Time-of-Use Electricity Savings

How time-of-use electricity savings work

Many electricity suppliers offer time-of-use tariffs with lower rates during off-peak hours—typically overnight—and higher rates during peak daytime periods. Electric vehicle owners who charge primarily during off-peak windows can reduce their annual electricity costs compared to charging at peak rates. This calculator computes the annual saving by comparing the cost of a blended charging schedule (part off-peak, part peak) against the cost of charging the same annual kWh volume entirely at the peak rate.

The formula

The tool applies a weighted average to determine blended cost:

Blended cost = kWh_year × (f × rate_off-peak + (1 − f) × rate_peak)

where f = (% charged off-peak) ÷ 100.

Peak-only cost = kWh_year × rate_peak

Annual saving = Peak-only cost − Blended cost

The calculator also reports an effective rate (blended cost ÷ kWh_year) to show the average price per kilowatt-hour under the mixed schedule.

Where this method is most accurate

The formula assumes that off-peak and peak rates remain constant throughout the year and that the user-entered percentage reflects actual charging behaviour. Real-world accuracy depends on correctly estimating annual kWh consumption—typically derived from miles driven and the vehicle's rated efficiency (miles/kWh or kWh/100 km)—and on the precision of the off-peak share. Tariffs with shoulder periods, seasonal rate changes, or demand charges are not modelled. Charging losses (AC-to-battery conversion inefficiency, typically 10–15%) are not included unless the user adjusts the annual kWh input upward.

What this tool does not do

It does not calculate the vehicle's energy consumption from mileage; users must supply total kWh per year. It does not account for standing charges, export payments under vehicle-to-grid schemes, or any tariff structure beyond a simple two-tier peak/off-peak split. The calculator does not model battery degradation, preconditioning energy draw, or the tapering of charge rates at high states of charge. It does not provide financial advice or recommend specific tariffs or suppliers.

Disclaimer

This calculator is an educational tool that performs arithmetic on user-supplied values. Outputs are estimates only. Actual electricity costs depend on tariff terms, metering accuracy, seasonal variation, and driving patterns. No calculation on this page constitutes financial, vehicle-purchase, or energy-contract advice. Users are responsible for verifying their own tariff rates and consumption data before making decisions.