EV charging time — methodology

What this calculator does

It estimates how long it takes to charge an EV between two states of charge (SoC) at a charger power you choose, and reports the energy and range added, the effective charging speed, and a full 0–100% charge time. It also compares the same charge across a range of charger powers, from a household outlet to DC fast charging.

The formulas

Energy and time

energy_needed     = battery_kwh * (target_soc - start_soc) / 100
effective_power   = charger_kw * charge_efficiency
charge_hours      = energy_needed / effective_power
full_charge_hours = battery_kwh / effective_power

We size the charge on energy delivered into the battery, then reduce the charger's rated power by a charging efficiency. Some energy is lost converting AC to DC for the battery, so the power that actually reaches the cells is a little lower than the charger's rating — which is why the real time is slightly longer than a naïve energy ÷ power.

Range added and charging speed

miles_added       = energy_needed * miles_per_kwh
charge_speed_mph  = miles_added / charge_hours

Charging speed is the miles of range added per hour. It is equivalent to miles_per_kwh × effective_power, independent of the size of the charge window.

Charger comparison chart

chart_hours(power) = energy_needed / (power * charge_efficiency)

for power ∈ kW. The bar nearest your charger power is highlighted.

We guard every division: a zero charger power or zero charging efficiency returns “no result” (an em dash) rather than infinity, and a charge window of zero correctly yields zero energy and zero time.

Default values

Every default below is editable in the calculator and shown with its source in the calculator’s assumptions table. The battery size, charge window and charger power are user inputs with no single authoritative default; the charging efficiency and EV efficiency are tied to dated sources.

Limitations — read these

Constant charging rate. We assume a steady rate for the whole session. Real DC fast charging slows as the battery fills — especially above ~80% state of charge — so the high-power times in the chart are best-case and a 0–100% DC session in practice takes noticeably longer than this model suggests. - No temperature or onboard-charger limits. Cold weather slows charging and raises losses, and a car’s onboard charger caps AC charging regardless of the wall supply. We don’t model either, so set the charger power to what your car can actually accept on AC. - Efficiency is a single lumped figure. The charging efficiency rolls all AC conversion losses into one number (90% by default). Actual losses vary with charger, power level and temperature.

How we keep it honest

The calculation logic lives in a small, pure function that is unit-tested against normal, edge and invalid inputs (for example, a zero charger power returns “no result” rather than infinity). If you spot an error, tell us and we’ll fix it.

What this calculator does

The formulas

Default values

How we keep it honest

Sources