EV Charging Time & Cost Calculator

Why charging estimates are usually wrong

When to use this: planning a road trip stop, deciding whether a bigger wallbox is worth fitting, working out whether an EV actually saves money against petrol, or checking why a charge took longer than the app promised.

Two mistakes account for nearly all of the error, and almost every calculator makes both.

1. Dividing energy by the advertised peak power. That power is delivered briefly and never again. The taper does most of the work in a real session.
2. Costing the energy that reaches the battery. You are billed at the meter, and between 6% and 20% of what you buy never gets there.

The taper

Lithium-ion cells accept current freely when empty and grudgingly when full. As the cells approach full charge their voltage rises and the risk of lithium plating grows, so the battery management system progressively cuts the current. Nothing is broken. It is the battery protecting itself, and it is why manufacturers quote 10–80% times and never 0–100%.

A Tesla Model 3 Long Range on a 250 kW post, broken into segments of state of charge — the energy per segment is identical, the time is not:

The 10→30% and 90→100% segments tell the story. The first adds 15.0 kWh in 4 min. The last adds half as much and takes 29 min.

What this does to the headline number

Dividing 52.5 kWh by 250 kW gives 13 min. The real 10–80% session is 26 min — the taper adds 104%, and the average power over the session is 123 kW against the 250 kW on the poster.

Push to 100% and the arithmetic gets worse. 10→80% is 26 min; 80→100% is a further 40 min, which is longer than the first seventy points of charge for 29% of the energy. The whole 10→100% run comes to 1 h 5 min.

The practical consequence on a road trip: leave at 80%. Two short stops beat one long one, because you spend the whole of both at the fast end of the curve.

Why the peak power figure misleads

The taper is not a fixed fraction of a car's peak power — it is a limit on the C-rate the pack will accept, which is why a car with a modest DC limit barely tapers at all. A Chevrolet Bolt EV asks 0.85C and never approaches the ceiling, so it holds near full power almost to 80%. A Tesla Model 3 Long Range asks 3.3C and runs into it early.

This is also why 800 V cars charge faster than their peak figures suggest. At the same power, twice the voltage means half the current, which means a quarter of the resistive heating — so the pack can sustain a high rate much further up the curve:

The Ioniq 5 and the Model 3 have similar peak figures and differ by minutes on the same session. The Rivian has a high peak and a mediocre curve. The Bolt has the lowest peak of the four and the highest ratio of average to peak, because it never gets near the limit.

Charging losses: what you pay for and what you get

Energy is lost converting AC to DC, warming the cells, running the thermal management system and keeping the 12 V electronics alive. Some of that overhead is roughly constant, which is what makes slow charging inefficient rather than efficient.

Level 1 is the case worth pausing on. It feels like the free option — no wiring work, an existing socket — and it wastes about a fifth of everything you buy. Over a year of daily commuting that is a meaningful sum, and it is invisible because it never appears as a line on anything.

AC charging is limited by the car, not the wallbox

The onboard charger inside the vehicle converts AC to DC, and its rating is a hard ceiling. Fitting a bigger wallbox to a car with a small onboard charger changes nothing at all.

The Leaf's 6.6 kW onboard charger means every wallbox above that size is wasted money. The Zoe, unusually, accepts 22 kW on AC and only 46 kW on DC — the exact opposite of the normal arrangement, and a car built around AC charging.

If your car does take 11 kW or more, that is a substantial circuit and a continuous load. Our wire size calculator applies the 125% continuous factor and the derating rules that decide what cable it needs.

Every listed vehicle

Usable capacity and real-world consumption, not gross capacity and optimistic test cycles. Times are 10–80% on a charge point large enough not to be the limit.

Charging against petrol

The comparison depends almost entirely on where you charge, and that is why both sides of the argument can produce a convincing number. Home overnight electricity and public rapid charging can differ by a factor of four for exactly the same kWh.

Do the comparison honestly: use your own tariff for the charging you actually do, remember that the meter figure is the one you pay, and compare it against the fuel price you actually pay rather than a national average. Our electricity bill calculator will give you the effective rate per kWh you are really on, including the standing charge — which is the number that belongs in this comparison, not the headline tariff.

A time-of-use tariff changes the picture more than anything else available to you. Shifting charging into an overnight window is usually the single largest saving an EV owner can make, and it costs nothing but a timer setting.

Battery care, briefly

• Daily use between 20% and 80% for a typical NMC pack. Charging to 100% before a trip is fine; doing it every night is not ideal.
• LFP is different. Manufacturers of LFP-equipped cars usually recommend a periodic charge to 100% to keep the state-of-charge estimate calibrated. Check which chemistry you have, because the advice genuinely inverts.
• Cold packs charge slowly. A winter DC session can take twice as long without preconditioning. Navigating to the charger in the car's own system usually triggers it.
• AC charging is gentler than DC, and most cars spend most of their life on it. Occasional fast charging is not something to worry about; exclusive fast charging is.