Traditionally, fuel for vehicles has been a "thing"; a tangible object that you can purchase and take home with you. It is measured in physical units, such as gallons or litres, or weight (in the case of fuels like coal). When we consume it, we measure the consumption by those same units. Hence we consider "miles per gallon" as a measurement of how efficient our car is.
As a relative measurement (to other cars) this is, of course, fine. As a consumer purchasing the fuel, it also gives us a view on the cost of motoring. However, it tells us nothing about the actual energy consumption, at least not directly or intuitively.
Energy is not measured in gallons or litres. The official SI unit of energy is the joule (J), but the reality is most people don't think in joules, and have no concept of how much a joule is. However, there are a number of other units of energy are commonly used, such as calorie and electronvolt. In the case of electrical power, energy is most often measured in killowatt hours (kWh). A kWh is also known as a "unit" of electricity. Most people have some understanding of kWh (or "units") as it relates directly to the power consumption of various appliances around the home, and to their electricity bill! (for reference, 1 kWh is equal to 3.6 megajoules)
Related to this, we can measure the rate of power use in watts (W). Power is the rate of energy change or transfer with 1 watt being equal to 1 joule of energy per second. In our case we use kilowatts (kW) which are 1,000 watts each.
We know, for instance:
- An average electric kettle in the UK draws 1.8kW of power
- A typical washing machine consumes around 0.7kW averaged across the wash cycle
- A 1 bar electric heater burns around 1 kW
- A typical filament electric light bulb burns 0.06kW (60 Watts)
With these as mental benchmarks, if we now look at driving a car. In my EV, driving steadily along a straight, flat road at 30mph, and can easily be burning 10kW. That's equivalent to 10 electric fires, or 5-6 electric kettles!
If I drive at that consumption rate for an hour, I will consume 10kWh of electricity. The maths are simple and obvious and it was quite startling to me how much power is required to propel a small car. If I accelerate, I can easily be burning 30 or 40 kW. That's a lot of power!
As I have shown above, consumption for an EV is easy to work out. It is, of course, significantly different (and more revealing) than for an ICE car. And, instead of mpg, we use mpkWh (miles per kilowatt hour, or per "unit" of electricity). In my example above I burned 10kWh for 30 miles and, hence, I am getting 3 mpkWh from my car. In fact, across a range of journeys which includes everything from edging along in traffic at 5mph to 70+ mph motorway driving, my car gives me 3.8 mpkWh.
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| My car trip computer showing average fuel consumption |
Others, who are more careful drivers than me, have achieved in the region of 4 mpkWh in the ZOE.
Of course, this helps us understand car energy consumption in relation to 1 bar fires and electric kettles, but how does it compare with good-old petrol engines? Petrol has a known energy density, and a calculation can be done to work out the amount of energy consumption, but this is rarely done. In general people merrily drive their cars without the slightest clue of how much energy it is consuming.
So let's do that calculation:
(EDIT: A friend of mine, +Francis Chin , pointed out an error in my calculations, so these have been updated)
A UK gallon of petrol typically contains around 156 megajoules of energy (132 MJ for a US gallon). If your car, on average, burns a gallon of fuel every miles (43 mpg) then every mile of travel consumes 1/43th of a gallon. That contains about 3.6 MJ of energy, which is equivalent to around 1kWh of electricity. Thus 43 mpg in an ICE is equivalent to 1 mpkWh in an electric car.
Of course, there are ICE cars with much better consumption than that, but in order to reach my, relatively heavy-footed, 3.8 mpkWh figure, an ICE would need to be 3.8 times better than 43 mpg. It would need to achieve 163.4 mpg!
And that is only considering the energy content of the fuel and ignoring the substantial energy costs of drilling, pumping, refining, and shipping. The energy used for refining alone is pretty boggling (as we shall see in my update below).
In reality even the most efficient ICE available to purchase today cannot reach half of that figure. And, bear in mind, this is real world driving, not some theoretical controlled test.
I also haven't considered cost in any of the above. Let's do that now:
A litre of unleaded petrol in the UK currently costs around £1.30. That is £5.91 per UK gallon. That is a cost of 3.79 pence per megajoule.
A "unit" of electricity in the UK currently costs about 13.5 pence (inc VAT). That is a cost of 3.75 pence per megajoule.
So, electricity is currently roughly the same price for the equivalent energy content.
However, petrol engines, even the most efficient modern ones, are only 35% efficient at best. The engines in electric cars are in excess of 80% efficient (The motor in the Telsa model S is claimed to have an efficiency of 88%). If we take the case most favourable to petrol, and calculate the cost of realising the energy stored in the fuel to an equivalent energy at the wheels, we end up with a cost of 10.83 pence per megajoule for a petrol car, and 4.69 pence per megajoule for an electric car. That's more than twice as much.
And petrol prices are rising faster than electricity prices.
The result of this is that, for an equivalent specification of car, the energy costs are around 3-5 times more for a ICE car than for an EV.
UPDATE (1 May 2014):
I've recently watched the latest episode of Robert LLewllyn's excellent Fully Charged Youtube show. At 4:55 he starts to discuss how much electricity is used to refine petrol.
Fully Charged - Watch from 4:55
The interesting take out from this is that petrol requires around 4.5 kWh of electricity per gallon to refine. That same power would allow me to drive 17 miles. So, straight away, compared to a petrol car, my electric car's energy consumption has a head-start equivalent to 17 mpg. So our hyper-efficient 60 mpg car above, is (relatively speaking) only doing the equivalent of 43mpg.
Put it another way: if you filled up your car up with 10 gallons of petrol, I could drive from London to Cardiff in my EV and might still consume less electricity than was used to refine the petrol in your tank. You would still be sitting on the petrol station forecourt, not even turned your engine on, I would be sitting in Cardiff, and you would still have burned more fossil fuel than me.
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