What Affects EV Efficiency
Here's a quick recap from other articles about the main things that impact your EV efficiency.
Changing the speed of an object - either from rest, or not, requires energy. The higher the rate of acceleration, the higher the energy demand. Once you reach your desired speed, the energy required to maintain is significantly reduced. Of course, this is true for all vehicles, not just EVs.
EPA studies dating back to the early 1980s indicated an nearly linear increase in fuel consumption with increasing acceleration . The initial studies were conducted on a 1979 Chevrolet Nova, with an inline 6 carbureted motor and 3-speed transmission, a platform that is quite different from modern vehicles. Despite differences from modern gasoline vehicles, the underlying principles remain consistent. Recent studies on EVs have demonstrated similar results, indicating that episodes of modest to high acceleration significantly compromise range and energy efficiency, regardless of absolute speed.
But, with an EV, speed itself presents a different efficiency problem.
Once you get to highway speeds, the aerodynamic drag of a vehicle is responsible for a large part of an electric car’s fuel consumption and may contribute up to 50% of the total vehicle fuel consumption. When looking at EPA numbers for conventional ICE vehicles, highway efficiency generally exceeds city efficiency. This trend is reversed for EVs, where stop-and-go city driving provides an opportunity for EVs to maximize efficiency and take advantage of regenerative braking.
If you think back to high school physics class, you may remember that the drag equation is proportional to the square of the vehicle’s speed. This means that speed is the primary contributor to drag force. Simply put, higher speeds create higher drag forces, which reduce efficiency. Based on experimentally determined variables, a theoretical efficiency curve for the Chevrolet Bolt EV is plotted below.
Auxiliary Power Drains
If you’ve ever popped the hood of a running ICE vehicle, you’re probably aware how much heat a gasoline engine creates. Although we typically think of this as wasted energy, in colder temperatures, this heat can be utilized to heat the cabin, reducing the need to generate heat from a secondary source. EVs, on the other hand, produce very little heat, and therefore require heat to be generated and pumped into the cabin when it’s cold. Additionally, since the battery and related components need to be kept warm, battery thermal management places additional power strain on the system.
Let’s look at the Chevrolet Bolt as an example. At temperatures below 50F, running the heater at ~70F while the car is warming up will require ~7kW of power. Once the vehicle and cabin warms up, the draw will drop into the 2-3kW range. For longer trips, this won’t have a significant impact on efficiency or range, but on short (especially frequent) trips, that power draw can have a significant impact.
The nice thing is that in a Bolt, and in many other EVs, the car will tell you how much energy is going to driving as opposed to warming you up!
Strategies to Improve Efficiency
Since speed has such a dramatic effect on aerodynamic drag, and efficiency, in an EV, they generally make excellent commuter vehicles. Above around 50 mph, drag starts to sap efficiency, although it is still significantly better than in ICE vehicles. In this section, we’ll detail some strategies to improve efficiency to maximize range, reduce costs, and reduce environmental impact.
Note that none of this is required to drive an EV, and that driving less efficiently is not bad, per se, but a lot of drivers like to know the tips and tricks to get the most range out of their cars.
- Speed - when possible, operating the vehicle below 50 mph is ideal.
- Acceleration - reduce erratic changes in speed and minimize absolute acceleration rates.
- Regenerative braking - avoid using the traditional brakes as much as possible. In practice, this means leaving enough space between you and the next car so that you can gently stop using regen and not have to slam on the traction brakes.
- Route - modern map applications such as Google Maps allow you to choose engine type, which can provide an optimal route for an EV.
- Auxiliaries - given the high demand to heat the vehicle, minimize the use of the heater. Precondition your car in the winter using the manufacturer’s app or in-car settings. Happily, the AC is not as energy intensive in an EV as it is in a gas car.
Note: driving uphill, like accelerating, is an efficiency killer in both gas and electric cars. It's OK if your speed decreases a bit if you're climbing a steep ascent!
Speed & Acceleration
The efficiency of gasoline vehicles at highway speeds is higher than at low speeds, despite increases in drag force. This is because gasoline vehicles are most efficient when they remain at a constant, (and moderate) speed. For EVs, motor efficiency is less speed-dependent. Since the drag force increases in proportion to the square of the speed, an electric car becomes exponentially less efficient at high speeds.
Thus, EVs are clearly best-suited for city driving. It’s not that they are inefficient at highway speeds, it’s just that the efficiency will drop dramatically. To maximize highway efficiency, try to avoid the fast lanes and travel at the slowest acceptable speed - generally 55-60 mph. With that said, maintaining the flow of traffic is important, so we do not encourage sacrificing safety for a slight increase in efficiency.
Acceleration rates also have an impact on efficiency, but it’s a complicated subject. EPA studies conducted on ICE vehicles in the 1980s indicated a large increase in fuel consumption as acceleration rates increased. These studies are limited in their relevance to modern gasoline vehicles, and certainly to EVs, but the underlying principles still apply. Getting to your desired speed faster will require more energy, as the demand on the vehicle increases. However, it is likely that acceleration has significantly less impact on EVs compared to ICE vehicles, given the high efficiency of electric motors. To make it more complicated, electric motors increase in efficiency above 50% load, which means they are the least efficient when operating at very low RPMs.
The image below is an example of the motor efficiency at different rotational speeds and torques for a Nissan LEAF.
The overall energy use at various acceleration rates is therefore a delicate balance between drag force, engine efficiency, and heat production that requires battery cooling.
In a traditional braking system, brake pads are pressed against rotors to produce friction, which results in a slowing of the vehicle. A byproduct of this interaction is the conversion of kinetic energy into heat, which is then lost to the atmosphere as wasted energy. Regenerative braking takes advantage of an electric motor's ability to run backwards, allowing the car to decelerate without using traditional friction brakes. While operating in the reverse direction, the motor also acts as an electric generator, producing electricity that then charges the battery. This process has the ability to recapture over 70% of the energy normally lost during braking, dramatically increasing the efficiency of an EV.
Taking full advantage of regenerative braking relies on two main things:
- driving conditions that allow for frequent braking, and
- allowing enough stopping distance that the vehicle does not utilize traditional braking.
City driving with stop-and-go traffic is where this style of braking really shines. However, even when driving at higher and/or constant speeds, regenerative braking can still be used to slow the car down on descents, turns, or when approaching zones of different speed limits. The trick is to leave enough room for the regenerative braking system to slow the car, which is usually a more gradual process than using the traction brakes.
Hopefully, we’ve made it abundantly clear so far that EVs are most efficient at low-to-moderate speeds and in driving conditions where regenerative braking can be maximized. This typically means avoiding high-speed highways and settling for back roads or city driving. Although this is not always feasible and will likely add time to your trip, choosing these optimized routes can save significant amounts of energy. Google Maps recently introduced an update that allows you to choose your engine type, which then offers energy optimized routes to your destination. Sometimes slowing down has its benefits.
Auxiliaries (aka Heat)
Low temperatures are known to have a greater effect on EV efficiency than ICE vehicle efficiency. This is primarily due to battery chemistry, but also a result of necessary heating and cooling of the battery components. Many EV owners report temperature-related range losses up 25-50%, depending on the particular climate and vehicle. One way to minimize these losses is by reducing dependence on cabin climate control.
In a Chevrolet Bolt, for example, simply running the heater on very cold days can require up to 7 kW of power. Depending on the specific climate, wearing extra layers and using heated seats are all ways to reduce the need for the vehicle's climate control.
Of course, preconditioning your car should go without saying. It is a first-line defense against cold weather energy loss.
Case Studies: Time Efficiency vs Cost Efficiency
Everything in life is a balance, and that includes balancing time efficiency and cost efficiency. A lower average speed will likely produce higher energy efficiencies, but the trip will take longer. In this section, we’re going to break down a few examples using real-world data from my Chevrolet Bolt EUV in the East Bay Area of Northern California.
Case Study 1: Commuting
A typical weekday for me consists of some work from home and some in the office. Generally, I go into 1-3 times per day, depending on the exact hours and schedule. This drive is ~5.5 miles each way, and is entirely city driving with light traffic but many traffic lights. In the late afternoon, when traffic is heavier, the drive will top out at ~18 minutes. In the early morning, it can be as short as 12 minutes. On average, I make 12 round trips per week, which equates to roughly 132 miles and 6 hours in the car.
On days when I’m in a hurry, I can shave ~1-2 minutes off per trip, by accelerating a bit faster, being less patient with slower drivers, and generally averaging a higher speed. At the end of a week, this could result in 20-30 minutes less time in the car.
But at what cost?
For the last 50 miles of commuting, I’ve averaged over 6 miles/kWh, a phenomenal efficiency for the Bolt, which is EPA rated at 3.8 miles/kWh. Electricity at home, where I primarily charge, is $0.256/kWh. From my own anecdotal evidence, driving faster - to save that 1-2 minutes - puts my efficiency much closer to 4 miles/kWh. At this efficiency, it would cost ~$736/year to charge the Bolt. At the current efficiency of 6 miles/kWh, this number drops to $300/year, an average savings of $36/month.
Now, I know what you’re thinking. Is it even worth it? Although a subjective topic, I’d suggest that it certainly is (more on this later). It requires very little effort and planning to accomplish this task, and provides me an opportunity to reel in that “always rushing” mentality that we so easily fall into.
Given the East Bay's proximity to national parks, I often take the Bolt on longer trips. Let’s use my recent trip to Yosemite National Park as an example of efficiency and cost. I charged the car to 95% the night prior, and headed East the following morning. I was staying in Wawona, so I took the southern route that cuts through Oakhurst, a small town that has a reliable DC fast charger. The drive was ~176 miles, and Google estimated it would take ~3 hours. I arrived after 3 hours and 25 minutes with ~33% battery, indicating ~39 kWh of energy was used. The calculated efficiency for this drive was ~4.1 miles/kWh. I averaged ~52 mph for this segment of the drive, which matches well with the theoretical curve above. It is worth noting that this drive is particularly hilly, with an elevation change of ~2300ft, something that is not taken into account in the calculations. For simplicity, let’s say that we’re off by ~10% from theoretical.
Now, in order to match the Google Maps estimated trip time of 3 hours, my average speed would need to increase to ~60 mph, which, when taking into account our 10% calculation adjustment, would result in an efficiency of ~3.6 miles/kWh. At this efficiency, I would use ~49 kWh of energy, and arrive with ~20% SoC, a charge level that makes me feel anxious.
In our real-world scenario, after 1 hour and 5 minutes of charging, the vehicle finished with 90% charge, which cost ~$20 for ~40 kWh of energy ($0.50/kWh). This yields an average price of $0.113/mile driven on the trip.
Let’s say I had driven 60 mph instead, and still wanted to charge back to 90% before continuing the trip. This would have required an extra ~10 minutes of charging and an additional $5 charging cost, increasing the cost to $0.142/mile. In this scenario, we are only saving a total time of ~15 minutes when driving and charging are considered, yet it costs more money, increases wear on the vehicle, and places the battery at a suboptimal SoC upon arrival. If I took the same drive even faster, the time and cost differences are even greater.
It is important to note that these numbers come from ideal environmental conditions. Extreme conditions, such as winter weather, further support the argument for slowing down to increase efficiency that has already taken a significant hit.
Is It Worth It?
At this point, many of you are probably thinking, “I’m already saving so much by switching from ICE to EV, it’s unnecessary to go through the trouble of trying to save more.” To an extent, I don’t disagree. However, if it’s feasible, safe, and does not add burden to your driving habits, what do you have to lose?
In the above examples, I’ve demonstrated that I can save an additional $400/year just on my commute, plus avoid feeling rushed going from A to B each day. The trade is that I have to spend an extra minute in the car each trip. Moreover, the road trip example demonstrated a savings of ~25% on a 175 mile trip, which was only ¼ of the driving I did that weekend. These small amounts add up over time, similar to making coffee at home each morning instead of making that small daily purchase.
As always, your mileage may vary. Factors to consider include
- Price per kWh that you pay
- Miles driven daily/annually
- Available routes to your destination (city vs highway)
- Flow rate of traffic on your route
- Sensitivity to extreme temperatures and comfort
At the end of the day, safety should be top priority. As drivers, we should always attempt to go with the flow of traffic, so in some cases, driving 55 mph on the highway may not be feasible. Wearing extra layers and using heated seats may not be sufficient to keep you warm when it’s really cold, so you may have to turn on the heater to stay comfortable. Your electricity rate at home may be 5 cents per kWh, so the savings accompanied with doubling your efficiency may be negligible. These are all factors to consider when deciding if adopting high efficiency techniques are suitable for your situation.
"Slow down and enjoy life. It’s not only the scenery you miss by going too fast — you also miss the sense of where you are going and why.” - Eddie Cantor