EV vs Petrol Running Costs: Common Mistakes, Tips and FAQs

Introduction

UK drivers juggle imperial gallons, pence per litre and pence per kWh. Converting everything to the same basis is the key to fair comparisons.

This article covers the most common mistakes people make with eV vs Petrol Running Costs, what really moves the result, and answers to the questions we hear most often.

Why it matters

Accurate motoring numbers help you budget journeys, claim mileage relief from HMRC and decide whether an electric car makes financial sense for you.

How it's calculated

Petrol cost uses miles ÷ MPG × 4.546 × price per litre. EV cost uses miles ÷ miles per kWh × price per kWh.

What changes the result most

We raised each input by 10% on its own, starting from a typical example where yearly saving with EV is £560.04. Here's what happened:

• Raising petrol price from 140 p/litre to 154 p/litre changes yearly saving with EV to £673.19 (+20.2%).
• Raising petrol car MPG from 45 to 49.5 changes yearly saving with EV to £457.18 (-18.4%).
• Raising electricity price from 25 p/kWh to 27.5 p/kWh changes yearly saving with EV to £502.90 (-10.2%).
• Raising miles per year from 8,000 to 8,800 changes yearly saving with EV to £616.05 (+10.0%).
• Raising eV efficiency from 3.5 miles/kWh to 3.85 miles/kWh changes yearly saving with EV to £611.99 (+9.3%).

The result is especially sensitive to petrol price – a 10% change there moves it by more than 10%, so get that figure right first.

Common mistakes to avoid

1. Using US MPG figures for UK calculations.
2. Ignoring depreciation and insurance in cost-per-mile estimates.
3. Forgetting to claim mileage allowance relief when your employer pays below HMRC rates.
4. Comparing EVs using public charging prices when you'll mostly charge at home.

A quick example

With the inputs below, yearly saving with EV comes out at £560.04.

Tips

• UK MPG uses imperial gallons (4.546 litres), which differ from US gallons.
• HMRC's approved mileage rate is 45p per mile for the first 10,000 business miles.
• Home EV charging on an off-peak tariff is usually far cheaper than public rapid chargers.
• Depreciation is often the largest hidden cost of owning a newer car.

Frequently asked questions

What's the quickest way to work out eV vs Petrol Running Costs?

Use our free eV vs Petrol Cost Calculator. It applies this method automatically: Petrol cost uses miles ÷ MPG × 4.546 × price per litre. EV cost uses miles ÷ miles per kWh × price per kWh.

What MPG should I use?

Use your real-world figure from your car's trip computer if you can; official figures are often optimistic.

Can I claim for commuting miles?

No. Normal commuting to a permanent workplace doesn't count as business mileage.

How often do HMRC mileage rates change?

Rarely. The 45p rate has been unchanged for many years, but always check GOV.UK.

Related calculators

• Fuel Cost Calculator
• MPG Converter
• Car Cost Per Mile Calculator
• Journey Time Calculator
• EV Charging Cost Calculator
• Car Depreciation Calculator

For the full method, read How to Calculate EV vs Petrol Running Costs in the UK: Step-by-Step Guide.