Charging Time, and Why the Last 20% Takes Forever
Charging time looks like division and is only division for part of the range. Understanding where the arithmetic stops being true is what makes a trip plan work.
The straightforward part
The charging time calculator works out the energy needed and divides by charger power, adjusted for efficiency losses (a 90% default, covering heat and conversion).
| Charger | 75 kWh, 20→80% |
|---|---|
| 7.4 kW (home AC) | 6 h 45 min |
| 50 kW (DC) | 1 h 00 min |
| 150 kW (DC rapid) | 20 min |
All three add the same 45 kWh. The difference is entirely the rate, and the spread — twenty minutes against nearly seven hours — is why charger type matters more than battery size for trip planning.
Where the last 20% goes
Take that 150 kW charger from 20% to 100% instead: 60 kWh added, 27 minutes. So the first 60% of the battery took 20 minutes and the last 20% took another 7 — the extra fifth costs 35% more time in the arithmetic alone.
In reality it is worse, and this is the calculator's main simplification. DC fast charging does not hold peak power across the range: the charge curve tapers, often substantially, well before 100%. A vehicle pulling 150 kW at 30% may be down to a fraction of that by 80%.
Which is why the 20–80% window is the one everyone quotes. It is the part of the curve where fast charging is actually fast.
The practical planning rule
On a long trip, two stops to 80% usually beat one to 100%. The arithmetic above understates the advantage, because it does not model the taper.
The exception is the last leg. Charging to 100% before the final stretch to a destination with no charger is a sensible use of slow charging — and doing it overnight on AC costs nothing but time you were asleep for.
What limits the rate
The charger's rating is a ceiling, not a promise. The actual rate is the minimum of what the charger can deliver, what the vehicle can accept, and what conditions allow.
Vehicle acceptance varies widely and is often well below the charger's headline figure. Battery temperature matters — a cold pack charges slowly until it warms, which is why some cars precondition on the way to a charger. And a shared or busy site may deliver less than its rating.
So a 150 kW charger and a car that accepts 100 kW is a 100 kW session, and neither number on its own predicts the stop.
Home versus rapid
The 7.4 kW figure looks bad in a table and is the right answer for most charging. Six hours forty-five is irrelevant when the car is parked for eleven hours overnight, and home rates are typically far cheaper than rapid charging — which, as the trip cost calculator shows, is where most of an EV's running-cost advantage actually comes from.
Rapid charging is for journeys, not for routine. Treating it as the normal way to charge makes an EV both slower and considerably more expensive than it needs to be.
Why the calculator still helps despite the taper
It gives a floor. Real sessions take at least the computed time and usually longer, so a plan built on it is optimistic in a known direction rather than in an unknown one.
It is also exactly right for AC charging, where the rate is flat — home charging genuinely is energy divided by power, and the 6 h 45 figure for a 20→80% top-up at 7.4 kW is what will happen.
Efficiency losses are real
The 90% default accounts for energy that never reaches the battery — heat, and AC-to-DC conversion where the car does it. That is why 45 kWh added to the pack draws rather more from the charger, and why the number on your bill exceeds the number on the dash.
Losses are generally higher on slow AC charging and in cold conditions, which is another reason winter costs more per mile than the consumption figure alone suggests.