Schedule EV charging around your overnight rate window
Time-of-use EV charging is simple in principle but easy to misjudge when a battery needs more energy than usual and the morning departure time is fixed. This EV off-peak charging schedule calculator is built for that overnight decision. Enter battery capacity, current and target state of charge, charger power, your utility’s off-peak hours, departure time, and both electricity rates. The result estimates the energy needed, charging duration, latest feasible start time, the kWh that fit in the lower-priced window, and the kWh that remain at the higher rate.
The timing can materially change the cost of adding the same battery energy. A session that remains within the overnight rate period uses the off-peak price for every kWh, while a session that continues after that window closes has an on-peak portion. The estimate makes that split visible so you can judge whether your usual plug-in routine is adequate or whether a different start time, target charge, charger power, or departure plan would be more economical.
This page specifically calculates the latest charging start that still reaches the target SOC by the entered departure time. It then checks the overlap between that latest-start session and the off-peak window. It does not search for a different start time to maximize cheap energy. If the displayed schedule includes on-peak charging, starting earlier may place more of the session inside an earlier overnight off-peak period when your real-world parking and charging access allow it.
EV battery, charger, rate, and time inputs explained
Battery capacity (kWh) is the battery size used for the estimate. With a 60 kWh battery, each percentage point represents 0.6 kWh in this simplified model. Current state of charge is the SOC when the vehicle is ready to charge, while desired state of charge is the SOC required before departure. Moving from 20% to 80% means adding 60 percentage points, or 60% of the entered battery capacity. Charging power (kW) is the sustained power expected from your charging equipment, not necessarily the vehicle’s maximum advertised AC or DC charging capability.
EV rate-window and departure times use decimal 24-hour notation. Enter 22 for 10:00 PM, 6 for 6:00 AM, and 7.5 for 7:30 AM. When the off-peak interval starts at 22 and ends at 6, the calculator treats it as an overnight window spanning midnight. The off-peak rate and on-peak rate are electricity prices in dollars per kWh, so they can be multiplied directly by the energy allocated to each period.
- Use decimal hours for minutes, such as 22.5 for 10:30 PM or 6.25 for 6:15 AM.
- Keep each SOC value from 0 to 100, with desired SOC higher than current SOC.
- Enter the charging power you expect the vehicle to receive during this session.
- For tariffs with several rate bands, use this as a two-rate planning approximation by entering the lowest period as off-peak and the remaining period as on-peak.
Small input errors can change an overnight EV plan substantially. Entering 7 for 7 AM versus 7 PM, using the wrong charger rating, or overlooking a rate-window boundary can move the apparent schedule by hours. Check the time convention, SOC difference, and utility prices before relying on the result.
How the EV off-peak charging calculation works
The EV schedule estimate first converts the requested SOC increase into battery energy. It multiplies battery capacity by the difference between target and current SOC expressed as a fraction. It then divides that required energy by charging power to estimate how many hours the session needs at constant power. These steps determine both the kWh to price and the time interval that must be fitted before departure.
The calculator places the end of that charging interval at the departure time, producing the latest feasible start. It measures the overlap between that interval and the entered off-peak window. Overlapping hours are converted to off-peak energy using charger power; any remaining required battery energy is on-peak energy. Total estimated cost is the sum of each energy amount multiplied by its corresponding rate.
Let C be battery capacity, Sstart current SOC, Send target SOC, and P charging power. The battery energy required and charging duration are:
After the rate-window overlap has been found, the cost calculation is:
The off-peak overlap is the key scheduling step. A longer charge, an earlier off-peak end, or a tighter departure deadline tends to increase the on-peak portion. Higher charging power shortens the interval and can allow more of the required battery energy to remain in the lower-cost window.
Worked example: a 60 kWh EV charging before a 7 AM departure
Consider a 60 kWh EV arriving at 20% SOC and needing 80% by 7:00 AM. With a 7.2 kW charger, an off-peak period from 22:00 to 6:00, an off-peak price of $0.10 per kWh, and an on-peak price of $0.25 per kWh, the requested SOC increase is 60 percentage points. The battery energy needed is 60 × (80 − 20) ÷ 100, or 36 kWh.
At 7.2 kW, 36 kWh requires 5 charging hours. Finishing at 7:00 AM makes 2:00 AM the latest start. The session runs from 2:00 AM through 7:00 AM, with four hours before the 6:00 AM off-peak cutoff. Those four hours deliver 28.8 kWh off-peak; the final hour delivers 7.2 kWh on-peak. The estimated cost is 28.8 × $0.10 plus 7.2 × $0.25, which equals $4.68.
This EV example distinguishes the latest possible start from the cheapest possible behavior. Starting at 2:00 AM is sufficient to meet the 7:00 AM departure target, but it leaves one hour at the on-peak rate. Starting earlier, if the vehicle is available to charge, could move that last hour into the 22:00–6:00 low-rate window.
Reading the EV charging schedule and departure comparison
The EV charging result reports the latest start time, off-peak energy, on-peak energy, and total cost. Use the start time with your vehicle or charger scheduling controls if its timing fits your routine. The energy split shows whether the selected rate window is long enough for the requested battery charge, while the cost shows the effect of the split at your entered electricity prices.
The comparison table below recalculates the same EV battery and rate assumptions for departures at 5, 7, and 9. It is useful for seeing how a different morning deadline changes the amount of energy that can fall in the off-peak period. A later departure can provide more time after the overnight rate window, but the table still bases each result on the latest start that reaches the target by that particular departure time.
EV charging assumptions and overnight scheduling limits
This EV planning calculator assumes constant charging power from the beginning to the end of the session. It does not model charging losses or power tapering near a high state of charge, so wall energy and billed cost may differ from the battery-energy estimate. Treat the result as a transparent planning estimate rather than an exact statement of what a utility bill will show.
Overnight off-peak windows crossing midnight are supported. An off-peak start of 22 and end of 6 represents 10:00 PM through 6:00 AM the following morning. The departure time is normalized in the same overnight timeline so that an early-morning departure is compared with the correct rate period.
If the required EV charging duration exceeds the available off-peak overlap, the calculator assigns the remainder to on-peak energy. If target SOC is not higher than current SOC, the form requests corrected values because the model is intended for a positive charging session. A zero or negative charging-power value is also rejected because charging time cannot be computed from it.
For a conservative real-world plan, leave enough time for charging behavior that this simplified model does not capture. The most useful question is usually whether your overnight window is comfortably large enough for the desired SOC, not whether the displayed dollar amount exactly matches every line item on a bill.
Ways to compare everyday EV off-peak charging plans
Use this EV off-peak calculator for a few focused checks: model a normal weekday target SOC, raise the target for a longer drive, and compare an earlier or later departure. You can also compare the charging power available from different home equipment or use seasonal utility rates. These tests reveal whether battery energy, charger speed, the departure deadline, or the rate difference is the main source of cost pressure.
The relationship is direct: more required kWh increases charging time and makes a spill into on-peak hours more likely. More charging power shortens the session. An earlier departure reduces scheduling flexibility, and a larger difference between on-peak and off-peak prices makes timing more consequential. With those relationships in view, scheduling overnight EV charging becomes a repeatable rate-window decision rather than a guess.
Enter EV battery, charger, and off-peak rate details
Use decimal hours on a 24-hour clock. For example, 22 = 10:00 PM, 6 = 6:00 AM, and 22.5 = 10:30 PM.
EV off-peak charging estimate
| Departure Hour | Off-Peak Energy (kWh) | Total Cost ($) |
|---|
Mini-game: Charge Window Rush
Optional, separate, and purely for fun: this arcade challenge turns the same overnight charging idea into a fast timing game. Blue bands are cheap off-peak energy, red bands are regular on-peak hours, and orange surge bands punish sloppy charging. Hold or tap only when the charger lines up with the blue window, build a streak, fill the battery before departure, and keep the charger heat under control.