EV Charging Time Calculator

How charging time is actually calculated

Every charging estimate comes down to one division: energy needed, divided by power delivered. The energy is the slice of the battery you want to fill — capacity multiplied by the gap between your current and target state of charge. The power is whichever is lower, the charger's output or your car's maximum intake. Between those two numbers sits the part most calculators ignore: charging losses, the electricity your meter records that never reaches the cells.

Worked example: 75 kWh pack, 20% to 80%, 7.4 kW home charger

The gap is 60% of 75 kWh, so 45 kWh has to end up in the battery. AC charging runs through the car's onboard charger, which converts alternating current to direct current and sheds heat doing it — assume 90% efficiency and you actually draw 45 / 0.9 = 50 kWh from the wall. Divide by 7.4 kW and you get 6.76 hours, or about 6 h 45 min. Plug it in at 10 pm and it finishes before 5 am, comfortably inside a cheap overnight tariff.

At a US average of $0.17/kWh that charge costs about $8.50 and adds roughly 158 miles at 3.5 mi/kWh — around 5.4 cents per mile. In the UK, an overnight EV tariff near 7p/kWh puts the same charge at about £3.50; on the standard cap near 25p it is closer to £12.50. Which tariff you are on changes the running cost of an EV more than the car you choose.

The same charge on a 150 kW DC rapid charger

DC rapid chargers bypass the onboard charger and feed the pack directly, so losses at the car drop to roughly 5%: 45 / 0.95 = 47.4 kWh drawn. Divide by 150 kW and the arithmetic says 19 minutes. Real life says 25 to 35 minutes, because no car holds its peak rate for the whole session. Rates ramp up as the pack warms, hold near peak through the middle of the curve, then taper as the state of charge climbs.

Why the taper above 80% matters

Charging speed is limited by how fast lithium ions can move into the anode without plating. As the battery fills, the acceptable current falls — sharply past 80% on most packs. A car pulling 150 kW at 30% may be down to 50 kW at 80% and 25 kW at 95%. In practice the last 20% of a rapid charge often takes as long as the first 60%, which is why this tool flags an honest warning whenever you ask for more than 80% on a 50 kW or faster charger. The flat-rate assumption is fine up to 80% and increasingly optimistic beyond it.

On AC there is no taper worth worrying about. A 7.4 kW wallbox is so far below what the pack can accept that the rate stays flat almost to 100%, which is why the "charge to 100% at home, stop at 80% on the road" rule exists.

Home versus rapid charging, in money

Take the same 45 kWh into the battery. At home on a 7p/kWh overnight rate in the UK, you draw 50 kWh and pay about £3.50. At a public rapid charger at 79p/kWh you draw 47.4 kWh and pay about £37 — more than ten times as much for the same miles, and roughly what a petrol car costs to run over that distance. The US spread is narrower but real: $0.17/kWh at home versus $0.48/kWh at a typical DC network is about $8.50 against $23.

The practical conclusion is that home charging is where the EV savings live, and public rapid charging is a convenience you buy on long trips. If you cannot charge at home, run the numbers with your local public tariff before assuming an EV is cheaper per mile than a hybrid.

Road-trip rule of thumb

Plan stops between 10% and 70-80%, not 0% to 100%. That band is where the charge curve is fastest and it keeps a safety buffer at the low end. For a car with a real-world 250-mile range, that means roughly 175 miles between stops and 20-30 minutes plugged in — about the length of a coffee and a comfort break. Two shorter stops almost always beat one long one.

What this estimate cannot know

Four things move the real number. Your car's maximum AC intake caps home charging: many EVs accept only 7.4 kW single-phase, so an 11 kW or 22 kW post gives no benefit. Battery temperature caps DC charging, and a cold pack can halve the rate until it warms — precondition on the way to a charger if your car supports it. Shared charging cabinets split power between two bays, so a "150 kW" post may deliver 75 kW when both stalls are busy. And the battery management system slows things down near full regardless of what the charger offers. Treat the output here as a good planning figure, then let the car's own estimate take over once you are plugged in.