EV Charging Time Calculator
Enter battery size, start and target charge, and the charger to get time, wall energy and a charger-level comparison.
Step-by-step
- Enter valid values to begin.
Simple taper model: the battery accepts constant power up to 80% state of charge, then the power falls linearly to the floor value at 100%. Real packs also taper with temperature, cell balance and the vehicle’s own charge curve, so treat the result as an estimate. AC power is limited by the lower of the supply and the onboard charger; DC power by the lower of the station and the vehicle. Efficiency is the wall-to-battery value (typically 88–92% AC, about 95% DC). Branch-circuit sizing follows NEC Article 625 (EVSE is a continuous load, 125%); verify the adopted edition and the vehicle manual.
EV Charging Time: Power, Losses and the 80% Taper
Core Engineering Principles
Charging time is energy divided by power, and for the first 80% of a pack it nearly is that simple. The trap is which power. A Level 2 station supplying 7.7 kW does not put 7.7 kW into the cells. The onboard charger and cooling take their cut, so we figure 88–92% on AC and about 95% on DC fast, where the rectifier sits in the cabinet. The utility bill follows the wall meter, not the battery. We also respect the weakest link: an 11 kW onboard charger turns a 48 A pedestal into an 11 kW charge, and a car that accepts 150 kW makes a 350 kW cabinet act like a 150 kW one.
Above about 80% state of charge the battery management system cuts current to hold cell voltage in check, so the last stretch takes far longer than the first. Our model keeps power flat to 80%, ramps it down linearly to a floor at 100%, and integrates the time exactly. Cold cells sag it further, so treat the answer as an estimate. That is why DC fast charging is quoted as 10–80%, and why we tell road-trip drivers to leave at 80%.
Taper: t = 0.2 C / (P k) × ln[(1 − k u1) / (1 − k u2)], u = (SoC − 80%) / 20%
NEC & Standard References
SAE J1772 defines the North American AC connector and the pilot signal that tells the car how many amps the station offers. IEC 62196 covers plugs and sockets elsewhere, and IEC 61851-1 sets the general requirements for conductive charging. NEC Article 625 treats EVSE as a continuous load, so the circuit is sized at 125% of the EVSE current: a 32 A charger needs a 40 A circuit. Confirm the adopted edition and local amendments.1. Supply = 240 × 32 = 7.68 kW, below the 11 kW onboard limit, so the car draws all of it.
2. Battery power = 7.68 × 0.90 = 6.91 kW.
3. Energy into the pack = 75 × 0.80 = 60.0 kWh; from the wall = 60 / 0.90 = 66.7 kWh.
4. 10–80%: 52.5 / 6.912 = 7.60 h (7 h 36 min).
5. Taper 80–90% adds 1.39 h: total 8.98 h, about 8 h 59 min, at 6.68 kW average and about 210 miles added.
6. Branch circuit = 1.25 × 32 = 40 A.
- Continuous-load rule. Never put a 32 A charger on a 32 A breaker; hours of sustained current overheat terminations first.
- Level 1 is a real fire risk on tired outlets. Twelve amps for ten hours cooks worn receptacles and thin extension cords.
- Cold batteries charge slowly. Below freezing the pack limits power until it warms.
- Check the real onboard limit. Many cars cap AC at 7.2 or 11 kW; a bigger wallbox buys nothing.
- Never defeat the pilot or ground checks. Cheater cables that bypass J1772 signalling remove the shock and overload protections.