EV Off-Peak Charging Schedule Calculator

Why Off-Peak Charging Matters

Electric utilities increasingly offer time-of-use rates, charging less for electricity during periods of low demand and more during evening peaks. For electric vehicle owners, timing when the car charges can dramatically affect operating costs. This calculator helps you plan your charging sessions so that as much energy as possible is drawn during cheaper off-peak hours while ensuring the battery reaches a desired state of charge before you hit the road.

The tool asks for your battery capacity and current state of charge, allowing it to compute the energy required to reach the target level. It then considers the maximum charging power of your home charger, translating energy needed into hours of charging. With the departure time you specify, the calculator determines the latest possible start time that still allows completion before departure while fitting within the off-peak window. If the required charging time exceeds the off-peak window, the remaining energy is assigned to on-peak rates.

Most online EV calculators estimate charging time or cost but do not guide you on scheduling. This page fills that niche by modeling how much energy can be shifted to cheaper periods and what savings result. The output includes a recommended start time, a breakdown of off-peak and on-peak energy, and the total cost for the session. A comparison table shows how changing the departure time affects off-peak energy usage and cost, helping you plan early departures or overnight trips.

The underlying math is straightforward yet often overlooked. Energy needed is given by battery capacity multiplied by the fraction of charge to be added. Charging time equals that energy divided by charger power. The cost equation appears as $C=\mathrm{E\_o}\mathrm{r\_o}+\mathrm{E\_p}\mathrm{r\_p}$, where E_o and E_p are energy portions charged off-peak and on-peak, with corresponding rates r_o and r_p. By maximizing E_o, drivers minimize cost.

For example, imagine a 60 kWh battery at 20% that needs to reach 80% before a morning commute. The charger supplies 7.2 kW, and off-peak hours run from 10 PM to 6 AM. The car must add 36 kWh, requiring five hours of charging. With a 7 AM departure, the latest start time is 2 AM, which lies inside the off-peak period. The entire session therefore occurs off-peak at $0.10 per kWh, costing $3.60. If you planned to depart at 5 AM, only three hours of off-peak charging would be available, leaving two hours to occur during higher rates, increasing the cost to $4.80.

The scenario table produced by the calculator offers a quick way to evaluate such trade-offs. It iterates through three departure times—5, 7, and 9 AM by default—to show how earlier departures reduce the portion of off-peak charging. You can edit the table generation logic if you want finer-grained comparisons. This visualization is particularly useful for households juggling multiple vehicles or planning longer trips.

Beyond cost savings, strategic charging helps stabilize the electrical grid. Charging overnight when demand is low reduces strain on generation and transmission infrastructure. Some utilities even offer rebates for smart chargers that automatically schedule sessions. Understanding the savings potential can justify the investment in such equipment.

There are, however, limitations. The calculator assumes a constant charging power and does not account for tapering at high state of charge, which some vehicles exhibit. It also ignores potential demand charges or minimum fees some utilities impose. Time-of-use periods may vary on weekends or holidays, and these complexities are beyond the scope of this simple tool.

If you're new to EV ownership, you may want to compare this optimization with our EV Charging Cost Calculator to see how basic charging costs are derived. For a broader perspective on where to charge, see the EV Home vs Public Charging Cost Calculator.

From an environmental standpoint, shifting charging to off-peak periods can also mean using cleaner energy mixes. Overnight electricity often comes from base-load generation sources such as wind or nuclear. By contrast, on-peak periods may rely more on fossil fuel peaker plants. Thus, your charging schedule influences not only your wallet but also the carbon intensity of your miles driven.

For households with rooftop solar, off-peak may coincide with low solar production, complicating the analysis. Some owners may choose to charge during midday when solar generation is abundant, even if rates are higher, effectively using their own electricity. This calculator is designed primarily for grid-powered scenarios but can be adapted by treating midday solar production as off-peak with an effective rate of zero.

Finally, remember that the recommended start time assumes the vehicle is plugged in and charging immediately. Smart charging apps may add delays, and vehicles may draw power for battery conditioning before charging begins. Always confirm schedules within your vehicle or charger interface.