True Efficiency In Cold Weather

True Efficiency In Cold Weather

This post follows my other one, but is more focused. Many people are thinking soley about range anxiety when we discuss energy. However, I am concerned about the overall efficiency. Obviously as a larger car it won't be as efficient as a smaller car. It also will have a bigger overhead because of it's range related to a bigger battery. However, add the weight and the extra charging to keep the battery warm and what might be lost overnight and where are we?

I own a Z4 with 115K miles. I got 29 mpg real world, with a mix of everything from lots of stop and go to some good highway miles. If we look only at the cost of energy with 3.60 per gallon (NJ prices) and 16 cents per kwh. I look at a "tank" from the Model S at ~250 miles and that would be 8.6 gallons of gas. So the gas would cost $31. Looking at a straight .16*85 it would cost $13.6 and those numbers look great. Now add roughly 10% for loss during charging and we are at $14.96. The model S is still 1/2 the cost. However, what happens in the cold in terms of wasted energy and the loss of driving range. If we look at 175 as a comfortable range with the heater, the rear defroster, etc. So then we are getting about 70% of the range. Assuming we reduce the gas cost to 70% of full we're at $21.7 vs. 14.96 and that's without warming the battery.

So the Model S still comes out ahead and certainly is much more car, but not as amazing as we would hope in gas cheap (dare I say) NJ and electric expensive.


Brian H | January 26, 2013

NJ? Check out a Lawrenceville, NJ, outfit, . In about 4-5 yrs they hope to start licensing 5MW generators making power at under ½¢/kWh. Try your numbers on that!

gregv64 | January 26, 2013

You just say 29 mpg real world mileage, but even with an ICE car winter mileage is much much lower than summer mileage. Is that your mileage averaged over the whole year, or what.

Getting Amped Again | January 26, 2013


Having been on this forum for a while now, I think I can classify Model S buyers as either:

1) Technophiles
2) Environmentalists
3) Non-Lemmings
4) Mixtures of 1-3 (I'm 50%-25%-25%)

IMO most see the cost savings as the gravy, not the mashed potatoes, and those who live in cold climates like their potatoes with just a bit of it.

campsalvage | January 26, 2013

Yes, it was averaged over the lifetime.
I am 1 & 2, but also a sports car guy.
I like saving the bucks, but I also question if there is too much wasted energy, is it really as environmental as we think when you add the battery impact and such.

DouglasR | January 26, 2013

Recalculate to compute cost per passenger-mile, figuring your S is seven passenger and your Z is (what?)


campsalvage | January 26, 2013

I hear ya. I am just spinning on it. I need to test drive the Tesla again in good conditions I think I will stop worrying. The thing is most of the time I drive my car, it's me and the dog. We've been driving my wife's care when we are together which gets 40+ MPG its a Jetta clean-diesel. If we get the Tesla, we'll switch to that for anything in range.

jat | January 26, 2013

I don't know what your electricity vs gas costs are, but for me I average about 1.7 cents/mile for electricity (that is assuming 87% efficiency from the wall to the battery, which is what I measured for the LEAF but don't have enough data for the Model S yet). That is with my marginal electric rate of 4.4 cents/kWh. Even with today's cheaper gas here (~$3/gal), that is about the same cost per mile as a gas car that got 176 mpg.

When it is really cold, that cost will go up for running the heater, but it certainly won't be a factor of 4 more unless you take nothing but short trips (and the heat isn't entirely free in a gas car either when it is really cold).

campsalvage | January 27, 2013

How do u get electric that inexpensively

Brian H | January 27, 2013

live in the right place. 7¢/kwh is local cost here, e.g.

campsalvage | January 27, 2013

I hear you but i never heard it that low. Right now most of my electric is fee. 30 solar panels, but i don't have excess capacity for an ev and have no room for more panels. Got the panels 5 years ago, and they weren't the most efficient and certainly aren't now. Free juice is of course the best.

Brian H | January 27, 2013

I think it's even lower in WA. All those mountain valleys full of water ...

DouglasR | January 27, 2013

I don't know anywhere that has an overall rate of 4.4 cents/kWh any more, but note that jat said this was a marginal rate. He could be talking about a time-of-day discounted rate, for example. Here in Seattle, the base rate is 4.66 cents/kWh for the first 10 kWh per day in summer or the first 16 kWh in winter. Thereafter the rate is 10.71 cents/kWh. Since I always exceed the base amount thresholds, I would say my marginal rate is 10.71 cents.

A bit north of us, Snohomish PUD has rates of 8.515 cents/kWh in summer and 8.852 cents/kWh in winter.

It all depends on rate structure.

Brian H | January 27, 2013

Last I'd looked (don't pay own bill directly, part of rental charge), Vancouver, BC's rates were flat (no off-peak) at between 7 & 8¢ back in the day, but are now higher. Interestingly, the marginal rate decreases with higher usage. Here's the calculator:

It appears to bottom out at 10.25¢/kWh.

DouglasR | January 27, 2013


Back in the day, utility managers at government-owned utilities were just like their brethren in the private sector: the more power they sold, the better they liked it. So their rate structures provided for declining block rates, i.e., unit prices that would decline with volume. Then came rate shock from the cost of adding so much capacity, along with environmental consciousness, and they were told they had to conserve. Thus the rate designs flipped and began to provide for increasing block rates. In addition, utilities were told that the purchase of conservation would be treated as the purchase of capacity.

I remember the head of the Bonneville Power Administration telling a story about how, when he and his staff would go to meetings in the region, the first thing they would do upon checking into their hotel rooms was to turn on all the lights and electric appliances. Times have changed.

jat | January 27, 2013

I am on the standard Georgia Power plan -

0-650 kWh - 5.2465 cents/kWh
650-1000 kWh - 4.5015 cents/kWh
1000+ kWh - 4.4190 cents/kWh

I have never had less than 1000 kWh in a month, so effectively any change in my electric use is at the top tier's rate.

In the summer, the rates will change to 5.2465 / 8.7211 / 9.0126, so my marginal rate will more than double from June-September.

There is also an EV rate - - which gets down to 1.3063 cents/kWh for super-off peak usage, but the on-peak rate increases to over 20 cents/kWh. Even with two EVs (my wife drives a LEAF) and even assuming I could charge them only during 12-7am, the savings on recharging would be dwarfed by the increased cost during peak times. I expect to spend about $35-40/mo charging both EVs ($70-75/mo in the summer).

The TVA region gets a lot of its power from hydroelectric and nuclear, which helps keep the cost low. Many things are more expensive in the northeast and it isn't just competitive pressures driving that.