EV Off-Peak Charging Schedule Calculator

Plan cheaper overnight charging without guessing

Electric-vehicle charging is one of those daily tasks that looks simple until a time-of-use electric bill arrives. Many households have a cheap overnight window, a more expensive daytime rate, and a departure deadline that does not move just because the car needs more energy than usual. This calculator is designed for that exact situation. You enter your battery size, current and desired state of charge, charger power, off-peak window, departure time, and energy rates. The tool then estimates how much energy the car needs, how long the session will take, when charging would need to begin to finish by your departure, and how much of that session lands in cheaper versus more expensive hours.

That last part matters more than many drivers expect. Two charging sessions that deliver the same number of kilowatt-hours can cost very different amounts depending on when they occur. A 36 kWh overnight session at a low off-peak rate can feel routine, while the same 36 kWh spilling into on-peak hours can noticeably raise the cost of the week. This page helps you see that tradeoff in plain numbers so you can decide whether your normal plug-in routine already works or whether shifting the start time, lowering the target charge, or using a faster charger would reduce cost.

It is also important to be clear about what this calculator does and does not do. The script on this page estimates the latest start time that still meets your departure target, then checks how much of that resulting session overlaps the off-peak window. In other words, it answers a practical question many drivers ask at night: “If I wait as long as possible and still want the car ready by the time I leave, how much of my charging will be cheap?” If that answer shows more on-peak energy than you want, the result gives you a strong hint that starting earlier could shift more charging into off-peak hours.

What each input means in real EV terms

Battery capacity (kWh) is the usable or quoted battery size you want to model. If your vehicle has a 60 kWh pack, entering 60 means each 1% of charge represents about 0.6 kWh of energy. Current state of charge is your present battery percentage when you plug in, and desired state of charge is the level you want before leaving. If you are going from 20% to 80%, you are adding 60 percentage points of charge, not 80% of the battery. Charging power (kW) is the rate your home charger actually delivers. A common Level 2 setup may provide around 7.2 kW, while other installations may be slower or faster.

The time fields use a 24-hour clock in decimal form. That means 22 is 10:00 PM, 6 is 6:00 AM, and 7.5 is 7:30 AM. This is especially helpful when your utility rate window crosses midnight. If the off-peak period starts at 22 and ends at 6, the calculator correctly interprets that as an overnight window from 10:00 PM through 6:00 AM the next day. Departure time is the moment the car must be ready. Off-peak rate and on-peak rate are your electricity prices per kilowatt-hour. Even a modest rate difference can create meaningful savings across months of routine charging.

• Use decimal hours when you need minutes, such as 22.5 for 10:30 PM or 6.25 for 6:15 AM.
• Keep SOC values between 0 and 100, and make sure the desired SOC is higher than the current SOC.
• Use the charger power you actually expect overnight, not the maximum the car can theoretically accept.
• If your utility has more than two price bands, this calculator still works as a first-pass planner by treating the cheapest period as off-peak and the rest as on-peak.

These details may sound small, but they are exactly where bad charging estimates usually begin. A time entered as 7 when you meant 7 PM, or a charger power copied from a brochure instead of your wall unit, can shift the result by hours or dollars. A good habit is to sanity-check every field once before pressing the button.

How the schedule estimate is calculated

The first step is the energy requirement. The calculator finds how much of the battery must be filled by multiplying battery capacity by the increase in state of charge. For an EV, that is the central physical quantity because electricity cost and charging time both flow from the number of kilowatt-hours you need to add. Once the required energy is known, the calculator divides by charger power to estimate charging hours. That gives the total length of the charging session if power remains constant throughout the session.

Next, the tool lines up that session with your departure time. Rather than asking when the charger should start in order to maximize off-peak energy, the page calculates the latest possible start that still reaches the target SOC by departure. Then it measures the overlap between that charging interval and the off-peak window. Whatever portion fits inside the low-cost window is counted as off-peak energy; the rest is counted as on-peak energy. Finally, the total cost is the off-peak energy multiplied by the off-peak rate plus the on-peak energy multiplied by the on-peak rate.

The EV-specific formulas can be written directly. Let C be battery capacity, S_start the current charge, S_end the target charge, and P charger power.

If you like seeing the same idea in a more abstract form, the page also preserves the general modeling formulas below. They describe the calculator as a function of several inputs and as a weighted total. In EV charging terms, those “weights” act like conversion factors and price rates that turn battery percentages and charging time into money and schedule decisions.

Worked overnight example

Suppose your EV has a 60 kWh battery, you arrive home at 20% state of charge, and you want 80% by morning. You charge with a 7.2 kW Level 2 charger. Your utility’s off-peak period runs from 22:00 to 6:00, your departure time is 7:00, the off-peak rate is $0.10 per kWh, and the on-peak rate is $0.25 per kWh. The calculator starts by finding the energy needed:

Energy needed = 60 × (80 − 20) / 100 = 36 kWh.

At 7.2 kW, the charging time is 36 / 7.2 = 5 hours. If the car must be ready at 7:00, the latest start time is 2:00. That means the session would run from 2:00 to 7:00. The off-peak window ends at 6:00, so only the four hours from 2:00 to 6:00 are off-peak. Four hours at 7.2 kW gives 28.8 kWh of off-peak charging. The remaining 7.2 kWh occurs from 6:00 to 7:00 at the higher on-peak rate. The estimated cost is therefore 28.8 × $0.10 + 7.2 × $0.25 = $4.68.

This example shows why the result is useful even when it is not a full optimization engine. If your goal is simply to learn the latest time you can wait before charging and still be ready by departure, the answer is 2:00. But if your goal is to push more of the session into cheaper hours, the example also tells you that starting earlier than 2:00 could move that final on-peak hour back into the off-peak window. In practical use, many drivers run several scenarios: one at their normal departure time, another with a slightly lower target SOC, and a third with a different charger power or departure hour.

How to read the result and scenario table

After you submit the form, the result line gives four pieces of information: the latest start time, the off-peak energy, the on-peak energy, and the total cost. The start time is especially helpful if your charger or vehicle app lets you schedule charging directly. The energy split tells you whether the cheap-rate window is wide enough for your planned session. The total cost combines that split with your utility rates, which is what makes the result actionable instead of merely descriptive.

The small scenario table underneath is a quick comparison aid for three departure hours. It keeps the same battery and rate assumptions but swaps the departure time to show how cost changes if you leave earlier or later. This is a realistic planning tool for commuters with flexible mornings. If the 9:00 departure is much cheaper than 7:00, that means the extra time before leaving allows more charging to happen inside the off-peak window. If the cost barely changes across the table, your session already fits mostly inside the cheaper period or your rate difference is relatively small.

Assumptions, limits, and useful edge cases

This calculator uses a clean planning model, so a few real-world details are simplified. It assumes constant charging power instead of tapering at high states of charge, which some EVs do near the top of the battery. It also does not add charging losses, which means the actual energy pulled from the wall may be slightly higher than the battery energy gained. If you need a conservative estimate, you can account for this by increasing the desired SOC slightly or mentally adding a small margin to the cost.

Another edge case involves off-peak windows that cross midnight, which is common and fully supported. If you enter an off-peak start of 22 and an off-peak end of 6, the page treats that as a window extending through midnight into the next morning. The departure time is likewise normalized so that early-morning departures are understood as the following day when appropriate. This is one of the most important scheduling details on the page because overnight charging almost always depends on midnight logic.

What if the off-peak window is too short for the energy you need? Then the calculator will still compute the session, but some energy will land on-peak. What if the desired SOC is not higher than the current SOC? The calculator correctly asks you to fix the inputs because no charging is needed or the target is invalid. What if charger power is zero? Again, the page stops and asks for a real value because dividing by zero would make the charging time meaningless.

For decisions with money attached, the best interpretation is not “this number will exactly match my bill” but “this is my planning estimate under clear assumptions.” That is enough to answer valuable questions such as whether your overnight window is wide enough, whether a faster home charger would shift more energy into cheap hours, or whether charging to 70% instead of 80% on ordinary weekdays would save enough to matter.

Practical ways to use the calculator

Many drivers only need three quick tests. First, use your usual arrival and departure pattern and see whether the latest start time feels convenient. Second, increase the target SOC for a long-drive day and see how much extra energy spills into on-peak hours. Third, compare two rate assumptions if your utility has seasonal pricing. These scenario checks are often more valuable than one perfect number because they show which variable is really driving cost: battery size, desired charge, charger speed, or departure pressure.

The broader lesson is straightforward. More required kWh increases both charging time and the risk of spilling outside the off-peak window. More charger power reduces charging time and makes it easier to stay inside the cheaper period. Earlier departures leave less schedule flexibility. Wider rate gaps make timing mistakes more expensive. Once you see those relationships clearly, EV charging becomes less of a late-night guess and more of a routine you can manage.