How to Use This Tool
Set the battery size, the state of charge you are starting and finishing at, and the charger. The answer separates energy that reaches the battery from energy you pay for, because they are not the same.
Charging losses
Not everything the meter records ends up in the pack. The rest becomes heat in the cable, the on-board charger, the battery itself, and the thermal management running while it all happens.
- Slow AC — typically 10–15% lost. A trickle charge from a domestic socket is the worst case, because the fixed overhead of keeping the car awake is spread over a small trickle of energy.
- Wallbox AC — around 10%.
- DC rapid — 5–8%, since the on-board charger is bypassed.
Putting 36 kWh into a 60 kWh pack at 90% efficiency draws 40.0 kWh from the meter. At 0.30 per kWh that is 12.00 rather than 10.80 — and the difference is invisible unless you compare the car's display with your electricity bill.
The rated power is a peak
A "150 kW" charger is rated at what it can deliver, not what your car will accept. The actual rate is whichever of these is lowest at that moment: the charger, the car's maximum DC rate, the cable, and what the battery management system is currently allowing.
That last one changes constantly. The pack accepts high current when it is fairly empty and reduces it as it fills, steeply above around 80%, to protect cell life. It also refuses full power when cold, which is why cars precondition the battery when a rapid charger is set as a destination.
So 20–80% on a 150 kW charger calculates to 15.3 minutes at full rate and takes rather longer in practice; 80–100% calculates to 5.1 minutes and typically takes closer to 20.
Usable capacity, not total
Manufacturers quote two numbers. Total capacity includes a buffer the car will never let you use, kept back to protect the cells at both ends. Usable capacity is what appears on the range display between 0% and 100%.
Use the usable figure here, because 20% on the dashboard means 20% of usable. Mixing them overstates both the time and the cost by whatever the buffer happens to be, which can be several kWh.
Comparing with petrol
The fair comparison is cost per distance rather than cost per unit of energy. A car using 18 kWh per 100 km at 0.30 per kWh costs 5.40 per 100 km before charging losses, and about 6.00 after them.
Two things distort this in both directions. Home charging on an off-peak tariff can be a small fraction of public rapid-charging prices, so the same car has wildly different running costs depending on where it is plugged in. And real consumption is worse than the official figure in cold weather, at motorway speed, and with the heating on — often by 30% or more, which affects the calculation more than the electricity price does.
