Energy efficiency is a ubiquitous term these days - and for good reason! Whether it’s your home or your car (or anything in between), the greater the efficiency the more effectively it works.
It’s more than likely that you’ve already done some research into electric vehicles - maybe even own one! But do you know how electric vehicle efficiency compares to fuel-burning cars? Or how electric vehicles use energy (i.e. where does the energy go)?
Let’s get plugged in…
When it comes to electric vehicles, energy efficiency means how far you can travel on a full charge. The simplest method to see an electric vehicle's efficiency is to work out the miles per kWh (the equivalent of miles per gallon). On average, most electric vehicles will cover between 3 - 4 miles per kWh.
If you need a refresher =
kW = kilowatt represents the rate of power (i.e. an EVs energy input and output). 1 kilowatt = 1000 watts.
kWh = kilowatt-hour represents the storage of energy (i.e. the capacity of an EV battery).
Need a bit more information? Check out our blog: kW v kWh – what's the difference?
To work out miles per kWh, you’ll need to do the following:
EV range ÷ kWh battery size = miles per kWh.
Remember, the higher the figure, the more efficient the vehicle.
For example, the Volkswagen ID.3 has a range of 280 miles and a battery size of 77 kWh. 280 ÷ 77 = 3.6 miles per kWh.
Whereas, the Volkswagen ID.Buzz has a range of 205 miles and a battery size of 77 kWh. 205 ÷ 77 = 2.6 miles per kWh.
Although the above method is handy, it’s worth taking it with a pinch of salt, as vehicles that offer more range are not necessarily the most efficient. For example, an SUV-style EV (i.e. big cars!) with a heavy battery will use more energy per mile, despite having a greater range.
For example, the Polestar 3 has a range of 305 miles and a battery size of 107 kWh. 305 ÷ 107 = 2.8 miles per kWh.
There are a few other factors that contribute to mileage efficiency, but we’ll come back to that a little later.
On top of being fossil fuel-burning guzzlers, petrol and diesel cars are estimated to only use 12 - 30% of the energy from the fuel you put in to get it moving. The rest (70 - 88%) of the energy is lost to inefficiencies (such as using fuel while in park or idle, and exhaust heat) or used to power accessories (e.g. heating, air conditioning, power steering, headlights etc.).
Comparatively, electric vehicles only use around 23% of its energy on powering accessories and the electrical drive system. This means electric vehicles are around 77% efficient.
The biggest difference? Regenerative braking.
As we explain in our blog: How do EVs work?
As a vehicle moves, it accumulates kinetic energy - and as GCSE physics textbooks will tell you, energy has to go somewhere. Unfortunately, in mechanical braking (used in ICE vehicles), kinetic energy is released as a waste product in the form of heat. This is why brake discs become overheated and worn over time.
Fortunately, EV brakes operate differently. Instead of converting the kinetic energy into a waste product, regenerative braking converts it into electrical energy, which is then stored in the battery. Not only is this more efficient, but it improves the range of your vehicle as energy is pushed back into the battery.
So, how does this work in practice?
Let's assume that you're driving down the road, and the traffic lights start to turn amber - you begin to press down on the brake pedal. As the brake pedal is engaged, the kinetic energy moving the vehicle is transferred to the motor, which slows the vehicle. As the motor rotates, it produces electricity that is pushed back into the EVs battery, ready to use when the traffic light turns green and the vehicle accelerates.
Although every electric vehicle is different - and with newer models employing more efficient technologies - it’s estimated that EVs use around:
13% of energy in electric drive system losses
10% of energy in charging losses - when an electric vehicle charges, the electrical energy from the charge point causes a chemical reaction in the lithium-ion battery, which causes heat as a result of power loss. To learn more about this process, check out our blog: How do EVs work?
0 - 7% of energy in electric accessories, power steering and heating/cooling
But 22% of energy losses are recouped by regenerative braking!
We’ve already discussed the internal mechanisms of electric vehicles, but what about other factors that affect efficiency?
Ambient temperature
Did you know that the optimum temperature for lithium-ion battery cells falls between 15 - 45 degrees Celsius? In the UK, winter temperatures average between 0 - 7 degrees Celsius - that’s between 8 to 15 degrees colder than a lithium battery can optimally perform. Due to the internal kinetics of the battery cell, colder temperatures slow the chemical reaction.
What does this mean in real life? 10 - 15% less driving range.
However, there are lots of ways to counteract this drop in range - to learn about this, check out our blog: How does cold weather affect EV battery capacity?
Payload/Weight
The same goes for petrol and diesel cars - the heavier your vehicle, the more energy it will take to get it moving. Thankfully, unless your payload is uncommonly heavy, there shouldn’t be too much effect on your battery range.
Battery age
Like most rechargeable batteries, performance will diminish over time, but please don't compare an EV battery to the likes of a mobile phone. EV batteries are hardy and car manufacturers are willing to guarantee it, typically for around eight years or 100,000 miles. That said, more and more EVs are proving to outlast their 8-year guarantee, and with the advancement of solid-state batteries, it shouldn’t be too long before EV batteries are remaining at peak optimisation for 30 years! This would also impact the range of an EV, with some reports estimating around a 50% increase.
Heavy acceleration (wind resistance and road quality)
According to the RAC, petrol and diesel cars are “most efficient at 45-50mph”. Comparatively, electric vehicles don’t have an optimal driving speed - built without an engine, they operate with a single-gear drive, meaning full torque (power) is instantly available. Unlike petrol and diesel cars which need to rev (burn fuel) to achieve more power.
However, much like petrol and diesel cars, the power to move an electric vehicle increases the faster it goes as it combats drag from wind resistance.
Moral of the story: the smoother you drive, the further you'll go.
To discover ways to extend your EV battery range (such as preconditioning, using eco-mode, charging when the battery is hot etc.), check out our blog: How does cold weather affect EV battery capacity?
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