How We Calculate Charging Time | Formulas and Sources

Every calculator runs a short formula in your browser and shows the steps under the result. These are the formulas and the defaults behind them.

Battery charging time

hours = Ah x (end % − start %) / (charger amps x efficiency)

Default efficiencies follow Battery University BU-808c: about 90% for lead-acid, over 99% for lithium-ion and 70% to 90% for NiMH (80% is used). The result assumes the charger holds its rated current. Real chargers slow down near full: lead-acid reaches about 70% in the fast stage and tops off over several more hours (BU-403), and lithium-ion tops off past about 85% (BU-409). For phones and power banks, mAh is converted to watt-hours at 3.7 V per cell, with 85% efficiency by default.

EV charging time and cost

hours = battery kWh x (end % − start %) / efficiency / min(charger kW, car's limit kW)

Charger power is volts times amps: 120 V x 12 A = 1.44 kW for Level 1, 240 V x 32 A = 7.68 kW for a typical Level 2 home charger. The default efficiency is 90%. The car's onboard charger caps AC charging; enter its limit if you know it. Cost multiplies the energy from the wall by your price per kWh. The example rate of 18 cents is rounded from the US residential average of 18.16 cents in August 2026 (EIA). Charger levels and typical range per hour come from the Department of Energy's Alternative Fuels Data Center. DC fast charging follows each car's charging curve, so the formula doesn't apply to it.

EV charging times by model

The EV charging time chart uses the EPA test data that fueleconomy.gov publishes for every vehicle (download page), model years 2018 and later. For each trim the EPA gives the range, the electricity used per 100 miles (measured at the wall, so charging losses are included) and the time to charge from empty on 240 V. Range times electricity use gives the energy a full charge draws from the wall. Dividing that by a charger's power gives the times for Level 1 and each Level 2 current. Dividing it by the EPA charge time gives the power the car accepted on the test; charger rows above that power are marked, because the data can't show whether the car takes more. Model pages show the newest model year's middle trim; every trim is listed lower on each page. The data is rebuilt once a year when new model years arrive.

Solar, current and runtime

Solar charging divides the watt-hours needed (Ah x volts x share to fill) by the panel's watts times a system efficiency, 75% by default, to give peak sun hours. Charging current uses C-rates: 0.1C to 0.3C for lead-acid, where makers recommend about 0.3C (BU-403), and 0.2C to 0.5C for lithium, inside BU-409's 0.5C to 1C for energy cells. Runtime multiplies Ah by volts and the usable share (50% for lead-acid, 80% for lithium by default), less 10% for an inverter if ticked, and divides by the load.

Capacitors

Charging through a resistor follows V(t) = V x (1 − e^(−t/RC)), so the time to a share p of the supply is −RC x ln(1 − p). Discharging follows V(t) = V0 x e^(−t/RC), so the time to a voltage V is RC x ln(V0 / V). Constant-current charging takes C x ΔV / I. Energy stored is ½CV². These are the standard RC circuit results.

Limits

Every result is an estimate for planning. Temperature, battery age, the charger's real output and the battery's own charging profile all change the time. Follow your battery and vehicle maker's instructions.