What charging actually costs
An electric car costs about five cents a mile to run on cheap home electricity and about four or five times that on a motorway rapid charger. The car is the same car. Almost everything about the running cost is decided by where you plug it in.
Two other things get lost in the usual comparison: you pay for more energy than reaches the battery, and the charger on the wall is often not the limiting factor.
You are billed for more than goes in
Charging is not lossless. At around ninety per cent efficiency, putting 56 kWh into a battery draws 62.22 kWh through the meter. The missing 6.22 kWh becomes heat in the cables, the charger and the battery itself, and you pay for it.
Over 200 miles at 16 cents a kWh that is a dollar of pure loss on a $9.96 charge — about ten per cent. It is not enormous, but it is real, and it is the reason a cost worked out from the battery capacity alone always comes out slightly too low.
Efficiency is worse in the cold and worse on slow AC charging, where the overheads run for longer relative to the energy delivered.
200 miles at 280 Wh/mile = 56 kWh into the battery
At 90% efficiency = 62.22 kWh off the meter
At $0.16/kWh = $9.96, of which $1.00 is loss
Cost per mile = $0.05
Work it out: EV Charging Cost Calculator →
The car is usually the bottleneck
People buy an 11 kW home charger and then find the car will only take 7.4 kW. The rest of the unit sits idle, because the limit is the on-board charger in the vehicle rather than the supply on the wall.
On a 60 kWh battery going from twenty to eighty per cent — 36 kWh added, 40 kWh drawn — that is five hours and twenty-four minutes at 7.4 kW rather than the three and a half hours the charger is rated for.
It is worth checking the car's AC charging rate before buying the charger, because the cheaper unit would have been just as fast. The distinction is between AC charging, which is limited by the car, and DC rapid charging, which is limited by whatever the post can deliver.
Work it out: EV Charging Time Calculator →
Why the last twenty per cent takes so long
Every charging estimate assumes a steady rate, and on rapid charging that assumption is generous. Charging slows sharply above roughly eighty per cent to protect the cells, so the last fifth can take about as long as the first three fifths did.
That is why public charging is quoted twenty to eighty rather than nought to a hundred, and why the sensible motorway strategy is more short stops rather than fewer long ones. Charging to full on a rapid unit is usually the worst value use of your time on the whole journey.
Cold cells charge more slowly too. A car that has stood outside overnight in winter may take half its rated rate until the battery warms, which is on top of everything else winter costs.
Comparing with petrol, honestly
Twelve thousand miles a year in a 30 mpg petrol car at $3.40 a gallon is $1,360 of fuel. The same distance in an electric car at 280 Wh a mile and 16 cents a kWh is $597.33. The electric car saves $762.67 a year — eleven cents a mile against five.
That comparison is energy only, and it should be labelled as such. Insurance, servicing, tax and depreciation all differ between the two: an EV generally wins on servicing and often loses more to depreciation, and neither appears here.
And the whole thing swings on the electricity price. Charging at home on a night rate and charging on a motorway are not the same argument — at rapid-charger prices the running cost advantage can disappear entirely. Anyone who cannot charge at home should run the comparison at the price they will actually pay.
Work it out: Electric vs Petrol Running Cost Calculator →
Questions
How much does it cost to charge an electric car?
On home electricity at 16 cents a kWh, roughly five cents a mile — about $9.96 for 200 miles. On a motorway rapid charger it can be four or five times that.
Do I pay for the energy lost in charging?
Yes. At about ninety per cent efficiency you are billed for 62 kWh to put 56 kWh in the battery. The rest is heat, and it is on your meter.
Why is my car charging slower than my charger allows?
Because AC charging is limited by the on-board charger in the car. A vehicle that accepts 7.4 kW will take 7.4 kW from an 11 kW unit and leave the rest unused.
Why does charging slow down near full?
To protect the cells. Above about eighty per cent the rate drops sharply, so the last fifth can take as long as the first three. That is why rapid charging is quoted 20 to 80 per cent.
Is an electric car cheaper to run?
On energy, usually by a wide margin — around $760 a year over 12,000 miles at home rates. That figure excludes insurance, servicing, tax and depreciation, and it depends entirely on where you charge.
Why does range drop so much in winter?
Cold costs twice: the battery is less willing to accept and deliver charge, and heating the cabin comes out of the same battery rather than from waste engine heat.