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About this tool
Calculate a lower-cost EV charging schedule around your time-of-use tariff windows.
The EV Charging TOU Optimizer works out the cheapest way to split an EV charging session across time-of-use tariff windows: it converts the energy you need at the battery into grid energy by dividing by your charging efficiency, turns that into hours at your charger's kW rating, then fills the cheapest priced window first and spills into the next cheapest until the hours are used up. It is for anyone on a TOU or day/night electricity tariff who wants to know what a full charge actually costs and whether it fits inside the off-peak block.
Open EV Charging TOU Optimizer on AltFTool — it loads instantly in your browser.
Enter "Energy needed at battery (kWh)", "Charger power (kW)" and "Charging efficiency (%)" — the defaults are 45 kWh, 7.2 kW and 90%.
List your blocks in "Tariff windows", one per line as Start-end | price per kWh, such as 00:00-06:00 | 5.5.
The Window / Price per kWh / Hours / Grid kWh / Cost table fills the cheapest window first, and the caption reports any hours that do not fit.
Energy is grossed up from the battery figure by your efficiency percentage, so the cost is based on the kWh the meter records, not the kWh the car receives.
Windows are sorted by price and filled in order, so the split shown is the lowest-cost arrangement of the hours you actually need.
If the session is longer than the windows you entered, the leftover hours are reported rather than quietly priced at the last rate.
Take the percentage of the pack you want to add and multiply by usable capacity — adding 60% to a 75 kWh pack is about 45 kWh at the battery. Enter that figure; the grid energy, which is what you pay for, is calculated from it using your efficiency setting.
AC home charging typically lands between 85% and 92%, so 90% is a reasonable default; lower it in cold weather, when battery preconditioning and cabin heating draw extra power that never reaches the cells. At 90% efficiency, 45 kWh at the battery means 50 kWh through the meter.
Divide grid energy by charger power: 50 kWh of grid energy at 7.2 kW is about 6.9 hours. Real sessions run a little longer because charging tapers near the top of the pack, and because the car may be limited by its onboard charger rather than by the wallbox.
Not exactly. It prices energy at the per-kWh rates you enter and assumes charging runs at full power for the whole session, so standing charges, taxes, demand charges, tapering above roughly 80% state of charge and any smart-charging pauses are not included. Check your supplier's tariff terms for the definitive rates and window times.
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