EV Range & Economy: Decoding the Numbers.

The key specification for EVs so far has been ‘range’. But how are the numbers measured, will range specific match reality, and can the number even be compared?

It turns out there are even 3 different standards for measuring range, and they give very different answers!

Further, why have we moved from ‘economy’ to ‘range’? Is there still even economy with EVs?

When you dig deeper, there is some really interesting revelations from numbers such as energy and power, that may be even more interesting than range. Surprises such are how little power cars normally use, and how huge amount of energy in the normal fuel tank is equivalent to around 600kWh, but will normally be used very inefficiently!

This is an exploration of the numbers behind EV specifications.

This is an explocan they be believed, and will twhat are they really saying? Range is the number of miles or kilometres an EV can travel on a single charge, and, like the the traditional counterpart like fuel economy rating for internal combustion cars, will vary significantly with speed, traffic and even temperature and road surface.

Energy: Gallons, Litres and kWh and the surprise behind EV range.

Translating Energy: Combustion Is So Inefficient.

The EPA has calculated the how much energy is in a typical gallon of fuel at 33.7 kWh. This calculation is needed by the EPA in order to calculate MPGe, and equivalent to MPG for fuel efficiency.

The conversion from US gallons to litres is one US gallon equals 3.7854 litres, therefore:

1 us gallon = 33.7 kWh, 1 litre = 8.9 kWh.

Actually 8.90262587837481 kWh per litre, but 8.9 is the best to use as the 33.7 would already have been rounded, plus the original figure would actual vary with octane rating and other facts, so in general, one decimal place is satisfactory.

Applying this conversion reveals a Toyota Prius with a 45 litre fuel tank, has over 4x the energy storage of a Tesla Model S 100. On the other side, the original Nissan Leaf with a 20 kWh battery had the equivalent energy storage of a 2.2 litre, or 0.6 US gallon, fuel tank!

Suddenly the inefficiency of internal combustion engine becomes very clear.

In the end, an electrical engine and drivetrain is at least 4x as efficient an internal combustion engine, and in practice, can probably be competitive with around 1/8 as much energy on board. Moving to an EV is like being able to move from a 16 gallon tank to a 2 gallon tank, or a 60 litre tank to a 7.5 litre fuel tank! However, given the cost of the battery based ‘fuel tank’ for EVs, it would be huge problem if the lift in efficiency was much less significant.

Translating Kilowatt hours Energy into Kilowatts or Horsepower.

With a tank of gasoline, it is very difficult to calculate how the energy in the tank translates to powering the car because not only is there the conversion of litres or gallons to kWh or ‘horsepower hours’, but then there is the other factor of the relatively small, and quite variable, percentage of the stored energy that will result as energy to power the car as a result of combustion. Efficiency will change with RPM, the gear in use, whether the engine was already warm and many other factors.

However with EVs, not only is the energy ‘in the tank’ already in useful units, and, as the conversion from stored energy to powering the car is very efficient, converting kWh to kW, or even if necessary ‘horsepower’, can be a useful calculation. Simply divide the number of kilowatt hours of battery used by the number of hours driving, and you

Calculating Average Energy In Use: Cars normally use very little of their power.

Being able to translate enables calculations such as average energy use under various driving conditions.

For example, the Tesla model 3(the current the best selling electric car) has either a 54kWh, a 62kWh, and 82kWh battery, with range for the RWD model of each at 381km/237m 430km/267m and 600km/373m on the WLTP test. Working with the 54kWh model, if the entire battery was to be consumed, then kWh to travel 381km. The figure I do not have is the average speed for the WLTP test. So I will calculate results for some possible values.

Using 50km/h, 381km divide by 50km/h gives 7.62 hours. 54kWh/7.62h = 7.08, which means if the average speed of the WLTP test was, 50km/h, the Model 3 would be only at an average power level of 7kw.

Using 80km/h, 381km divide by 80km/h gives 4.76 hours. 54kWh/4.76h = 11.34, which means if the average speed of the WLTP test was, 80km/h, the Model 3 would be only at an average power level of less than 12kw. Since it seems safe to assume the average speed for the test would no higher than 80km/h once freeway and other factors are taken into account, this tells us that despite the 54kWh Model 3 having a maximum power of 211kw (286 hp), the average power in use during the WLTP test cycle is only around 12kw. The maximum power of an engine is so much more than the typical power. And conversely, typical power being used is so much less than I would have thought.

If fact a poster I found online from VW suggests a vehicle would use a total of 5 kW at a constant 50 km/h, 16kw at a constant 100 km/h and 82 kw at 200 km/h.

Note the maximum output of the 54 kWh Tesla model 3 is 211 kw, given nothing, not even an EV, is perfectly efficient, the Tesla would be consuming slightly more than 211 kw when using that maximum power. If continuously using maximum power, as for example when repeatedly testing 0-100 km (0-60 mph) times, then a 54 kWh battery could only produce power for 54/211 hours, or 0.2559 hours, which at just over a 1/4 of an hour is less than 16 minutes to operation to completely flatten the battery.

Power, Economy & Efficiency: The different Power of EVs and Combustion engine.

Terminology of ‘power’: Why use Kilowatts not Horsepower of EVs.

When we by a new car, one key specification is the ‘power’ measured in kilowatts (kW) in most countries but in horsepower in the US (hp) which is also called brake horsepower in the UK (bhp). Horsepower numbers can be converted to kW by dividing by 1.34, so 134 hp is equal to 100 kW. For those accustom to horsepower, here is a page for conversions.

While horsepower sort of makes sense because before cars horses were used for transport, as electric vehicles increase in popularity, using kilowatts make more sense than ever. Most people in the world already measure the power of automobile engines in kW, but everyone, even in the US or UK, buys their electricity in kilowatts. In Electric vehicles, battery specifications are in kilowatts hours for everyone. Considering engine power in kilowatts, by dividing by 1.34 to get kW if you normally use horsepower, allows for some calculations that were not possible previously.

Heat Is an Enemy of Efficiency.

Internal combustion engines have been refining and improving their efficiency for years and years, but there are certain barriers to efficiency that just cannot be surmounted.

The very nature of combustion is that the energy is produced as heat. Heat is the reason combustion is so inefficient. The heat from the combustion makes the gas inside the cylinders expand rapidly, and that expansion drives the pistons and the pistons turn the crankshaft which eventually turns the wheels. However, much the energy that goes into heat is energy not used to turn the wheels, but solely in making things hot. There is even so much heat, that a sophisticated cooling system is needed to disperse the heat. This generation of heat is the primary inefficiency of internal combustion.

Internal Combustion Engines: Least Efficient at Lower Speed.

Power comes from ‘revs’. The term ‘rev head’ refers to someone who enjoys powerful internal combustion engines, because with any given engine, the more revolutions of the engine up to the limits of operation, the more power.

This dependence on ‘revs’ creates the situation where the power output, and efficiency of the engine are determined by engine speed. To achieve operation within an ideal engine speed range, at different vehicle speeds a number of different gear ratios is required. At any time, the lowest possible ‘gear’ will deliver the best power, but the worst ratio of engine speed to vehicle speed. The higher the gear, the better the ratio of engine speed to vehicle speed, increasing vehicle efficiency, that the high gear will only correspond to best engine efficiency at higher speeds.

The result is that a 200kw car engine will be more efficient when operating at 100kw (half power) than when operating at 50kw (1/4 power). If only 100kw is required, than there will be a fuel saving at 50kw because an engine at 50% will be more efficient. If all else is equal, the greater then engine power, the lower the percentage of power required to produce 50 kw, so the less efficient the engine will be when operating to produce 50kw. We see this as a constant speed of 100km/h (65 mph), a car with a more powerful engine will use more fuel if all else is equal. This means this more powerful engine is less efficient at this level than the less powerful engine.

As calculated above, normal speeds require relatively low power. The more powerful the engine, the further outside optimum operation at low speeds. So with a powerful engine, best efficiency would be gained by driving fast, in only air resistance didn’t increase energy requirements such, the gain in engine efficiency will not prevent requiring a lot of fuel to generate the required energy. This results in a less powerful engine being more efficient when operating at low power. The result is a lose-lose. With a powerful engine, drive fast and the engine is efficient, but going fast is not efficient, drive slow because going slow is efficient and the engine is less efficient. The result is there is an efficiency cost to having a powerful engine, and the lower the actual power requirements, the greater that efficiency cost. The result is there is also an optimum speed, not too fast for the air resistance to be too significant, and not too slow for engine efficiency and rolling resistance to be too significant.

Speed: The enemy of Energy Consumption.

Obviously the faster you travel, the more energy that is required. But what can be surprising is how much more energy. As you can see from the graph here, air resistance force vs speed produces an exponential graph. Double the speed produces not double the air resistance, but over 4x the air resistance. The greatest efficiency in terms of air resistance will occur when a vehicle is barely moving. From the perspective of air resistance, the slower a vehicle moves, the more efficient the movement can be.

Also note power required at different speeds.

If the only factor was air resistance, then travelling at just 1 km/h would be the ultimate way to extend range and reach a fuel stop, but rolling resistance, which presents a much flatter curve and does not start from zero, changes the equation, and moves the ‘optimum speed’ upwards. The graph to the right showing the percentage coming from rolling and air resistance as speed increases has a balance point of around 50km/h, however the optimum speed would change depending on tyres and other factors determining rolling resistance, and Cd or drag coefficient affecting air resistance.

A trap is that many of us have for years been driving combustion engine vehicles, where while it is still clear that going faster requires more energy, just how much more energy is hidden by the inefficiencies of the drive train at lower speeds. Move to an EV, and it becomes more transparent how rapidly energy requirements increase as speed increases.

Brakes: Wasted Energy.

The case for Hybrids.

Driving in cities produces inefficiencies because of the frequent stopping. Whenever a car decelerates using brakes, all the energy used to accelerate the car was lost as heat in the brakes, and causes wear on the brakes. So lots of decelerating in city traffic, means lots of lost energy. Hybrid cars can capture much of that energy by using an electric motor/generator operating as a generator to slow the car and capture the energy, and reuse the energy captured to help re-accelerate the car by operating the motor/generator in ‘motor’ mode when getting back to speed. This makes hybrid vehicles more efficient in cities, but often no more efficient at constant speed on a highway when using the same size combustion engine, as there is no braking energy available to be captured when driving on the highway. The only solution for better fuel economy on the highway, is a smaller combustion engine, which can be acceptable if the electric motor/generator provides additional power when needed.

EVs: ‘Extreme Hybrids’.

An EV is basically a hybrid where the battery and motor/generators have grown to the point where the combustion engine is no longer required. The ability to capture energy when decelerating is improved by the larger motor/generators.

EV vs Combustion engine: Drive differently to get optimum efficiency.

FactorEVCombustion Engine
Most Efficient OperationAll Engine and Vehicle SpeedsHigh vehicle speed: High engine speed and highest gear
Most Fuel Efficient Speed.Lowest drag/speed ratio.Raised from lowest drag/speed ratio by higher efficiency of drive train at higher speeds
Example Optimum Speed40 km/h90 km/h
Efficiency ‘penalty’ from more powerful enginesOnly from increased weight, and from increased electrical resistance of engine coils.The more powerful the engine, the lower the efficiency of operation at lower loads.
StrengthsAcceleration across all speeds, efficiency at all loads allows for greater economy even at lower speedsHigher speed for maximum economy, higher maximum speed relative to acceleration.
WeaknessesEconomy will decrease more than with combustion engines as speed increases, lower top speed relative to acceleration.Less acceleration, particularly initial acceleration relative to engine power.

The major factors outcomes are:

  1. Although EVs are at all times far more energy efficient than combustion engine vehicles, this has the greatest impact at lower speeds where combustion engine vehicles are least efficient, resulting in EVs being more economical in city or lower speed driving, then when driven on the freeway, or at higher speeds.
  2. EVs are far more efficient in stop start traffic than traditional, non-hybrid, combustion engine vehicles.
  3. The most efficient driving speed for an EV is significantly lower than the optimum speed for a non-EV.
  4. EVs are more efficient on the urban cycle, combustion vehicles are more efficient on the highway cycle.
  5. Having a powerful combustion engine results in a greater increase of energy consumption during normal driving than having a more powerful electric vehicle.

Generally, once past an optimum speed, the faster you travel, the lower the range of any vehicle. For EVs the optimum speed is lower.

The bad news: EVs provide more incentive to drive slower to save energy.

The good news: At any speed, the energy cost of driving and EV is much lower.

What really matters: Range (km,miles) or Economy(l/100km,mpg)?

Answer, range only really matters with it is critical, and historically with EVs, it has been critical a lot.

Most of us are familiar with fuel economy, either as l/100km or mpg. While there is an equivalent ‘electric’ fuel economy measure of kWh/100 km or kWh/100 miles, instead most specifications provide range or even MPGe.

Why the different focus?

Previously when considering a car, the size of the fuel tank is something I did check, but it was not a key specification. Could you imagine there being two models of a car previously where the most significant difference between the two models was the size of the fuel tank? Yet today with electrical vehicles, battery capacity is often the key difference between models.

When trip computers added a ‘range’ function, I have found it useful, but use it far more often when range becomes critical. Now consider that first Nissan Leaf which had a 20 kWh battery yet was considered remarkable at the time. Back then, and still with come cars today, range always feels critical.

So far, range has been the key specification, but as the products mature, efficiency measures may become increasingly important. There seems to be a standard emerging of the 350kW charger, and the range a car can gain from 20 minutes on a 350kW charger will be very much determined by efficiency, as all cars could gain the same kWh in the same time, and then it becomes how far they go with those kWh. In the end there is a ratio between driving time, which drains the battery and recharging time refilling the battery. If recharging speeds are fixed, then the most efficient car will have the best ratio of time on the road vs recharging during a road trip.

Range: The Key Specification?

Overview: Caution when comparing range, all ranges are not equal!

Even though there are standards, not all range standards are the same, and what you get may vary more with how fast you go than range does with a combustion vehicle.

To compare two vehicles, check range is specified against the same standard.

Particularly watch out for specifications quoting the NEDC standard, as it will give inflated range numbers and suggest a longer range.

NEDC was designed back in the 80s and last updated in around 2019, back when 20kWh batteries like the original Nissan leaf were just a dream. The estimate is fine for very short range vehicles which will normally be confined to use in 40 km/h (25 mph) zones or even lower speeds, this was the best use when the standard was designed. However NEDC is being deliberately phased out, so anyone using in 2021 is either trying to exaggerate the range, or using it for a tiny vehicle like an electric smart for jammed cities such Beijing where it could still make sense.

How Range Is Measured.

Range is measured on a dynamometer, as in the images shown above and to the left (except without measuring emissions), and the official figure is measured by the testing authority, not the manufacturer. However, each manufacturer will normally have equipment to do their own tests, and can publish their own results prior to official testing. Despite that a car on a dynamometer is not moving, each test system has to also factor in aerodynamic efficiency, and that can introduce variance between measuring equipment, so manufacturer figures may not be precise.

The problem with using a real world test to measure range, is that conditions in the real world vary day to day, so the test for each car would be slightly different. The fact that results in the real world will be different day to day highlights the fact that the test results are only approximate as a prediction of the real world, but do allow an accurate comparison of different vehicles.

While many of us have become accustom to how official fuel economy data may vary from our experience, this needs to be learnt again for official range data. Like fuel economy, data is calculated as separate ‘highway’ and ‘urban’ figures, but often only presented as a single combined value.

Unlike fuel economy for gasoline or diesel vehicles, many electric vehicles are often more economical in the urban tests than on the highway.

Real World: Why Ratings May Be Flawed.

A significant problem with range specifications, is that range varies significantly with speed. In fact, EV range can vary more with speed than internal combustion engine range does, as internal combustion engine drive trains a less efficient in lower gears. So even a highway mileage economy figure will not be of much use if you are travelling at a different speed that the measurement. Here is some comparison data from some cars at different speeds. I will add more comparisons as I find them:

Note keeping the speed in the 80-100km range doubled the range with the Ioniq 5. The impact is, that as can be seen with the Ioniq 5, travelling only on the freeway at 130km/h will drastically reduce range. While this also does drastically reduce a petrol or diesel cars range as well, there is typically far more range to waste.

The critical range for EVs is road trip range. When used for local transport, range can be in practice less of a problem than with petrol cars. The problem that needs solving is highway high speed range, and that is where official figures can be least accurate.

EPA: USA ‘Typical’ pattern, more highway, less urban.

In the US, the official range is calculated by the Environmental Protection Agency. In the USA the pattern of heading directly to the nearest highway, and then exiting the highway system near the destination results in a greater percentage of time being on the highway than is used for the European WLTP test. The process is described here in detail.

How is EPA rated range calculated?
Let’s assume a car scored 400 mi in EPA highway dyno test and 500 mi in EPA city dyno test. The first step is to calculate EPA highway and city range by multiplying the dyno scores by 0.7. EPA Highway range would be 4000.7= 280 miles and EPA city range would be 5000.7= 350 miles. Then you take 45% of highway range and 55% city range to calculate the combined range which is also known as EPA rated range. In this example, EPA rated range would be 2800.45 + 3500.55= 319 miles.

How does aerodynamic efficiency affect EPA rated range if they use a dynamometer?
Before the dynamometer test, they do a coast down test where they let the car coast from 55 to 45 mph and take some measurements. Then they enter these measurements into dynamometer settings. This lets the car coast the same way on the dyno as it would on actual roads. That’s how aerodynamic efficiency plays a role.

https://teslike.com/

Noteworthy is that stated range represents only 70%, or in special cases 75%, of the calculated maximum range. This reduction is to allow for degradation of the battery due to temperature

WLTP: Newer (2017) European measure, more urban, less highway.

The Worldwide Harmonised Light-Vehicles Test Procedure includes a range test for electric vehicles, and is required for vehicles to be sold in Europe. The standard was introduced in 2015, and has been mandated since 2017. This standard recognises that EVs are now also being used on freeways, a use not really even practical back when NEDC was designed.

NEDC: Older (1980s), Flattering New European Driving Cycle.

Designed in the 1980s, NEDC was a mandatory test for vehicles in Europe from 1992 to 2017 when it was replaced by the WLTP test procedure. When NEDC first was used for electric cars, their limited range made them suitable for only shorter distances and lower speeds, and the test was designed around specific applications in cities. As EVs became viable for general use, a new test reflecting the same mix of speeds

Comparisons.

VehiclekwHEPA km(miles)WLTPNDECkw/100km0-100Fast Charge min ( %)Max Charger kw
Hyundai Kona Electric64415(258)48254675 (to 80%) 50
Hyundai Ioniq 558/7318 (to 80%)350
Jaguar iPace90376(234)47022?4.840 (to 80%)100
Tesla Model 3 (Std Rng) 54354(220)381429165.3170
Tesla Model 3 (Std Rng+)54430155.3170

Chargers (section still being added)

All EVs can technically connect to any charger although in some cases an adapter is required. If I find exceptions, I will add them here.

In practice, there also needs to be an account and arrangement to use the charger, and as far as I know the Tesla ‘supercharger‘ charge network is completely exclusive to Tesla cars.

Cars have a rate of charge, and chargers have a maximum rate of charge. This means if a vehicle has a ‘max charger’ specification above of 100kw, then it will charge at less than its faster rate at an older 50kw charger, and wont take full advantage of a 350kw charger, but can still be charged at these higher capability chargers.

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