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Calculator / Cheap-Rate Window
Plan overnight charging

Cheap-Rate Window

Estimate how much of a home charge lands inside your off-peak window and how much spills into the standard rate.

The vehicle’s usable battery capacity in kWh, not its advertised gross capacity.
The battery percentage when the car is plugged in.
The battery percentage you want to reach by the end of the session.
The home charger’s maximum output. Most UK wallboxes are around 7.4 kW; a three-pin plug is roughly 2.3 kW.
The vehicle’s maximum AC charging rate from its specification. The lower of this and Charger power limits the speed.
Electricity drawn from the grid but not stored in the battery. Leave at 10% if unsure.
Your off-peak unit price in pence per kWh, excluding the standing charge.
Your normal daytime unit price in pence per kWh, excluding the standing charge.
The time your tariff’s cheaper electricity period begins. Check the exact terms because some tariffs use several separate windows.
The time the cheaper period ends. Overnight windows that cross midnight are handled automatically.
The time charging can actually begin—not necessarily when you arrive home if the car or charger is scheduled.

Can your EV finish charging during the off-peak rate?

This tool compares the energy your battery needs with the useful charging time available between the tariff’s start and end times. Overnight windows that cross midnight are handled automatically.

Use the effective charging speed

A charger rated at 7.4 kW may be limited by the car, the supply or charging losses. Enter the charger output and your vehicle’s AC limit so the estimate uses the slower of the two.

If the target does not fit

The result shows the estimated battery percentage achievable before the cheap window closes. You can then lower the target, start with more charge or allow some charging at the standard rate.

Use the main EV charging calculator to price the session, or calculate the charge needed for a journey.