Electric cars 1 – baseline figures: electric cars vs an economical diesel

Small electric cars
Carwow recently tested 6 small electric cars. They were able to travel between 3.7 and 5.2 miles per kWh and had ranges between 113 and 229 miles. That’s a factor of 2 in the range but the battery sizes varied from 28.5 kWh to 52 kWh. Roughly, if you have twice the battery size, you can go twice as far. The best was the Renault Zoe which, with the 52 kWh battery, has a mass of about 1500 kg, or a ton and a half.

Larger electric cars
A Tesla Model S has a mass of 2350 kg (2.3 tonnes) with the largest (100 kWh) battery. Its range is claimed to be 315 miles. So the Model S is able to travel 315 miles/100 kWh = 3.15 miles per kWh.

Why do larger electric cars, with larger range, do fewer miles per kWh? The answer is simple. If you have a larger battery it’s much heavier (technically a larger mass). The Renault Zoe 52 kWh battery has a weight of 326 kg. The Tesla S 100 kWh battery has a weight of 625 kg, roughly twice that of the Renault Zoe for twice the capacity. If the battery has a larger mass then the whole car needs to have a larger mass because it needs to have larger motors to accelerate that mass, larger brakes to slow the mass down, stronger structures to support the mass, etc.

The biggest electric fuel tank on the market
So far as I am aware, the 100 kWh battery of the Tesla model S is the biggest car electrical fuel tank on the market. It has a capacity of 100 kWh.

Comparing electric car figures with a modern diesel car
Let’s compare the Tesla battery with the modest 47 litre fuel tank from my own medium-sized Renault Megane.
Diesel fuel has energy of 10 kWh per litre. So the the 47 litre tank has an energy capacity of 470 kWh. That sounds great until you realise that the efficiency of a diesel engine is about 30% compared with the near 100% efficiency of electrical motors. So let’s multiply by 30% (0.3) to work out the effective capacity of the Renault Megane fuel tank.
30% x 470 kWh = 141 kWh.
Gosh, that’s interesting. A medium-sized diesel car has nearly 50% more energy capacity than the largest electric car.
Moreover, since diesel has a density of about 0.8 kg per litre, the mass of the diesel tank plus the fuel will be about 47 kg for its energy capacity of 141 kWh, compared with the Tesla battery’s mass of 625 kg for an energy capacity of 100 kWh.
Diesel tank: 3 kWh/kg.
Electric battery: 0.16 kWh/kg.
The diesel fuel tank stores nearly 20 times as much per kilogram than the electric battery. In fact, for reasons I may tackle on a later post, the difference is even bigger than this.

Diesel car fuel consumption
The 2016 Renault Megane diesel is quiet, comfortable and has good economy figures.
It has a mass of 1500 kg, about the same smaller electric Zoe. (The Zoe’s mass is larger because the battery has a large mass.) Over around 8364 miles of mostly long-distance driving, it averaged 56 mpg with an average speed of about 30 mph. That’s a slower speed, and more accelerating and braking than the cars in the Carwow test.
8364 miles at 56 mpg is 8364/56 = 149.4 gallons for those 8364 miles.
Since there are 4.55 litres per gallon, that is 149.4 x 4.55 = 680 litre.
Each litre has 10 kWh of energy but diesel engines are only 30 % efficient, so we get 3 kWh of useful work out of each litre of diesel.
Total amount of useful work out = 3 kWh/litre x 680 litres = 2040 kWh for 8364 miles.
So energy needed per mile = 8364 miles/2040 kWh = 4.1 miles/kWh.
So we can see that the medium-sized Renault Megane travels as many miles per available kWh as a typical small electric car.

Broad facts worth remembering.
A small car will travel about 5 miles per kWh.
A large car will travel about 3 miles per kWh.
Medium cars, as you might imagine, are somewhere in the middle.
If you want to travel 300 miles in a large car, you need 100 kWh.
That is as simple as that.
A 50 kWh battery, powering a small car which does 5 miles/kWh, will give a range of 50 kWh x 5 miles/kWh = 250 miles, but there isn’t yet a small electrical car that will do that: the Renault Zoe, travelling at a constant speed along a motorway, only did 229 miles.

Other tests on the Megane
I did two trial runs on quiet motorways to measure fuel consumption at fixed speeds.
At 48 mph I averaged 72 mpg.
At 58 mph I averaged 58 mpg.
These figures point to the reasonableness of the overall 56 mpg figure experienced in practice.