EV Charging Time Calculator

Related tools: Battery Capacity Calculator, Battery Runtime Calculator, and EV Charger Load Calculator.

An EV Charging Time Calculator estimates how long a vehicle needs to charge across a defined state-of-charge window. It converts the usable battery energy required between Start charge and Target charge into the grid energy required at the selected Charging efficiency, then divides that grid energy by Charger power.

The resulting Estimated charging time supports practical EVSE planning: comparing Level 2 charging options, estimating dwell time at a residence or workplace, reviewing expected branch-circuit demand, and evaluating whether an existing service, feeder, or panel load study needs further investigation. The calculated time is an energy-and-power estimate; conductor ampacity, overcurrent protection, EVSE nameplate requirements, voltage drop, terminal ratings, and applicable NEC or AHJ requirements remain separate installation decisions.

State of Charge and Battery Energy

Battery capacity is the usable vehicle battery-energy basis in kWh. Start charge and Target charge establish the charging window, expressed as a percentage-point difference rather than a percentage increase.

\(\displaystyle \text{SOC window} = \text{Target charge} - \text{Start charge}\)

\(\displaystyle \text{Battery energy added} = \text{Battery capacity} \times \frac{\text{SOC window}}{100}\)

A 20% to 80% charge is a 60-percentage-point SOC window. It does not mean 60% more energy than the battery already contains. With a 75 kWh usable battery basis, the energy stored in the battery during that window is:

\(\displaystyle 75\text{ kWh} \times \frac{80 - 20}{100} = 45\text{ kWh}\)

The calculator reports this value as Battery energy added.

Charger Power and Grid Energy

Charger power is the average charging power delivered from the charging source, in kW. Charging power is the rate of energy transfer, so a higher kW value reduces the estimated time when all other inputs remain unchanged.

Charging efficiency accounts for energy losses between the grid and the energy retained in the vehicle battery. Those losses can occur in EVSE-related delivery equipment, vehicle onboard charging equipment, battery conditioning, and other charging-system loads. The calculator uses charging efficiency to estimate the energy drawn from the grid:

\(\displaystyle \text{Grid energy used} = \frac{\text{Battery energy added}} {\text{Charging efficiency}/100}\)

Charging efficiency changes both Grid energy used and Estimated charging time in this calculation. It does not change the battery energy required to move through the selected SOC window.

For electrical planning, charger power should represent realistic average delivered power rather than a maximum value that will not be sustained. Supply-voltage variation, vehicle charge acceptance, battery temperature, load sharing, and charge taper can reduce actual average kW.

Charging-Time Formula

The calculator calculates charging time from estimated grid energy and average charger power:

\(\displaystyle \text{Estimated charging time} = \frac{\text{Grid energy used}} {\text{Charger power}}\)

With energy in kWh and power in kW, the result is hours:

\(\displaystyle \frac{\text{kWh}}{\text{kW}} = \text{h}\)

This approach is useful for normal charging windows where the selected Charger power reasonably represents the average rate. It is less precise near a high target SOC, where many EVs reduce charging power as the battery approaches full charge. A 20% to 80% estimate is generally more representative of routine charging than an estimate to 100%, but the vehicle’s actual charging curve still governs field performance.

Calculation Example

Use the following inputs:

Field	Value
Battery capacity	75 kWh
Start charge	20%
Target charge	80%
Charger power	9.6 kW
Charging efficiency	90%
Efficiency	3 mi/kWh

First, calculate the SOC window:

\(\displaystyle 80\% - 20\% = 60\text{ percentage points}\)

Then calculate battery energy added:

\(\displaystyle 75\text{ kWh} \times 0.60 = 45\text{ kWh}\)

Apply charging efficiency to determine grid energy used:

\(\displaystyle \frac{45\text{ kWh}}{0.90} = 50\text{ kWh}\)

Calculate estimated charging time:

\(\displaystyle \frac{50\text{ kWh}}{9.6\text{ kW}} = 5.2083\text{ h}\)

Using the optional vehicle Efficiency input, calculate estimated miles added:

\(\displaystyle 45\text{ kWh} \times 3\text{ mi/kWh} = 135\text{ mi}\)

Result	Value
Battery energy added	45 kWh
Grid energy used	50 kWh
Estimated charging time	5.2083 h
Estimated miles added	135 mi
SOC window	60 percentage points

The 50 kWh grid-energy estimate is greater than the 45 kWh battery-energy result because the 90% charging-efficiency input assigns 5 kWh to charging losses. At an average Charger power of 9.6 kW, that grid energy requires approximately 5.21 hours.

Miles Added Estimate

Efficiency is an optional vehicle-efficiency input in mi/kWh. When entered, it converts Battery energy added into Estimated miles added:

\(\displaystyle \text{Estimated miles added} = \text{Battery energy added} \times \text{Efficiency}\)

The miles-added result uses battery energy added, not grid energy used. Grid losses affect utility energy consumption and charging time, but they do not add usable propulsion energy to the battery.

Vehicle efficiency varies with speed, ambient temperature, HVAC use, terrain, payload, tire condition, and driving style. Use a representative mi/kWh value for the expected operating conditions rather than a published range rating when estimating practical mileage.

Electrical Installation Limits

The calculation does not size an EV branch circuit, feeder, service, conductor, raceway, or overcurrent protective device. Charger power alone does not establish required conductor AWG or kcmil, ampacity, breaker rating, terminal temperature rating, insulation temperature rating, adjustment factor, correction factor, or voltage-drop performance.

An EVSE installation requires separate review of:

• EVSE nameplate ratings and manufacturer installation instructions.
• Available voltage, phase, and actual supply configuration.
• Branch-circuit or feeder load calculation and service capacity.
• Conductor ampacity after applicable ambient-temperature and current-carrying-conductor adjustments.
• Termination ratings, overcurrent protection, disconnecting means, grounding and bonding, and required working clearances.
• Raceway fill, routing, physical protection, and voltage-drop design for long runs.
• The adopted electrical code and local AHJ requirements.

Use the calculator to estimate charging duration, grid energy, and expected range added. Use the actual EVSE, vehicle, installation conditions, and governing code requirements to make the electrical design decision.