Energy
What exactly are you paying for when you charge your car?
You are buying energy. Each kWh of energy costs you about 50p at a public chargepoint or less than 10p on the cheapest home tariff, and takes your car about 4 miles.
Spot the difference!
kWh
Little k, big W, little h. This is a measure of energy, and relates to how far your car goes. "How many kWh are in my battery?" is similar to "How much fuel is in my tank?".
The numbers in front of kWh are usually two digits, but could be anything depending on battery size or how much you are depleting or recharging your battery. Only top of the range EVs have capacities over 100 kWh.
kW
Little k, big W. This is a measure of power, and relates to how quickly energy flows into your car when you charge. We never thought about how quickly fuel flowed into a tank because filling up took a matter of seconds.
You might see the numbers 3, 7, 11, 22, 50, 100, 150, or 350 in front of kW. This is because there are standard charging speeds.
From money to miles
Charging
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↓
Filling up
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Each litre of petrol contains about 9kWh of chemical energy.
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Efficiency, efficiency, efficiency
Have a look at the bottom arrow in each flowchart above. EVs can go around three miles further on a kWh of energy than fuel cars can! Electric powertrains are much more efficient than fuel powertrains.
A kWh of electrical energy tends to cost more than a kWh of chemical energy in fuel, as per the top arrows in each flowchart above. But not always! If you can charge overnight at home for less than 15p/kWh, then you are getting electrical energy cheaper than fuel energy, and each unit is taking you about four times as far. This means your mileage is cheap as chips!
If you really want to compare mileage costs, then you need to take account of efficiencies of both types of car, incorporating the two little arrows above into one big arrow:
The rule of thumb is that petrol costs 13.5p/mile, and you need to divide your charging tariff by 4 to get p/mile. E.g. 54p/kWh is roughly 13.5p/mile.
Efficiency varies! Factors that influence EV efficiency include car weight, technical design, outside temperature, and driving environment (e.g. urban stop-start v motorway pedal-down).
Range
Unless your wallet's empty or your car breaks down, the middle point of the above flowcharts limits how far you can go on a single charge/tank.
Like efficiency, battery capacities and tank sizes vary hugely, but if we take 40kWh as the average battery capacity, and 60l as the average tank capacity, then this comes out at around 160 miles in the EV versus 600 miles in a petrol car.
The larger the battery capacity, the larger the range, but there are no two ways about it - you will need to charge more often than you refilled.