Choose a consistent comparison period
Begin with how long you expect to keep the vehicle and how many miles you expect to cover. Compare cars over the same period, mileage and financing basis. A monthly lease figure cannot be meaningfully set against an outright purchase price without accounting for the initial payment, contract length, mileage allowance and what you own at the end.
For a purchased car, depreciation is the difference between acquisition cost and eventual disposal value. The latter is uncertain, not a number a calculator can know in advance. Use several residual-value scenarios rather than assuming that either a strong or weak used market will persist. If financing is involved, account for borrowing costs without counting the same vehicle capital twice.
Calculate billed energy, not just battery energy
At an assumed 10,000 annual miles and 4 miles per battery kWh, the car consumes 2,500 kWh from its battery. If wall-to-battery charging efficiency is assumed to be 90%, approximately 2,778 kWh must be supplied. That difference is an energy loss, not extra driving. It can vary with equipment, temperature, charging rate and vehicle overheads.
The following examples apply one assumed blended electricity price to all billed energy. They are not current tariff quotes or claims about any named operator. Real public-charging billing may measure energy differently from a household meter, and session or parking fees may be additional. Keep your actual invoices if you want a defensible running-cost record.
| Assumed blended rate | Annual electricity | Energy cost per mile |
|---|---|---|
| £0.15/kWh | £416.67 | 4.17p |
| £0.25/kWh | £694.44 | 6.94p |
| £0.60/kWh | £1,666.67 | 16.67p |
Expand the arithmetic and exclusions
Annual battery energy = 10,000 ÷ 4 = 2,500 kWh. Supply energy = 2,500 ÷ 0.90 = 2,777.78 kWh. Multiply by the assumed price, then divide the annual cost by 10,000 to obtain pounds per mile. Values are rounded at presentation. Standing charges, subscriptions, parking, installation, taxation and vehicle costs are excluded.
Weight the charging mix honestly
A driver with a driveway should not assume that every mile will be powered at an overnight rate. Holidays, workplace charging and motorway trips can change the average. Calculate the share of billed energy obtained through each source and multiply each share by its corresponding price. Weight by energy, not merely by the number of sessions.
A fictional mix of 80% at £0.20/kWh and 20% at £0.70/kWh produces a blended £0.30/kWh before separate fees. If you cannot charge at home, do not borrow the home-charging economics of someone who can. Conversely, a public-heavy example should not be presented as every household’s likely experience.
Put the other bills back in
Obtain an insurance quote for the exact model before placing an order. Include tyres, servicing, inspections, roadside assistance and any charging subscriptions you expect to retain. Electric vehicles avoid some combustion-engine maintenance items, but they are not maintenance-free. Tyre size, vehicle weight, driving style and repair arrangements can materially affect bills.
Home installation deserves its own line. A simple wallbox price may exclude groundworks, long cable runs, supply upgrades or unusual parking arrangements. Ask for a written, property-specific quote and clarify what support remains if the equipment supplier changes its service. Do not assume the installation cost is recovered unless your expected savings over your ownership period support that conclusion.
Check current tax rules for your jurisdiction and registration circumstances. UK Vehicle Excise Duty, company-car treatment and local charges should be verified against official guidance rather than an old forum post. This guide deliberately does not freeze a tax amount into a generic example that may be wrong for your vehicle.
Compare the decision you actually face
Replacing a working car early is different from choosing between two cars you already need to buy. For the first decision, account for the value and likely remaining service of the existing vehicle. For the second, compare total expected ownership costs and practical suitability. Fuel savings alone do not settle either question.
Keep environmental claims similarly bounded. Lifecycle outcomes depend on manufacturing, battery production, electricity supply, vehicle size and lifetime use. An electric drivetrain has no tailpipe exhaust emissions during driving, but that does not make manufacturing or electricity generation impact-free. A smaller efficient vehicle, fewer unnecessary miles and longer useful service can all belong in a sustainable transport decision.