September 20, 2009
Tesla Roadster Battery Charging And Efficiency

Dr. Robert Wilder describes the charging of his Tesla Roadster's battery.

But before you knock the Roadster for increasing our energy demand, remember: We're not paying a penny for gasoline. And the Roadster has supercar performance and a correspondingly large battery. This battery holds 54 kWh, giving this car great speed and a good range but therefore needing much (solar) 'juice' -- certainly more than a smaller EV that might be used mainly for short trips or inter-city commuting and errands.

Due to cooling and other losses in charging, filling from empty takes about 68 kWh, or 26% more than 54 kWh the battery holds. This 68 kWh is the seminal amount; it quantifies how much truly is needed. We'll reference this number to determine how far we can go from power of the sun alone.

What I find most interesting here: Charging up the Tesla requires 26% more electricity than the battery holds. 26% additional gets wasted. Batteries getting charged heat up. That heat is waste.

What I want to know: Will the Chevy Volt and other pluggable hybrids and pure electric cars have similar amounts of electricity waste when charging their batteries? Does anyone reading this have some data on battery charging efficiency for other lithium battery chemistries?

Wilder charges his Tesla at night when electric rates are cheaper. But he lives in an area where electric power prices are quite high.

Crucially, we do all EV charging overnight because with Time Of Use (TOU) meter rates, the cost here is 'only' 18 cents/kWh during off-peak hours at night.

By contrast, a peak rate is far higher at 30 cents/kWh from 11 a.m. to 6 p.m., when our PV makes surplus power from the sun and sells it back to the utility, giving us a credit on our bill.

So even though he has photovoltaic panels he charges his Tesla at night since his daytime electricity is worth more to sell to his local electric utility.

In sunnier areas at sunnier times of the year really cheap PV could eventually make late morning the cheapest time to buy electricity. The big spike in demand happens in the late afternoon in warmer climes. If the price declines in PV continue then eventually this trend might cause a decline in electric power prices in the morning and a sharper spiking of electricity prices in the late afternoon and early evening.

Of course, given enough electric cars and sufficient battery longevity the late afternoon electric power price spike could be dampened by selling electric power from car batteries out onto the grid.

The range on a Tesla depends heavily on how fast you drive. You can go over 200 miles if you drive slowly enough. A blog post by Tesla CTO JB Straubel shows how fast drag increases and electric power usage doubles as the Tesla Roadster goes faster.

To cruise at 60 mph takes about 15kW. However, if you double that to 30kW you will only accelerate to about 80mph — far less than twice as fast. And if you double it again to 60kW you will accelerate to about 107 mph using 4 times as much power as you did at 60mph, yet you’d only travel about 1.8 times as fast.

Check out the first graph at that page. The Tesla is using slightly over 250 Watt-hours per mile at 60 mph but at 30 it drops to only 150 Wh per mile and bottoms out at about 135 Wh per mile around 17 mph. So the big losses in efficiency occur over 60 mph.

Share |      Randall Parker, 2009 September 20 03:22 PM  Energy Electric Cars

anonyq said at September 20, 2009 5:33 PM:

"So even though he has photovoltaic panels he charges his Tesla at night since his daytime electricity is worth more to sell to his local electric utility."

You write it like it is something unexpected but isn't that what one would expect to happen.

Randall Parker said at September 20, 2009 5:48 PM:


Over several years of writing posts I've learned:

1) There are lots of things obvious to me that are not obvious to many of my readers. So I have to state more things than I feel like stating.

2) If I jump over links in chains of reasoning and state conclusions someone will pipe up claiming to not see my conclusion as correct or obvious.

3) Stating things out loud introduces the point in order to trigger a discussion about it. Sure, it makes sense to sell the PV power during the day and charge from the grid at night. But the consequences of those facts have a lot lower chance of being explored in the discussion if I do not mention the obvious.

Dan said at September 21, 2009 7:25 AM:

One comment on the 26% waste-heat while charging: I've had a roadster now for about 4 months, and the amount of heat generated (both percentage-wise and in absolute terms) while charging is a strong function of how fast you charge. The worst case for waste heat is when the battery is completely recharged in ~3.5hours using the "high-power" 70A/220v connector which is wall-mounted and custom wired into your garage. The best case is with the 110v/12A connector which will only give you about 50miles of range with overnight charging. Since I only drive about 50 miles a day (and rarely on the weekends), I use the 110v on a daily basis. Much of the time, the battery cooling system doesn't even turn on ... with the 220v, it turns on before the car even starts recharging the batteries.

BTW: I have solar too, and sell the electricity during the day while recharging overnight. It's not so much a time-of-use thing with e- pricing; I recharge the car when I get home at night to drive it the next morning. i.e. when I charge has more to do with the time-of-use for the vehicle than with charging costs.


odograph said at September 21, 2009 7:50 AM:

I saw a Tesla in the wild yesterday, a first for me. In OC, on PCH.

That makes them seem a bit more "real." Of course I saw an Audi R8 at the same time ... I guess neither car is "real" for my budget.

Ned said at September 21, 2009 11:53 AM:

Dan -

Could you expand on your experience with the Tesla roadster? A 70A/220V charger is quite a bit of juice. How much did it cost? Did you require additional wiring to bring this much power into your house? The figures furnished by Tesla are based on no heater or air conditioner usage. For many parts of the country, this is totally unrealistic most of the year. What is the effect on range of turning the heater/AC on? Does battery peeformance deteriorate with age? If so, how quickly? How much does battery replacement cost?


Dan said at September 21, 2009 4:58 PM:


Can't help you much here, if you posted the questions to I expect you could get a good answer.

From my experience: I don't have a 70A/220v in my house (but have used one on occasion at other locations), I use a 50A/220v feed when I need a fast recharge. I ran a seperate circuit from my fusebox for it, but did not need to upgrade the feed-in to my house. The first few hundred owners received a "high-power connector" with the car, but now Tesla gives the 110v connector standard and a 220v for an extra ~$1k (details on the website). I think most owners have figured out that unless you are on a roadtrip, then there is no need for fast recharging; as long as the battery gets recharged overnight it's more than sufficient. Usually the 110v is good enought for that (on a standard 18A fuse).

I haven't noticed any real change in range with either AC or heat, but in San Diego neither is used that often in a convertible. Haven't heard of any other owners complaining about it either; I expect the power draw for AC/heat is small compared with that for the engine. Some complain that the AC is underpowered since it is also used to cool the battery in hot climates (top priority) before providing cool air to the driver.

The battery will certainly deteriorate with age, but no one seems to know how much. When I signed up for the car (a couple years back), the battery was expected to last for 5 years at 80% of original capacity. That's now up to 7 years; I think it may go up a bit more as they get more lifetime data. Battery replacement is (I think) $15k prepaid, but the Tesla website would tell you for sure.


anonyq said at September 21, 2009 5:31 PM:

Randall Parker, It was more that i have the feeling that you don't understand the basics of the electricity market. It is hard about the future without that basic knowledge.

Garson O'Toole said at September 22, 2009 1:07 PM:

Much thanks to the roadster owner for sharing his experiences. I noticed something odd about the marketing of EVs like the Tesla. An FAQ at the Tesla Motors website discusses the cost per mile of the electricity needed to run the Tesla automobile:

… a cost of less than two cents per mile driven (using off-peak charging rates with a time-of-use meter. To calculate your exact cost-per-mile, contact your local electricity provider).
This sounds intriguingly cheap. But what about the cost of battery replacement? The Edmunds website cites a newsletter from Tesla CEO Elon Musk for the following information:
... battery pack in the Tesla Roadster would cost about $36,000 to replace today, but should last at least 100,000 miles, or about 7 years. … Pay $12,000 now, Musk wrote, and reserve a replacement battery for seven years from now (if your original lasts longer, there will be a pro-rated refund; if you need a new one sooner, you pay a little more).
I think that means the cost for the battery is: $12,000/100,000 miles = 12 cents per mile. So the cost of the battery pack dominates the cost of the electricity by a multiplicative factor of six. (A more sophisticated calculation would account for the time-value of money. The $12,000 is prepaid 7 years in advance while the electricity is paid for in increments over 7 years.)

On the other hand, if an electrical vehicle is being compared to a gasoline or diesel based vehicle then the requirement for significant maintenance work on the fossil fuel systems during 100,000 miles must be factored in to its cost per mile. Apparently electrical motors are easier to maintain.

Nick G said at September 22, 2009 3:31 PM:


GM says that the Volt requires .25KWH per mile at the outlet, and .2KWH/mile from the battery, so we see 20% waste (or a 25% "markup") in the charging process.

As I understand this, there's about 7% waste in the battery, and about 13% in the AC->DC converter. I suspect that the converter could be more efficient, and that there's a cost tradeoff for this.

Efficiency is also likely to improve with later generations - especially aerodynamics (the most important factor with an electric drivetrain, where regenerative braking greatly reduces acceleration/braking losses), but also peripheral loads. The importance of aerodynamics can bee seen with the ultra-streamlined Aptera, which is expected to use only .07 KWH per mile.

Randall Parker said at September 22, 2009 7:42 PM:

Nick G,

You sure there's a 13% loss from AC->DC conversion? I wonder whether an external converter without a weight or vibration constraint would be more efficient. Does the Volt take AC? Or is the conversion made outside of the car?

Dan's thinking the charging rate causes charging efficiency to vary. I wonder if all that variation is in the battery itself and if he's correct.

Nick G said at September 23, 2009 2:39 PM:


I haven't seen any mention in 2nd gen li-ion specs of the charging rate causing charging efficiency to vary. Dan says that charging at 14.4KW generates much more heat than charging at 1.3KW, which certainly makes sense even if charging efficiency is constant.

The Volt does take AC, at least for 110V. I would expect that a larger, external converter would be more efficient - I'm not sure whether the 220V 14.4KW charging station also does conversion.

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