What Does the EV Tariff Calculator & Break-Even Do?
This tool compares the annual electricity bill under a flat-rate tariff and a tariff with separate peak and off-peak prices. It includes the home as well as the car, because an attractive overnight charging rate can be offset by a higher price for daytime household electricity. The result shows annual totals, the saving at your chosen charging pattern and how the comparison changes as the EV's off-peak share varies.
How to Use This Calculator
- Enter annual non-EV household kWh separately from EV mileage. Do not include the car in both your household consumption and the mileage-derived charging estimate. Enter the household's off-peak energy share independently.
- Enter EV efficiency, charging efficiency and the proportion of EV energy expected in the cheap window. Use annual energy shares and check whether the tariff applies the cheap rate to the whole home or only specific charging.
- Enter the flat tariff's unit rate and daily standing charge, then the alternative's peak rate, off-peak rate and standing charge. Use pence in all price fields and compare quotes on the same tax basis.
- Calculate the annual totals and inspect the off-peak scenarios. If the threshold is outside 0–100%, read the explanation: one tariff may win throughout the feasible range, or never win under these inputs.
The Maths & Assumptions Behind the Tool
Let H be non-EV household kWh and E be purchased EV kWh. The flat bill is (H + E) × flat rate ÷ 100, plus 365 daily standing charges. Each alternative energy block uses peak rate × peak share plus off-peak rate × off-peak share. The tool varies the EV share while holding household share constant and solves where the bill difference reaches zero. Equal rates, reversed rates and zero EV mileage require separate interpretations rather than division by zero.
Worked example
Start with 3,200 household kWh, 10,000 EV miles, 3.5 miles/kWh and 90% charging efficiency. The EV buys 3,174.60 kWh from the meter. Compare a flat 26p/kWh tariff with 31p peak and 8p off-peak. Both standing charges are 50p/day. Assume no household electricity shifts off-peak and 85% of EV energy does.
The standard bill is £1,839.90/year. The EV-tariff bill is £1,537.99/year, saving £301.90 before rounding. EV charging must be at least 43.65% off-peak to match the standard bill. These are illustrative rates, not an available tariff quote.
Why This Matters for UK Drivers
There are two different off-peak shares. One belongs to non-EV household electricity, such as appliances that can be scheduled into the cheap window. The other belongs to EV charging. Keep them separate and base each on energy, not time alone. Running an appliance for an hour does not imply the same kWh as charging a car for an hour. The model applies the entered off-peak rate to both shares, so check whether your actual tariff permits that assumption.
Eligibility and tariff terms remain outside the arithmetic. A cheap rate may require particular equipment, a compatible car, a smart meter or a provider-controlled schedule. Some offers apply a special price only to eligible vehicle charging while the rest of the home follows another rate. Model only the prices and periods offered to you. If the saving is small, exit fees, missed cheap windows or a modest change in winter efficiency may matter more than the headline rate.
Interpreting the result in practice
Compare annual quotes, not just the cheapest unit rate. A small peak-rate increase applied to a large household load can outweigh the car's overnight saving. Conversely, shifting a suitable appliance into the cheap window can affect the household side independently of EV charging. Only include a shift you expect to achieve, and follow the appliance's safety instructions.
The model uses 365 days for the annual standing charge and constant prices over that period. It does not forecast future tariffs or model seasonal price changes. Exit fees, equipment costs and introductory offers need their own treatment. If a provider gives a personalised annual estimate, reconcile its assumed kWh and charging split with yours before attributing a difference to an error.
Frequently asked questions
Why include household electricity?
Because changing to an EV tariff can change the price of electricity used by the rest of your home, not just the car.
What does the break-even percentage mean?
It is the minimum share of your EV charging that needs to be billed at the off-peak rate for the EV tariff's annual total to equal the standard tariff, based on your inputs.
What about gas?
This tool compares electricity only.
Compare the whole electricity bill
What the number tells you
An EV tariff can offer very cheap overnight charging while increasing the daytime unit rate. Looking only at the EV rate can therefore be misleading. The relevant question is whether savings on off-peak charging outweigh any extra cost on the rest of the household electricity use.
Use realistic inputs
Use a realistic household daytime consumption figure and estimate how much EV energy can genuinely be shifted into the cheap window. Smart charging can make a high off-peak share achievable, but work patterns, charger availability and battery needs may prevent every kWh from being bought at the lowest rate.
What is outside the calculation
Standing charges are included in both annual totals. Run a second scenario for winter efficiency, household consumption and the energy you can realistically move off-peak. Gas, exit fees and charger installation are excluded.