Calculate how long it takes to charge an EV from battery capacity, charge level, and charger power, covering Level 1, Level 2, and DC fast charging.
EV Charging Time Formula
T = (C * (SOCt - SOCc) / 100) / (P * eff)
- T is the charging time in hours
- C is the usable battery capacity in kilowatt-hours (kWh)
- SOCt is the target state of charge in percent
- SOCc is the current state of charge in percent
- P is the charger power in kilowatts (kW)
- eff is the charging efficiency as a decimal, typically about 0.90 for AC charging and 0.93 for DC fast charging
The calculator’s default mode uses this formula directly. It multiplies the battery capacity by the percentage of charge you need, then divides by the charger power after accounting for efficiency losses. If you enter a vehicle maximum charging power, the calculator caps the charger power at that value, which matters for AC charging because the car’s onboard charger, not the wall unit, often sets the real limit. For DC fast charging past 80%, the calculator models the slowdown that nearly all EVs apply to protect the battery: half speed from 80% to 90% and quarter speed from 90% to 100%.
The second mode solves the same relationship for power instead of time:
P = (C * (SOCt - SOCc) / 100) / (T * eff)
Enter how many hours you have, and the calculator returns the charger power in kW you need, along with whether a Level 1 outlet, a Level 2 charger, or DC fast charging fits that requirement. This is useful when shopping for a home charger or deciding whether an overnight window is long enough.
Charging Levels and Typical Speeds
The table below shows common charger power levels and how long each takes to run a typical 20% to 80% session on a 75 kWh battery. Range added per hour assumes a vehicle efficiency of 3 mi/kWh.
| Charger | Power | Range added per hour | 20-80% time (75 kWh) |
|---|---|---|---|
| Level 1 (120V outlet) | 1.4 kW | 3 to 4 mi | about 36 hours |
| Level 2 (16A) | 3.7 kW | about 10 mi | 13h 31m |
| Level 2 (32A) | 7.4 kW | about 20 mi | 6h 45m |
| Level 2 (48A) | 11.5 kW | about 31 mi | 4h 21m |
| Level 2 (80A) | 19.2 kW | about 52 mi | 2h 36m |
| DC fast | 50 kW | up to 140 mi | 58m |
| DC fast | 150 kW | up to 400 mi | 19m |
| DC fast | 250 kW | session limited | 12m |
Charging is never 100% efficient, and the loss depends on the charger type and the weather. The next table shows typical efficiency by scenario and what that means for a session that puts 45 kWh into the battery. This is why your wall meter reads higher than the energy your car reports gaining.
| Scenario | Typical efficiency | Wall energy to add 45 kWh |
|---|---|---|
| DC fast, mild weather | 93% to 95% | 47 to 48 kWh |
| Level 2, mild weather | 88% to 92% | 49 to 51 kWh |
| Level 2, below freezing | 80% to 88% | 51 to 56 kWh |
| Level 1, any weather | 75% to 85% | 53 to 60 kWh |
Level 1 loses the most because the car’s electronics draw a fixed overhead the whole time it is plugged in, and a slow charge stretches that overhead over many more hours. Cold weather adds losses at any level because part of the power goes to heating the battery instead of charging it. If you want the calculator to reflect winter conditions, open the advanced options and lower the efficiency to 80% to 85%.
Example Problems
Example 1: You have a 75 kWh battery at 20% and want to reach 80% on a 7.4 kW Level 2 charger at 90% efficiency. The energy needed is 75 * (80 – 20) / 100 = 45 kWh. The effective charging rate is 7.4 * 0.90 = 6.66 kW. The charging time is 45 / 6.66 = 6.76 hours, or about 6 hours 45 minutes, and the wall draws 45 / 0.90 = 50 kWh.
Example 2: You need a 60 kWh battery to go from 10% to 80% during a 5 hour window and want to know what charger to buy. The energy needed is 60 * (80 – 10) / 100 = 42 kWh. The battery-side rate is 42 / 5 = 8.4 kW, and dividing by 0.90 efficiency gives 8.4 / 0.90 = 9.33 kW. A 9.6 kW (40A) Level 2 charger covers this with a small margin.
Frequently Asked Questions
Why do DC fast chargers slow down after 80%?
As the battery fills, empty spots for lithium ions become scarce, and forcing a high current into a nearly full pack causes heat and long-term damage. The car’s battery management system tapers the charging power, typically starting around 80% state of charge, which is why a 10% to 80% session is far faster per kWh than the last 20%. On road trips it is usually quicker to stop at 80% and charge again later than to wait for 100%.
Why is my actual charging time longer than the calculated time?
The most common reasons are cold weather, which diverts power to battery heating, an onboard charger that accepts less power than the wall unit can supply, and charging curves that ramp power up slowly at the start of a session. Shared chargers can also throttle when multiple cars plug in. Enter your vehicle’s maximum charging power in the advanced options and lower the efficiency in winter to bring the estimate closer to reality.
Should I enter total or usable battery capacity?
Use usable capacity when you know it. Manufacturers reserve a buffer, often 3% to 8% of the total pack, that the state of charge display never touches, so percentages shown in the car map to the usable figure. If you only know the total capacity, the calculator will slightly overestimate charging time, which is a safe direction for planning.
