a rip off.

The absolute best deal I get is that there are several places to get electricity at no cost to me.  My work provides free charging.  Quite a few public chargers are still free, although I expect that to change over time.  The Tesla superchargers blast a lot of electricity into my car very quickly - all at no additional cost to me.

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