Results tagged “energy efficiency” from Tom Saxton's Blog

EV Efficiency: Tesla Roadster and Nissan LEAF Compared

How do the Tesla Roadster and Nissan LEAF compare in energy use?

Tesla Roadster owners have been driving electric for a couple of years now and have built up knowledge about how much energy is required for many different routes and driving scenarios. New Nissan LEAF owners could perhaps benefit from what Roadster owners have learned, especially in the near term while charging stations are few and far between.

On August 4, 2011, we did a test to answer a couple of questions:

How does energy use in a Nissan LEAF compare to a Tesla Roadster?

Does knowing how much energy a Roadster uses for a certain drive help a LEAF owner plan the charge needed for a long drive?

The Plan

To take a first stab at figuring things out, Cathy and I joined up with her parents, Jim and Barbara Joyce, to drive a Nissan LEAF and a Tesla Roadster on an interstate freeway up a mountain pass. We wanted to compare just the two cars and eliminate as many other variables as possible. We drove up together so we had identical road and weather conditions, put the cars on cruise control to minimize driver differences, and restricted ourselves to using the fan but not air conditioning. From Roadster data collected on previous drives and also a recent LEAF drive up the same pass, we were pretty confident it could be done from the Joyces' home even cruising at 70 mph. We were right.

snoq-70-cars.jpgThe Route

We started at the Joyce residence near where Washington State Highway 18 meets Interstate 90 at Exit 25. Their LEAF started with a full charge. We drove to I-90, recorded trip and energy data at the stop light at the base of the on-ramp, accelerated up to 70 mph, then locked on cruise control. We exited I-90 at Exit 52 and recorded trip and energy data at the bottom of the off-ramp. We puttered around the summit for a bit, got some lunch, then reversed the route, again recording data at the bottom of the on-ramp getting back onto I-90 and again after exiting the freeway back at exit 25.

The Results

The graphs below show energy use for both vehicles up the pass from exit 25 to 52, a distance of 27 miles with a 2,000 foot elevation gain, then the descent back down from exit 52 to exit 25.

snoq-70-energy.png The graph shows that the LEAF used about 6% more energy than the Roadster on the way up and about 13% more energy on the way down. Both vehicles used about twice as much energy on the way up as the way down, although that ratio depends on the slope and speed. For a sufficiently steep road and slow descent, an electric vehicle can actually gain net energy driving downhill. At 70 mph, we did not see a lot of energy production, just low energy driving. At slower speeds, more energy would have been produced on the steep sections of the descent.

The LEAF averaged 2.7 miles per kWh (376 Wh/mi) on the way up and 4.8 mi/kWh (233 Wh/mi) on the way down, for an average of 3.3 mi/kWh (305 Wh/mi).

The Roadster averaged 2.8 miles per kWh (355 Wh/mi) on the way up and 5.5 mi/kWh (206 Wh/mi) on the way down, for an average of 3.6 mi/kWh (271 Wh/mi).

How Much Charge is Needed to Drive a LEAF Up to Snoqualmie Pass?

The LEAF doesn't give an indication of the state of charge to any useful precision, so we could only measure energy use from the trip miles and miles per kWh supplied by the LEAF. In terms of how much charge we used, the LEAF started with a full charge and ended back home with one bar showing and 4 miles on the generally worse-than-useless guess-o-meter. This included under 10 miles of driving between the freeway and home. It was a little surprising that the LEAF charge got so low given that the home-to-home energy use was only about 18 kWh, but the reported 24 kWh capacity of the battery is probably measured at a discharge rate that's lower that what's needed to climb the pass at 70 mph. Also, we know the LEAF hides some reserve charge from the driver.

From this data I conclude that starting from a full charge in Snoqualmie or North Bend, a LEAF can easily make it up and down the mountain at the speed limit without climate control. With climate control on, a bit slower speed may be required.

With a DC Quick Charge to 80% at North Bend, it could probably be done by anyone starting in the greater Seattle metro area.

Having Level 2 charging at the summit would be a big help. Even Level 1 would make a difference for someone spending the day skiing at the pass and wanting to get home with little or no charging on the way back.

Driving at lower speeds would use less charge. Really efficient driving, including better use of regenerative braking on the way down, would further decrease the charge needed.

Comparing the Nissan LEAF and Tesla Roadster

The curb weight of the Roadster is about 2,700 lbs, compared to the LEAF at 3,350 lbs. So the LEAF weighs about 25% more than the Roadster. The LEAF has a more aerodynamic shape, but has a much larger frontal cross-sectional area, so I would expect the LEAF to also have more aerodynamic drag. At freeway speeds, one would expect the aerodynamic drag to be a bigger factor in energy use, but doing a significant climb increases the importance of vehicle weight.

Because of how these two issues interact under different conditions, these numbers tell the story only for this specific drive on this route at this speed. Other drives are likely to give different results, so more tests are needed to get the full picture. It would also be interesting to do the same drive with multiple LEAFs and Roadsters to see how much variation there is between vehicles of the same model.

Data Method and Repeatability

We did everything we could both to minimize the difference between the two side-by-side drives and also standardize the drive so it could be repeated later under either similar or different conditions.

It was warm enough that we had to run the car fans to stay comfortable, but we were able to avoid use of the air conditioning.

We were able to maintain 69 to 70 mph pretty well, with a couple of exceptions. Below are graphs of the Roadster's speed versus time. The LEAF speed profile would be similar, with one exception on the descent, described below.

snoq-70-ascent.pngOn the way up, a few minutes after we got onto I-90, we ran into a clump of traffic we had to maneuver through, which slowed us down a little for a few minutes around the 10-minute mark.

snoq-70-descent.pngOn the way down, just a couple of miles from exiting I-90, the Roadster got boxed in between an RV at the same speed in the center lane and a slower vehicle entering just ahead of us. Rather than speed up to jump ahead of the slower vehicle (which would have used a bunch of extra energy), we slowed down sharply to let the vehicle in ahead of us. The LEAF was far enough ahead that it avoided this problem.

Tesla Roadster Energy Reporting and Efficiency

For the month of November, I drove the Roadster 762.2 miles. That's mostly with just me in the car driving a variety of city and highway miles. I tend to drive enthusiastically most of the time, but the month also included a roundtrip drive to Longview, WA on cruise control at 55 mph.

During the month, I put about 247.8 kWh into the car from the wall (213.3 kWh metered from my garage plus approximately 34.5 kWh from an unmetered NEMA 14-50 outlet in Longview). That's 325.1 Wh/mi and includes charging losses, battery pack self discharge, heater, headlights, etc. That's my wall-to-wheel number and is based completely on things I can measure.

From July 25th to August 27th, I drove the Roadster 696 miles and pulled 234 kilowatt hours (kWh) from the grid, giving us 336 Wh/mi. That included some hot weather and four 1/4 mile runs at Pacific Raceways.

On individual charges, I see efficiency vary from 240 Wh/mi to over 400 Wh/mi, and obviously much higher for things like drag racing.

I charge consistently at 240V and 40A at home. In Longview it was 230V and 40A. Because of charging overhead, I assume I would get slightly better charging efficiency if I charged at home at 70A. So, my numbers are just that, my numbers. Another driver would get different numbers depending on driving, weather, road conditions, and charging habits.

The EPA estimates documented in the paperwork for our car say 260 Wh/mi city and 290 Wh/mi highway. I've seen information from early 2008 Roadsters that had the EPA numbers and 340 and 360 Wh/mi.

You may have heard Roadster owners talk about numbers well below my 330 Wh/mi numbers. These are most often the number reported by the car's info screen which are not wall-to-wheel numbers, and in fact are (as far as I know) not at all documented as to what that number means. I have figured out some things about the numbers reported by the car, which I'll now explain.

For the month of November, the Roadster's trip meter says that I used 207.9 kWh, and thus 272.8 Wh/mi. But what does that mean? Did I push 207.9 kWh into the motor, or is that net of energy pushed back into the pack from regenerative braking (regen)? Does it include energy used to run the accessories and/or running the coolant pump and fans during charging?

On the "Energy History" screen, the Roadster tells me my "net energy used" for the month was 233 kWh and that I got 26 kWh from regen. What does "net" mean? I would assume that "net" means "net of regen," i.e., power from battery pack minus power into battery pack from regen. Except, if I compare those numbers to what the trip meter says, I notice that 233 - 26 = 207, which is suspiciously close to the energy use number reported on the trip meter.

From that, I infer that the trip meter's number is net energy use from the battery pack (power drawn minus regen put back in), and thus the so-called "net energy" from the energy use screen is really the gross energy pulled from the battery pack including energy that went into the pack from both wall charging and regen charging.

Do these numbers include the energy spent on accessories? Is the difference between what I put in through charging (247.8 kWh) and the car's reported net energy use (207.9 kWh) just charging losses or does that also include accessory use? I have no idea.

The only number I can stand behind, and the only number I can compare with other electric vehicles, is the wall-to-wheel number. The efficiency number reported on various of the Roadster's info screens is useful for understanding how driving style and conditions affect efficiency and for predicting/optimizing range, but is seemingly useless in any other context.

I believe the same is true of any efficiency number for the Leaf given out by Nissan, or any other EV manufacturer or driver, unless that number is as clearly defined and directly measured as the wall-to-wheel number.

It used to be that the Tesla screen reported an energy number after each charge that was much lower that what was actually drawn from the wall. I suspect that was the energy that actually made it into the battery pack, but I never saw it defined by Tesla. More recent firmware versions are reporting a number that is close to the number I read from the wall meter (and averaging multiple consecutive readings together agrees to within 1% of the wall reading). This is a big step forward for drivers who want to monitor their actual wall-to-wheel energy use and efficiency, but don't want to go to the expense of installing a dedicated meter. It would be a real benefit to the Tesla community if Tesla would (a) define the number they currently report and (b) make the energy drawn from the wall across multiple charges easily available.

Regarding range on a single charge, my personal record is 192 miles driven with a passenger in 100+ degree weather starting with a bit less than a full charge and ending with 10 miles of range left. On the trip back from Longview in cool weather, I drove 136.9 miles using cruise control at 55 mph using 55% of the battery. To the extent that you can extrapolate that to the full battery, that figures out to about 249 miles of range. On the trip down to Longview earlier the same day, also using cruise control at 55 mph, it was raining and colder, so I had the wipers, headlights and heater on and used 65% of the battery pack, for an extrapolated range of 208 miles.

My car is a 2008 Tesla Roadster with firmware version "3.4.15 15" (upgraded from "3.4.13 15" on 11/15/2009).

Edited at 10:23 pm on 12/13 to correct typo in second paragraph.