EV Time-of-Use Charging Savings Calculator
Why timing your EV charging matters
Electric vehicles have flipped the script on household energy use. Charging an EV can double a home’s electricity consumption, and utilities are adapting by rolling out time-of-use (TOU) rate plans that reward customers for shifting demand away from peak hours. For EV owners, enrolling in the right rate plan can slash fuel costs, reduce grid strain, and cut carbon emissions because off-peak power often comes from cleaner resources. Yet many drivers still charge as soon as they arrive home, paying peak prices that erode the savings advantage over gasoline. This calculator helps you quantify the stakes by comparing a flat-rate plan with a TOU plan using your actual driving and charging habits.
We start by translating annual miles into energy demand using your vehicle’s efficiency and charging losses. Because onboard chargers and Level 2 equipment are not perfectly efficient, the calculator accounts for wasted energy by increasing the required kWh accordingly. You can then specify how much of your charging you can shift into super off-peak windows (often midnight to 6 a.m.) and regular off-peak periods. The remaining percentage is automatically attributed to peak periods. Combined with rate inputs, the model determines monthly and annual costs under both tariff structures. It also estimates the breakeven off-peak share you need to match your current flat-rate spending.
Carbon intensity varies by time of day because peak demand typically triggers gas peaker plants while off-peak periods feature more wind or nuclear generation. By entering emissions intensities for peak and off-peak windows—values available from utility data portals or grid operators—the calculator estimates the carbon dioxide equivalent (CO2e) impact of your charging strategy. This insight helps climate-conscious drivers align savings with sustainability.
How the calculator works
The core of the model is the energy requirement derived from your annual driving distance. We compute monthly energy needs as:
where \(M\) is annual miles and \(k\) is efficiency in kWh per mile. The factor \(\lambda\) represents charging losses as a decimal. The total monthly energy is allocated across super off-peak, regular off-peak, and peak periods based on the shares you provide. Those shares must sum to 100%, and the calculator validates your inputs accordingly.
Monthly costs under the flat rate equal \(E_{month} \times R_{flat}\). Under TOU, we multiply each energy portion by its respective rate and then add any demand charge. Because some utilities offer managed charging credits that reduce or replace the base demand charge, the calculator lets you enter both the standard demand charge and the managed charge so you can compare the two outcomes side by side. The annual cost is simply the monthly cost multiplied by 12. Savings are the difference between flat-rate and TOU annual costs. The breakeven off-peak percentage solves for the portion of energy that must shift out of peak hours for TOU to match flat pricing:
The actual computation in the script adjusts for super off-peak pricing and ensures the result stays between 0% and 100%. Carbon calculations multiply the energy in each period by its emissions intensity and sum the results, then compare the total to a baseline scenario where all energy is charged at peak intensity.
Worked example
Consider a driver who logs 12,000 miles per year in a crossover rated at 0.28 kWh per mile. With 12% charging losses, the monthly energy requirement is roughly 313 kWh. The flat rate is $0.18/kWh, while the TOU plan charges $0.32/kWh at peak, $0.12/kWh off-peak, and $0.08/kWh during super off-peak hours. The driver can schedule 20% of charging during super off-peak and 50% during off-peak, leaving 30% during peak. Demand charges are not currently applied. Under the flat rate, monthly EV fueling costs are $56.34, or $676 annually. Under TOU, the blended rate works out to $0.152/kWh, yielding $47.58 per month and $571 annually. The driver saves about $105 per year, while shifting energy away from peak hours reduces emissions from 141 kg CO2e annually (all-peak baseline) to 107 kg—a 24% reduction.
If the driver can only manage 40% off-peak charging, the savings vanish and TOU becomes slightly more expensive because the higher peak rate dominates, even before accounting for any residual demand charge. The calculator will flag this by showing a breakeven off-peak share of approximately 55% for the chosen rates. Armed with that insight, the driver might invest in a smart charging scheduler or adjust the time they plug in to reach the necessary threshold.
Input tips and validation
The interface enforces sensible ranges to ensure the calculations remain realistic. Efficiency should reflect EPA ratings or real-world averages. Charging losses typically range from 8% to 16% for Level 2 charging; higher losses might indicate a 120-volt Level 1 setup. Demand charges are rare for residential customers but increasingly common for commercial fleets, so the field is optional. If your utility offers only a single off-peak window, set the super off-peak share to zero. The calculator automatically checks that the combined super off-peak and off-peak percentages do not exceed 100%; any remainder is treated as peak charging.
Comparing scenarios
The table below summarizes three sample charging profiles to highlight how schedule flexibility shapes the economics.
| Scenario | Off-peak share | Monthly TOU cost | Annual savings vs flat | Annual CO2e |
|---|---|---|---|---|
| Night owl | 85% | $42 | $171 | 96 kg |
| Balanced | 70% | $47 | $105 | 107 kg |
| Busy commuter | 45% | $55 | -$12 | 128 kg |
These figures use the default inputs provided in the form. They demonstrate that high off-peak utilization is critical to maximizing savings. Smart charging timers, utility-controlled managed charging programs, or simple behavioral changes—plugging in later in the evening—can deliver meaningful cost and emissions benefits.
Understanding carbon implications
EVs are already cleaner than gasoline cars in most regions, but the timing of charging still matters. Peak hours often rely on natural gas plants with higher marginal emissions intensity. Our carbon calculation compares total emissions from your actual charging schedule with a baseline where all energy is consumed at the peak intensity. The reduction figure helps you quantify the incremental climate benefit of managed charging. If you live in a grid region with abundant nighttime wind power, the difference can be substantial. Conversely, in hydro- or nuclear-heavy grids with flat emissions profiles, the carbon impact may be minimal; the calculator will reflect that by showing similar totals under both scenarios.
Limitations and next steps
This tool provides a high-level view and does not replace a detailed utility bill analysis. TOU plans can include seasonal rate changes, minimum bills, and demand charges triggered by maximum 15-minute usage rather than average monthly consumption. We assume constant rates and a single vehicle. If you operate multiple EVs or have rooftop solar, your optimal plan may differ. Some utilities also offer EV-specific credits, rebates for installing smart chargers, or critical peak pricing events that temporarily raise rates. Incorporate those incentives by adjusting the demand charge or off-peak percentages to approximate their effect. Finally, carbon intensities fluctuate hourly; using regional average data provides a reasonable estimate but not a definitive lifecycle analysis.
By experimenting with the inputs, you can prepare for conversations with your utility, justify investment in scheduling technology, or coach new EV owners on best practices. The calculator reinforces that electrified transportation works best when paired with smart charging habits.