Thursday, January 26, 2017

Martin Karo: How green was my Tesla | Power Line

So how much energy does it take to move a Tesla, say, 1000 miles, as opposed to a similarly-sized luxury car? Calculating the latter is fairly easy: using a roughly equivalent car (in size and status) as a baseline, a BMW 740i/Li, it gets (according to the DOE) 24 mpg combined, or 21/29 city/highway. 1000 miles /24MPG = 41.7 gallons.
Now for the Tesla. A Tesla Model S uses about 38 KwH of power to go 100 miles, so to go 1000 miles, easy math, the car needs 380 KwH of electricity. The figures vary very little between city, highway and combined, because electric motors use no power when idling and are more linear in application. The main difference is air drag at speed.
Well, it’s not exactly “no power when idle.” There’s a parasitic power loss. A Tesla uses power just sitting there, running its internal computers and whatnot. Teslas used to consume 4.5 KwH per day standing still, but Tesla claims to have improved that to 1 KwH per day. There’s also the need to heat the battery, and heat the cabin; a gasoline motor uses waste heat for the latter and nothing for the former. Given that the average car is driven 15,000 miles per year, it would take 24 days to drive that far, so add another 24 KwH to the Tesla’s consumption for parasitic loss, and add another 5 KwH per day for battery heating and climate control over that period. (The EPA tests are measured with the car at operating temperature and the climate controls off.) So the Tesla uses 380 + 24 + 120 = 524 KwH over that time and distance.
That figure is not bad at electric power rates, but the issue is planetary efficiency – how green is it? How much fuel does a powerplant use to create that much electricity? The petroleum equivalent of that at the powerplant is 13.76 KwH per gallon of petroleum equivalent (figures from the EIA), so generating the power to move the Tesla that far takes 524/13.76 = 38.08 gallons.
But there’s many a slip ‘twixt the cup and the lip, and with electric cars there are several. First, transmission power loss consumes between 8% and 15% of the power just moving it from point of generation to point of use.
In California, the average figure is 9%. Add another 1% for the resistive power loss from where the power enters the home to when it gets to the Tesla’s charger. Let’s total it at 10%. So it takes 38.08 x 1.1 = 41.9 gallons to generate the amount of power the Tesla will use and then get it to the Tesla. But it takes even more than that, because the charging process itself is only about 85% efficient. (Tesla claims 91% efficiency, but real world experience seems to be more like 70 – 80%.) So 41.9 /0.85 = 49.28 gallons (678 KwH, if you were still counting those).
Liberals frequently care more about feelings than facts, and your smug Tesla-owning frenemy will never admit it, but in day to day usage, the big BMW is actually 18% more efficient, and 18% kinder to the planet. (Don’t get too cocky, Mr. 7 Series: at a US average 12 cents per KwH, the electricity cost to the Tesla owner for 1000 miles works out in total to about $81, as opposed to $98 for the gasoline. The reason the Tesla is less efficient, but still cheaper to run, is that the power company pays a lot less for fuel than the automobile driver does. But when the issue is green impact, not greenbacks, the BMW wins handily.)
Ah, but your frenemy retorts after mulling it over, “MY Tesla can run on solar power! And I can put solar panels on my roof! It’s free, I tell you! My S runs FREE!”
Not really. The average solar panel produces about 10 watts per square foot. So some quick and dirty math: taking out of the equation the long-distance power transmission losses, and spreading out the power generation evenly over the time period, how much square footage would our Green Californian need to power his Tesla? 524 KwH for 24 days, as established above, plus 2% for transmission power loss at the solar panel and house level, and accounting for the 85% charger efficiency, you need 628,800 watt-hours. Dividing that by 24, you need 26,200 watt-hours per day.
You get about five hours of useful sun power production per day, so you need to get 5,240 watts per hour. You lose about 20% of your electricity in large systems; and accounting for the fact that the sun also doesn’t shine every day, add another 15% for reserve capacity, so you need 7,532.5 watts per hour capacity to account for efficiency losses and those rainy days. At top efficiency, that means you need 753 square feet of solar panels. At an installation price of$7 – $9 per watt (average of $8), the Green Man needs to spend over $60k for that much power. If he’s off the grid (i.e., stores the power instead of using net metering via his local utility), the storage system cost is on top of that. 753 square feet is a lot of ugly acreage, but it’s doable.
Of course, no self-respecting Green Weenie would settle for powering his car by the sun, but his house by Con Edison. And with the average efficient house using 1 KwH per hour, i.e., 24 KwH per day, the house needs 4.8 KwH capacity, and considering efficiency losses and reserve requirements, that means 6.9 KwH for the house. So to power both the Tesla and the house, Green Man needs at least 1,443 square feet of power production, at a cost of $115,000. But even using a Tesla-only setup, $60k would buy 25,641 gallons of gasoline (at the current US average price of $2.34 per gallon). The Big BMW could travel, on that much fuel, 24,000 x 24 MPG = 615,384 miles. Game, set and match – Munich and Detroit. Sad!

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